vnf.scad

//////////////////////////////////////////////////////////////////////
// LibFile: vnf.scad
//   The Vertices'N'Faces structure (VNF) holds the data used by polyhedron() to construct objects: a vertex
//   list and a list of faces.  This library makes it easier to construct polyhedra by providing
//   functions to construct, merge, and modify VNF data, while avoiding common pitfalls such as
//   reversed faces.  It can find faults in your polyhedrons.  Note that this file is for low level manipulation
//   of lists of vertices and faces: it can perform some simple transformations on VNF structures
//   but cannot perform boolean operations on the polyhedrons represented by VNFs.
// Includes:
//   include <BOSL2/std.scad>
// FileGroup: Advanced Modeling
// FileSummary: Vertices 'n' Faces structure.  Makes polyhedron() easier to use.
// FileFootnotes: STD=Included in std.scad
//////////////////////////////////////////////////////////////////////


// Section: Creating Polyhedrons with VNF Structures
//   VNF stands for "Vertices'N'Faces".  VNF structures are 2-item lists, `[VERTICES,FACES]` where the
//   first item is a list of vertex points, and the second is a list of face indices into the vertex
//   list.  Each VNF is self contained, with face indices referring only to its own vertex list.
//   You can construct a `polyhedron()` in parts by describing each part in a self-contained VNF, then
//   merge the various VNFs to get the completed polyhedron vertex list and faces.

/// Constant: EMPTY_VNF
/// Description:
///   The empty VNF data structure.  Equal to `[[],[]]`.
EMPTY_VNF = [[],[]];  // The standard empty VNF with no vertices or faces.


// Function: vnf_vertex_array()
// Synopsis: Returns a VNF structure from a rectangular vertex list.
// SynTags: VNF
// Topics: VNF Generators, Lists
// See Also: vnf_tri_array(), vnf_join(), vnf_from_polygons(), vnf_from_region()
// Usage:
//   vnf = vnf_vertex_array(points, [caps=], [cap1=], [cap2=], [style=], [reverse=], [col_wrap=], [row_wrap=], [triangulate=]);
// Description:
//   Creates a VNF structure from a rectangular vertex list, creating edges that connect the adjacent vertices in the vertex list
//   and creating the faces defined by those edges.  You can optionally create the edges and faces to wrap the last column
//   back to the first column, or wrap the last row to the first.  Endcaps can be added to either
//   the first and/or last rows.  The style parameter determines how the quadrilaterals are divided into
//   triangles.  The default style is an arbitrary, systematic subdivision in the same direction.  The "alt" style
//   is the uniform subdivision in the other (alternate) direction.  The "flip1" style is an arbitrary division which alternates the
//   direction for any adjacent pair of quadrilaterals.  The "flip2" style is the alternating division that is the opposite of "flip1". 
//   The "min_edge" style picks the shorter edge to
//   subdivide for each quadrilateral, so the division may not be uniform across the shape.  The "quincunx" style
//   adds a vertex in the center of each quadrilateral and creates four triangles, and the "convex" and "concave" styles
//   choose the locally convex/concave subdivision.  The "min_area" option creates the triangulation with the minimal area.  Degenerate faces
//   are not included in the output, but if this results in unused vertices they will still appear in the output.
// Arguments:
//   points = A list of vertices to divide into columns and rows.
//   ---
//   caps = If true, add endcap faces to the first AND last rows.
//   cap1 = If true, add an endcap face to the first row.
//   cap2 = If true, add an endcap face to the last row.
//   col_wrap = If true, add faces to connect the last column to the first.
//   row_wrap = If true, add faces to connect the last row to the first.
//   reverse = If true, reverse all face normals.
//   style = The style of subdividing the quads into faces.  Valid options are "default", "alt", "flip1", "flip2",  "min_edge", "min_area", "quincunx", "convex" and "concave".
//   triangulate = If true, triangulates endcaps to resolve possible CGAL issues.  This can be an expensive operation if the endcaps are complex.  Default: false
// Example(3D):
//   vnf = vnf_vertex_array(
//       points=[
//           for (h = [0:5:180-EPSILON]) [
//               for (t = [0:5:360-EPSILON])
//                   cylindrical_to_xyz(100 + 12 * cos((h/2 + t)*6), t, h)
//           ]
//       ],
//       col_wrap=true, caps=true, reverse=true, style="alt"
//   );
//   vnf_polyhedron(vnf);
// Example(3D): Both `col_wrap` and `row_wrap` are true to make a torus.
//   vnf = vnf_vertex_array(
//       points=[
//           for (a=[0:5:360-EPSILON])
//               apply(
//                   zrot(a) * right(30) * xrot(90),
//                   path3d(circle(d=20))
//               )
//       ],
//       col_wrap=true, row_wrap=true, reverse=true
//   );
//   vnf_polyhedron(vnf);
// Example(3D): Möbius Strip.  Note that `row_wrap` is not used, and the first and last profile copies are the same.
//   vnf = vnf_vertex_array(
//       points=[
//           for (a=[0:5:360]) apply(
//               zrot(a) * right(30) * xrot(90) * zrot(a/2+60),
//               path3d(square([1,10], center=true))
//           )
//       ],
//       col_wrap=true, reverse=true
//   );
//   vnf_polyhedron(vnf);
// Example(3D): Assembling a Polyhedron from Multiple Parts
//   wall_points = [
//       for (a = [-90:2:90]) apply(
//           up(a) * scale([1-0.1*cos(a*6),1-0.1*cos((a+90)*6),1]),
//           path3d(circle(d=100))
//       )
//   ];
//   cap = [
//       for (a = [0:0.01:1+EPSILON]) apply(
//           up(90-5*sin(a*360*2)) * scale([a,a,1]),
//           wall_points[0]
//       )
//   ];
//   cap1 = [for (p=cap) down(90, p=zscale(-1, p=p))];
//   cap2 = [for (p=cap) up(90, p=p)];
//   vnf1 = vnf_vertex_array(points=wall_points, col_wrap=true);
//   vnf2 = vnf_vertex_array(points=cap1, col_wrap=true);
//   vnf3 = vnf_vertex_array(points=cap2, col_wrap=true, reverse=true);
//   vnf_polyhedron([vnf1, vnf2, vnf3]);
// Example(3D): Building a Multi-Stage Cylindrical Ramp
//   include <BOSL2/rounding.scad>
//   major_r = 50;
//   groove_profile = [
//       [-10,0], each arc(points=[[-7,0],[0,-3],[7,0]]), [10,0]
//   ];
//   ramp_profile = [ [-10,25], [90,25], [180,5], [190,5] ];
//   rgroove = apply(right(major_r) * xrot(90), path3d(groove_profile));
//   rprofile = round_corners(ramp_profile, radius=20, closed=false, $fn=72);
//   vnf = vnf_vertex_array([
//       for (a = [ramp_profile[0].x : 1 : last(ramp_profile).x]) let(
//           z = lookup(a,rprofile),
//           m = zrot(a) * up(z)
//       )
//       apply(m, [ [rgroove[0].x,0,-z], each rgroove, [last(rgroove).x,0,-z] ])
//   ], caps=true, col_wrap=true, reverse=true);
//   vnf_polyhedron(vnf, convexity=8);
function vnf_vertex_array(
    points,
    caps, cap1, cap2,
    col_wrap=false,
    row_wrap=false,
    reverse=false,
    style="default",
    triangulate = false
) =
    assert(!(any([caps,cap1,cap2]) && !col_wrap), "col_wrap must be true if caps are requested")
    assert(!(any([caps,cap1,cap2]) && row_wrap), "Cannot combine caps with row_wrap")
    assert(in_list(style,["default","alt","quincunx", "convex","concave", "min_edge","min_area","flip1","flip2"]))
    assert(is_matrix(points[0], n=3),"Point array has the wrong shape or points are not 3d")
    assert(is_consistent(points), "Non-rectangular or invalid point array")
    assert(is_bool(triangulate))
    let(
        pts = flatten(points),
        pcnt = len(pts),
        rows = len(points),
        cols = len(points[0])
    )
    rows<=1 || cols<=1 ? EMPTY_VNF :
    let(
        cap1 = first_defined([cap1,caps,false]),
        cap2 = first_defined([cap2,caps,false]),
        colcnt = cols - (col_wrap?0:1),
        rowcnt = rows - (row_wrap?0:1),
        verts = [
            each pts,
            if (style=="quincunx")
                for (r = [0:1:rowcnt-1], c = [0:1:colcnt-1])
                   let(
                       i1 = ((r+0)%rows)*cols + ((c+0)%cols),
                       i2 = ((r+1)%rows)*cols + ((c+0)%cols),
                       i3 = ((r+1)%rows)*cols + ((c+1)%cols),
                       i4 = ((r+0)%rows)*cols + ((c+1)%cols)
                   )
                   mean([pts[i1], pts[i2], pts[i3], pts[i4]])
        ],
        allfaces = [
            if (cap1) count(cols,reverse=!reverse),
            if (cap2) count(cols,(rows-1)*cols, reverse=reverse),
            for (r = [0:1:rowcnt-1], c=[0:1:colcnt-1])
               each
               let(
                   i1 = ((r+0)%rows)*cols + ((c+0)%cols),
                   i2 = ((r+1)%rows)*cols + ((c+0)%cols),
                   i3 = ((r+1)%rows)*cols + ((c+1)%cols),
                   i4 = ((r+0)%rows)*cols + ((c+1)%cols),
                   faces =
                        style=="quincunx"?
                          let(i5 = pcnt + r*colcnt + c)
                          [[i1,i5,i2],[i2,i5,i3],[i3,i5,i4],[i4,i5,i1]]
                      : style=="alt" || (style=="flip1" && ((r+c)%2==0)) || (style=="flip2" && ((r+c)%2==1)) || (style=="random" && rands(0,1,1)[0]<.5)?
                          [[i1,i4,i2],[i2,i4,i3]]
                      : style=="min_area"?
                          let(
                               area42 = norm(cross(pts[i2]-pts[i1], pts[i4]-pts[i1]))+norm(cross(pts[i4]-pts[i3], pts[i2]-pts[i3])),
                               area13 = norm(cross(pts[i1]-pts[i4], pts[i3]-pts[i4]))+norm(cross(pts[i3]-pts[i2], pts[i1]-pts[i2])),
                               minarea_edge = area42 < area13 + EPSILON
                                 ? [[i1,i4,i2],[i2,i4,i3]]
                                 : [[i1,i3,i2],[i1,i4,i3]]
                          )
                          minarea_edge
                      : style=="min_edge"?
                          let(
                               d42=norm(pts[i4]-pts[i2]),
                               d13=norm(pts[i1]-pts[i3]),
                               shortedge = d42<d13+EPSILON
                                 ? [[i1,i4,i2],[i2,i4,i3]]
                                 : [[i1,i3,i2],[i1,i4,i3]]
                          )
                          shortedge
                      : style=="convex"?
                          let(   // Find normal for 3 of the points.  Is the other point above or below?
                              n = (reverse?-1:1)*cross(pts[i2]-pts[i1],pts[i3]-pts[i1]),
                              convexfaces = n==0
                                ? [[i1,i4,i3]]
                                : n*pts[i4] > n*pts[i1]
                                    ? [[i1,i4,i2],[i2,i4,i3]]
                                    : [[i1,i3,i2],[i1,i4,i3]]
                          )
                          convexfaces
                      : style=="concave"?
                          let(   // Find normal for 3 of the points.  Is the other point above or below?
                              n = (reverse?-1:1)*cross(pts[i2]-pts[i1],pts[i3]-pts[i1]),
                              concavefaces = n==0
                                ? [[i1,i4,i3]]
                                : n*pts[i4] <= n*pts[i1]
                                    ? [[i1,i4,i2],[i2,i4,i3]]
                                    : [[i1,i3,i2],[i1,i4,i3]]
                          )
                          concavefaces
                      : [[i1,i3,i2],[i1,i4,i3]],
                   // remove degenerate faces
                   culled_faces= [for(face=faces)
                       if (norm(cross(verts[face[1]]-verts[face[0]],
                                      verts[face[2]]-verts[face[0]]))>EPSILON)
                           face
                   ],
                   rfaces = reverse? [for (face=culled_faces) reverse(face)] : culled_faces
               )
               rfaces,
        ],
        vnf = [verts, allfaces]
    ) triangulate? vnf_triangulate(vnf) : vnf;


// Function: vnf_tri_array()
// Synopsis: Returns a VNF from an array of points.
// SynTags: VNF
// Topics: VNF Generators, Lists
// See Also: vnf_vertex_array(), vnf_join(), vnf_from_polygons(), vnf_from_region()
// Usage:
//   vnf = vnf_tri_array(points, [row_wrap], [reverse])
// Description:
//   Produces a VNF from an array of points where each row length can differ from the adjacent rows by up to 2 in length.  This enables
//   the construction of triangular VNF patches.  The resulting VNF can be wrapped along the rows by setting `row_wrap` to true.
//   You cannot wrap columns: if you need to do that you'll need to merge two VNF arrays that share edges.  Degenerate faces
//   are not included in the output, but if this results in unused vertices they will still appear in the output.
// Arguments:
//   points = List of point lists for each row
//   row_wrap = If true then add faces connecting the first row and last row.  These rows must differ by at most 2 in length.
//   reverse = Set this to reverse the direction of the faces
// Example(3D,NoAxes): Each row has one more point than the preceeding one.
//   pts = [for(y=[1:1:10]) [for(x=[0:y-1]) [x,y,y]]];
//   vnf = vnf_tri_array(pts);
//   vnf_wireframe(vnf,width=0.1);
//   color("red")move_copies(flatten(pts)) sphere(r=.15,$fn=9);
// Example(3D,NoAxes): Each row has two more points than the preceeding one.
//   pts = [for(y=[0:2:10]) [for(x=[-y/2:y/2]) [x,y,y]]];
//   vnf = vnf_tri_array(pts);
//   vnf_wireframe(vnf,width=0.1);
//   color("red")move_copies(flatten(pts)) sphere(r=.15,$fn=9);
// Example(3D): Merging two VNFs to construct a cone with one point length change between rows.
//   pts1 = [for(z=[0:10]) path3d(arc(3+z,r=z/2+1, angle=[0,180]),10-z)];
//   pts2 = [for(z=[0:10]) path3d(arc(3+z,r=z/2+1, angle=[180,360]),10-z)];
//   vnf = vnf_join([vnf_tri_array(pts1),
//                     vnf_tri_array(pts2)]);
//   color("green")vnf_wireframe(vnf,width=0.1);
//   vnf_polyhedron(vnf);
// Example(3D): Cone with length change two between rows
//   pts1 = [for(z=[0:1:10]) path3d(arc(3+2*z,r=z/2+1, angle=[0,180]),10-z)];
//   pts2 = [for(z=[0:1:10]) path3d(arc(3+2*z,r=z/2+1, angle=[180,360]),10-z)];
//   vnf = vnf_join([vnf_tri_array(pts1),
//                    vnf_tri_array(pts2)]);
//   color("green")vnf_wireframe(vnf,width=0.1);
//   vnf_polyhedron(vnf);
// Example(3D,NoAxes): Point count can change irregularly
//   lens = [10,9,7,5,6,8,8,10];
//   pts = [for(y=idx(lens)) lerpn([-lens[y],y,y],[lens[y],y,y],lens[y])];
//   vnf = vnf_tri_array(pts);
//   vnf_wireframe(vnf,width=0.1);
//   color("red")move_copies(flatten(pts)) sphere(r=.15,$fn=9);
function vnf_tri_array(points, row_wrap=false, reverse=false) =
    let(
       lens = [for(row=points) len(row)],
       rowstarts = [0,each cumsum(lens)],
       faces =
          [for(i=[0:1:len(points) - 1 - (row_wrap ? 0 : 1)]) each
            let(
                rowstart = rowstarts[i],
                nextrow = select(rowstarts,i+1),
                delta = select(lens,i+1)-lens[i]
            )
            delta == 0 ?
              [for(j=[0:1:lens[i]-2]) reverse ? [j+rowstart+1, j+rowstart, j+nextrow] : [j+rowstart, j+rowstart+1, j+nextrow],
               for(j=[0:1:lens[i]-2]) reverse ? [j+rowstart+1, j+nextrow, j+nextrow+1] : [j+rowstart+1, j+nextrow+1, j+nextrow]] :
            delta == 1 ?
              [for(j=[0:1:lens[i]-2]) reverse ? [j+rowstart+1, j+rowstart, j+nextrow+1] : [j+rowstart, j+rowstart+1, j+nextrow+1],
               for(j=[0:1:lens[i]-1]) reverse ? [j+rowstart, j+nextrow, j+nextrow+1] : [j+rowstart, j+nextrow+1, j+nextrow]] :
            delta == -1 ?
              [for(j=[0:1:lens[i]-3]) reverse ? [j+rowstart+1, j+nextrow, j+nextrow+1]: [j+rowstart+1, j+nextrow+1, j+nextrow],
               for(j=[0:1:lens[i]-2]) reverse ? [j+rowstart+1, j+rowstart, j+nextrow] : [j+rowstart, j+rowstart+1, j+nextrow]] :
            let(count = floor((lens[i]-1)/2))
            delta == 2 ?
              [
               for(j=[0:1:count-1]) reverse ? [j+rowstart+1, j+rowstart, j+nextrow+1] : [j+rowstart, j+rowstart+1, j+nextrow+1],       // top triangles left
               for(j=[count:1:lens[i]-2]) reverse ? [j+rowstart+1, j+rowstart, j+nextrow+2] : [j+rowstart, j+rowstart+1, j+nextrow+2], // top triangles right
               for(j=[0:1:count]) reverse ? [j+rowstart, j+nextrow, j+nextrow+1] : [j+rowstart, j+nextrow+1, j+nextrow],                        // bot triangles left
               for(j=[count+1:1:select(lens,i+1)-2]) reverse ? [j+rowstart-1, j+nextrow, j+nextrow+1] : [j+rowstart-1, j+nextrow+1, j+nextrow], // bot triangles right
              ] :
            delta == -2 ?
              [
               for(j=[0:1:count-2]) reverse ? [j+nextrow, j+nextrow+1, j+rowstart+1] : [j+nextrow, j+rowstart+1, j+nextrow+1],
               for(j=[count-1:1:lens[i]-4]) reverse ? [j+nextrow,j+nextrow+1,j+rowstart+2] : [j+nextrow,j+rowstart+2, j+nextrow+1],
               for(j=[0:1:count-1]) reverse ? [j+nextrow, j+rowstart+1, j+rowstart] : [j+nextrow, j+rowstart, j+rowstart+1],
               for(j=[count:1:select(lens,i+1)]) reverse ? [ j+nextrow-1, j+rowstart+1, j+rowstart]: [ j+nextrow-1, j+rowstart, j+rowstart+1],
              ] :
            assert(false,str("Unsupported row length difference of ",delta, " between row ",i," and ",(i+1)%len(points)))
          ],
       verts = flatten(points),
       culled_faces=
           [for(face=faces)
               if (norm(verts[face[0]]-verts[face[1]])>EPSILON &&
                   norm(verts[face[1]]-verts[face[2]])>EPSILON &&
                   norm(verts[face[2]]-verts[face[0]])>EPSILON)
                   face
           ]
    )
    [flatten(points), culled_faces];



// Function: vnf_join()
// Synopsis: Returns a single VNF structure from a list of VNF structures.
// SynTags: VNF
// Topics: VNF Generators, Lists
// See Also: vnf_tri_array(), vnf_vertex_array(), vnf_from_polygons(), vnf_from_region()
// Usage:
//   vnf = vnf_join([VNF, VNF, VNF, ...]);
// Description:
//   Given a list of VNF structures, merges them all into a single VNF structure.
//   Combines all the points of the input VNFs and labels the faces appropriately.
//   All the points in the input VNFs will appear in the output, even if they are
//   duplicates of each other.  It is valid to repeat points in a VNF, but if you
//   with to remove the duplicates that will occur along joined edges, use {{vnf_merge_points()}}.
//   .
//   Note that this is a tool for manipulating polyhedron data.  It is for
//   building up a full polyhedron from partial polyhedra.
//   It is *not* a union operator for VNFs.  The VNFs to be joined must not intersect each other,
//   except at edges, or the result will be an invalid polyhedron.  Similarly the
//   result must not have any other illegal polyhedron characteristics, such as creating
//   more than two faces sharing the same edge.
//   If you want a valid result it is your responsibility to ensure that the polyhedron
//   has no holes, no intersecting faces or edges, and obeys all the requirements
//   that CGAL expects.
//   .
//   For example, if you combine two pyramids to try to make an octahedron, the result will
//   be invalid because of the two internal faces created by the pyramid bases.  A valid
//   use would be to build a cube missing one face and a pyramid missing its base and
//   then join them into a cube with a point.
// Arguments:
//   vnfs = a list of the VNFs to joint into one VNF.
// Example(3D,VPR=[60,0,26],VPD=55,VPT=[5.6,-5.3,9.8]): Here is a VNF where the top face is missing.  It is not a valid polyhedron like this, but we can use it as a building block to make a polyhedron.
//   bottom = vnf_vertex_array([path3d(rect(8)), path3d(rect(5),4)],col_wrap=true,cap1=true);
//   vnf_polyhedron(bottom);
// Example(3D,VPR=[60,0,26],VPD=55,VPT=[5.6,-5.3,9.8]): Here is a VNF that also has a missing face.
//   triangle = yrot(-90,path3d(regular_ngon(n=3,side=5,anchor=LEFT)));
//   top = up(4,vnf_vertex_array([list_set(right(2.5,triangle),0,[0,0,7]),
//                               right(6,triangle)
//                               ], col_wrap=true, cap2=true));
//   vnf_polyhedron(zrot(90,top));
// Example(3D,VPR=[60,0,26],VPD=55,VPT=[5.6,-5.3,9.8]): Using vnf_join combines the two VNFs into a single VNF.  Note that they share an edge.  But the result still isn't closed, so it is not yet a valid polyhedron.
//   bottom = vnf_vertex_array([path3d(rect(8)), path3d(rect(5),4)],col_wrap=true,cap1=true);
//   triangle = yrot(-90,path3d(regular_ngon(n=3,side=5,anchor=LEFT)));
//   top = up(4,vnf_vertex_array([list_set(right(2.5,triangle),0,[0,0,7]),
//                                right(6,triangle)
//                               ], col_wrap=true, cap2=true));
//   full = vnf_join([bottom,zrot(90,top)]);
//   vnf_polyhedron(full);
// Example(3D,VPR=[60,0,26],VPD=55,VPT=[5.6,-5.3,9.8]): If we add enough pieces, and the pieces are all consistent with each other, then we can arrive at a valid polyhedron like this one.  To be valid you need to meet all the CGAL requirements: every edge has exactly two faces, all faces are in clockwise order, no intersections of edges.
//   bottom = vnf_vertex_array([path3d(rect(8)), path3d(rect(5),4)],col_wrap=true,cap1=true);
//   triangle = yrot(-90,path3d(regular_ngon(n=3,side=5,anchor=LEFT)));
//   top = up(4,vnf_vertex_array([list_set(right(2.5,triangle),0,[0,0,7]),
//                                right(6,triangle)
//                               ], col_wrap=true, cap2=true));
//   full = vnf_join([bottom,
//                     for(theta=[0:90:359]) zrot(theta,top)
//                    ]);
//   vnf_polyhedron(full);
// Example(3D): The vnf_join function is not a union operator for polyhedra.  If any faces intersect, like they do in this example where we combine the faces of two cubes, the result is invalid and will give rise to CGAL errors when you add more objects into the model.
//   cube1 = cube(5);
//   cube2 = move([2,2,2],cube1);
//   badvnf = vnf_join([cube1,cube2]);
//   vnf_polyhedron(badvnf);
//   right(2.5)up(3)color("red")
//         text3d("Invalid",size=1,anchor=CENTER,
//         orient=FRONT,h=.1);
function vnf_join(vnfs) =
    assert(is_vnf_list(vnfs) , "Input must be a list of VNFs")
    len(vnfs)==1 ? vnfs[0]
    :
    let (
        offs  = cumsum([ 0, for (vnf = vnfs) len(vnf[0]) ]),
        verts = [for (vnf=vnfs) each vnf[0]],
        faces =
            [ for (i = idx(vnfs))
                let( faces = vnfs[i][1] )
                for (face = faces)
                    if ( len(face) >= 3 )
                        [ for (j = face)
                            assert( j>=0 && j<len(vnfs[i][0]),
                                    str("VNF number ", i, " has a face indexing an nonexistent vertex") )
                            offs[i] + j ]
            ]
    )
    [verts,faces];



// Function: vnf_from_polygons()
// Synopsis: Returns a VNF from a list of 3D polygons.
// SynTags: VNF
// Topics: VNF Generators, Lists
// See Also: vnf_tri_array(), vnf_join(), vnf_vertex_array(), vnf_from_region()
// Usage:
//   vnf = vnf_from_polygons(polygons, [eps]);
// Description:
//   Given a list of 3D polygons, produces a VNF containing those polygons.
//   It is up to the caller to make sure that the points are in the correct order to make the face
//   normals point outwards.  No checking for duplicate vertices is done.  If you want to
//   remove duplicate vertices use {{vnf_merge_points()}}.  Polygons with zero area are discarded from the face list by default.
//   If you give non-coplanar faces an error is displayed.  These checks increase run time by about 2x for triangular polygons, but
//   about 10x for pentagons; the checks can be disabled by setting fast=true.  
// Arguments:
//   polygons = The list of 3D polygons to turn into a VNF
//   fast = Set to true to skip area and coplanarity checks for increased speed.  Default: false
//   eps = Polygons with area small than this are discarded.  Default: EPSILON
function vnf_from_polygons(polygons,fast=false,eps=EPSILON) =
   assert(is_list(polygons) && is_path(polygons[0]),"Input should be a list of polygons")
   let(
       offs = cumsum([0, for(p=polygons) len(p)]),
       faces = [for(i=idx(polygons))
                  let(
                      area=fast ? 1 : polygon_area(polygons[i]),
                      dummy=assert(is_def(area) || is_collinear(polygons[i],eps=eps),str("Polygon ", i, " is not coplanar"))
                  )
                  if (is_def(area) && area > eps)
                    [for (j=idx(polygons[i])) offs[i]+j]
               ]
   )
   [flatten(polygons), faces];




function _path_path_closest_vertices(path1,path2) =
    let(
        dists = [for (i=idx(path1)) let(j=closest_point(path1[i],path2)) [j,norm(path2[j]-path1[i])]],
        i1 = min_index(column(dists,1)),
        i2 = dists[i1][0]
    ) [dists[i1][1], i1, i2];


function _join_paths_at_vertices(path1,path2,v1,v2) =
    let(
        repeat_start = !approx(path1[v1],path2[v2]),
        path1 = clockwise_polygon(list_rotate(path1,v1)),
        path2 = ccw_polygon(list_rotate(path2,v2))
    )
    [
        each path1,
        if (repeat_start) path1[0],
        each path2,
        if (repeat_start) path2[0],
    ];


/// Internal Function: _cleave_connected_region(region, eps)
/// Description:
///   Given a region that is connected and has its outer border in region[0],
///   produces a overlapping connected path to join internal holes to
///   the outer border without adding points. Output is a single non-simple polygon.
/// Requirements:
///   It expects that all region paths be simple closed paths, with region[0] CW and
///   the other paths CCW and encircled by region[0]. The input region paths are also
///   supposed to be disjoint except for common vertices and common edges but with
///   no crossings. It may return `undef` if these conditions are not met.
///   This function implements an extension of the algorithm discussed in:
///   https://www.geometrictools.com/Documentation/TriangulationByEarClipping.pdf
function _cleave_connected_region(region, eps=EPSILON) =
    len(region)==1 ? region[0] :
    let(
        outer   = deduplicate(region[0]),             //
        holes   = [for(i=[1:1:len(region)-1])         // deduplication possibly unneeded
                      deduplicate( region[i] ) ],     //
        extridx = [for(li=holes) max_index(column(li,0)) ],
        // the right extreme vertex for each hole sorted by decreasing x values
        extremes = sort( [for(i=idx(holes)) [ i, extridx[i], -holes[i][extridx[i]].x] ], idx=2 )
    )
    _polyHoles(outer, holes, extremes, eps, 0);


// connect the hole paths one at a time to the outer path.
// 'extremes' is the list of the right extreme vertex of each hole sorted by decreasing abscissas
// see: _cleave_connected_region(region, eps)
function _polyHoles(outer, holes, extremes, eps=EPSILON, n=0) =
    let(
        extr = extremes[n],    //
        hole = holes[extr[0]], // hole path to bridge to the outer path
        ipt  = extr[1],        // index of the hole point with maximum abscissa
        brdg = _bridge(hole[ipt], outer, eps)  // the index of a point in outer to bridge hole[ipt] to
    )
    brdg == undef ? undef :
    let(
        l  = len(outer),
        lh = len(hole),
        // the new outer polygon bridging the hole to the old outer
        npoly =
            approx(outer[brdg], hole[ipt], eps)
            ?   [ for(i=[brdg:  1: brdg+l])   outer[i%l] ,
                  for(i=[ipt+1: 1: ipt+lh-1]) hole[i%lh] ]
            :   [ for(i=[brdg:  1: brdg+l])   outer[i%l] ,
                  for(i=[ipt:   1: ipt+lh])   hole[i%lh] ]
    )
    n==len(holes)-1 ?  npoly :
    _polyHoles(npoly, holes, extremes, eps, n+1);

// find a point in outer to be connected to pt in the interior of outer
// by a segment that not cross or touch any non adjacente edge of outer.
// return the index of a vertex in the outer path where the bridge should end
// see _polyHoles(outer, holes, extremes, eps)
function _bridge(pt, outer,eps) =
    // find the intersection of a ray from pt to the right
    // with the boundary of the outer cycle
    let(
        l    = len(outer),
        crxs =
            let( edges = pair(outer,wrap=true) )
            [for( i = idx(edges) )
                let( edge = edges[i] )
                // consider just descending outer edges at right of pt crossing ordinate pt.y
                if(    (edge[0].y >  pt.y) //+eps)
                    && (edge[1].y <= pt.y)
                    && _is_at_left(pt, [edge[1], edge[0]], eps) )
                    [ i,
                      // the point of edge with ordinate pt.y
                      abs(pt.y-edge[1].y)<eps ? edge[1] :
                      let( u = (pt-edge[1]).y / (edge[0]-edge[1]).y )
                      (1-u)*edge[1] + u*edge[0]
                    ]
             ]
    )
    crxs == [] ? undef :
    let(
        // the intersection point of the nearest edge to pt with minimum slope
        minX    = min([for(p=crxs) p[1].x]),
        crxcand = [for(crx=crxs) if(crx[1].x < minX+eps) crx ], // nearest edges
        nearest = min_index([for(crx=crxcand)
                                (outer[crx[0]].x - pt.x) / (outer[crx[0]].y - pt.y) ]), // minimum slope
        proj    = crxcand[nearest],
        vert0   = outer[proj[0]],    // the two vertices of the nearest crossing edge
        vert1   = outer[(proj[0]+1)%l],
        isect   = proj[1]            // the intersection point
    )
    norm(pt-vert1) < eps ? (proj[0]+1)%l : // if pt touches an outer vertex, return its index
    // as vert0.y > pt.y then pt!=vert0
    norm(pt-isect) < eps ? undef :         // if pt touches the middle of an outer edge -> error
    let(
        // the edge [vert0, vert1] necessarily satisfies vert0.y > vert1.y
        // indices of candidates to an outer bridge point
        cand  =
            (vert0.x > pt.x)
            ?   [ proj[0],
                  // select reflex vertices inside of the triangle [pt, vert0, isect]
                  for(i=idx(outer))
                      if( _tri_class(select(outer,i-1,i+1),eps) <= 0
                          && _pt_in_tri(outer[i], [pt, vert0, isect], eps)>=0 )
                        i
                ]
            :   [ (proj[0]+1)%l,
                  // select reflex vertices inside of the triangle [pt, isect, vert1]
                  for(i=idx(outer))
                      if( _tri_class(select(outer,i-1,i+1),eps) <= 0
                          &&  _pt_in_tri(outer[i], [pt, isect, vert1], eps)>=0 )
                        i
                ],
        // choose the candidate outer[i] such that the line [pt, outer[i]] has minimum slope
        // among those with minimum slope choose the nearest to pt
        slopes  = [for(i=cand) 1-abs(outer[i].x-pt.x)/norm(outer[i]-pt) ],
        min_slp = min(slopes),
        cand2   = [for(i=idx(cand)) if(slopes[i]<=min_slp+eps) cand[i] ],
        nearest = min_index([for(i=cand2) norm(pt-outer[i]) ])
    )
    cand2[nearest];


// Function: vnf_from_region()
// Synopsis: Returns a 3D VNF given a 2D region.
// SynTags: VNF
// Topics: VNF Generators, Lists
// See Also: vnf_vertex_array(), vnf_tri_array(), vnf_join(), vnf_from_polygons()
// Usage:
//   vnf = vnf_from_region(region, [transform], [reverse]);
// Description:
//   Given a (two-dimensional) region, applies the given transformation matrix to it and makes a (three-dimensional) triangulated VNF of
//   faces for that region, reversed if desired.
// Arguments:
//   region = The region to convert to a vnf.
//   transform = If given, a transformation matrix to apply to the faces generated from the region.  Default: No transformation applied.
//   reverse = If true, reverse the normals of the faces generated from the region.  An untransformed region will have face normals pointing `UP`.  Default: false
// Example(3D):
//   region = [square([20,10],center=true),
//             right(5,square(4,center=true)),
//             left(5,square(6,center=true))];
//   vnf = vnf_from_region(region);
//   color("gray")down(.125)
//        linear_extrude(height=.125)region(region);
//   vnf_wireframe(vnf,width=.25);
function vnf_from_region(region, transform, reverse=false, triangulate=true) =
    let (
        region = [for (path = region) deduplicate(path, closed=true)],
        regions = region_parts(force_region(region)),
        vnfs =
            [
                for (rgn = regions)
                let(
                    cleaved = path3d(_cleave_connected_region(rgn))
                )
                assert( cleaved, "The region is invalid")
                let(
                    face = is_undef(transform)? cleaved : apply(transform,cleaved),
                    faceidxs = reverse? [for (i=[len(face)-1:-1:0]) i] : [for (i=[0:1:len(face)-1]) i]
                ) [face, [faceidxs]]
            ],
        outvnf = vnf_join(vnfs)
    )
    triangulate ? vnf_triangulate(outvnf) : outvnf;



// Section: VNF Testing and Access


// Function: is_vnf()
// Synopsis: Returns true given a VNF-like structure.
// Topics: VNF Manipulation
// See Also: is_vnf_list(), vnf_vertices(), vnf_faces()
// Usage:
//   bool = is_vnf(x);
// Description:
//   Returns true if the given value looks like a VNF structure.
function is_vnf(x) =
    is_list(x) &&
    len(x)==2 &&
    is_list(x[0]) &&
    is_list(x[1]) &&
    (x[0]==[] || (len(x[0])>=3 && is_vector(x[0][0],3))) &&
    (x[1]==[] || is_vector(x[1][0]));


// Function: is_vnf_list()
// Synopsis: Returns true given a list of VNF-like structures.
// Topics: VNF Manipulation
// See Also: is_vnf(), vnf_vertices(), vnf_faces()
//
// Description: Returns true if the given value looks passingly like a list of VNF structures.
function is_vnf_list(x) = is_list(x) && all([for (v=x) is_vnf(v)]);


// Function: vnf_vertices()
// Synopsis: Returns the list of vertex points from a VNF.
// Topics: VNF Manipulation
// See Also: is_vnf(), is_vnf_list(), vnf_faces()
// Description: Given a VNF structure, returns the list of vertex points.
function vnf_vertices(vnf) = vnf[0];


// Function: vnf_faces()
// Synopsis: Returns the list of faces from a VNF.
// Topics: VNF Manipulation
// See Also: is_vnf(), is_vnf_list(), vnf_vertices()
// Description: Given a VNF structure, returns the list of faces, where each face is a list of indices into the VNF vertex list.
function vnf_faces(vnf) = vnf[1];



// Section: Altering the VNF Internals


// Function: vnf_reverse_faces()
// Synopsis: Reverses the faces of a VNF.
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_quantize(), vnf_merge_points(), vnf_drop_unused_points(), vnf_triangulate(), vnf_slice(), vnf_unify_faces() 
// Usage:
//   rvnf = vnf_reverse_faces(vnf);
// Description:
//   Reverses the orientation of all the faces in the given VNF.
function vnf_reverse_faces(vnf) =
    [vnf[0], [for (face=vnf[1]) reverse(face)]];


// Function: vnf_quantize()
// Synopsis: Quantizes the vertex coordinates of a VNF.
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_reverse_faces(), vnf_merge_points(), vnf_drop_unused_points(), vnf_triangulate(), vnf_slice() 
// Usage:
//   vnf2 = vnf_quantize(vnf,[q]);
// Description:
//   Quantizes the vertex coordinates of the VNF to the given quanta `q`.
// Arguments:
//   vnf = The VNF to quantize.
//   q = The quanta to quantize the VNF coordinates to.
function vnf_quantize(vnf,q=pow(2,-12)) =
    [[for (pt = vnf[0]) quant(pt,q)], vnf[1]];



// Function: vnf_merge_points()
// Synopsis: Consolidates duplicate vertices of a VNF.
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_reverse_faces(), vnf_quantize(), vnf_drop_unused_points(), vnf_triangulate(), vnf_slice(), vnf_unify_faces() 
// Usage:
//   new_vnf = vnf_merge_points(vnf, [eps]);
// Description:
//   Given a VNF, consolidates all duplicate vertices with a tolerance `eps`, relabeling the faces as necessary,
//   and eliminating any face with fewer than 3 vertices.  Unreferenced vertices of the input VNF are not dropped.
//   To remove such vertices uses {{vnf_drop_unused_points()}}.
// Arguments:
//   vnf = a VNF to consolidate
//   eps = the tolerance in finding duplicates. Default: EPSILON
function vnf_merge_points(vnf,eps=EPSILON) =
    let(
        verts = vnf[0],
        dedup  = vector_search(verts,eps,verts),                 // collect vertex duplicates
        map    = [for(i=idx(verts)) min(dedup[i]) ],             // remap duplic vertices
        offset = cumsum([for(i=idx(verts)) map[i]==i ? 0 : 1 ]), // remaping face vertex offsets
        map2   = list(idx(verts))-offset,                        // map old vertex indices to new indices
        nverts = [for(i=idx(verts)) if(map[i]==i) verts[i] ],    // this doesn't eliminate unreferenced vertices
        nfaces =
            [ for(face=vnf[1])
                let(
                    nface = [ for(vi=face) map2[map[vi]] ],
                    dface = [for (i=idx(nface))
                                if( nface[i]!=nface[(i+1)%len(nface)])
                                    nface[i] ]
                )
                if(len(dface) >= 3) dface
            ]
    )
    [nverts, nfaces];


// Function: vnf_drop_unused_points()
// Synopsis: Removes unreferenced vertices from a VNF.
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_reverse_faces(), vnf_quantize(), vnf_merge_points(), vnf_triangulate(), vnf_slice(), vnf_unify_faces() 
// Usage:
//   clean_vnf = vnf_drop_unused_points(vnf);
// Description:
//   Remove all unreferenced vertices from a VNF.  Note that in most
//   cases unreferenced vertices cause no harm, and this function may
//   be slow on large VNFs.
function vnf_drop_unused_points(vnf) =
    let(
        flat = flatten(vnf[1]),
        ind  = _link_indicator(flat,0,len(vnf[0])-1),
        verts = [for(i=idx(vnf[0])) if(ind[i]==1) vnf[0][i] ],
        map   = cumsum(ind)
    )
    [ verts, [for(face=vnf[1]) [for(v=face) map[v]-1 ] ] ];

function _link_indicator(l,imin,imax) =
    len(l) == 0  ? repeat(imax-imin+1,0) :
    imax-imin<100 || len(l)<400 ? [for(si=search(list([imin:1:imax]),l,1)) si!=[] ? 1: 0 ] :
    let(
        pivot   = floor((imax+imin)/2),
        lesser  = [ for(li=l) if( li< pivot) li ],
        greater = [ for(li=l) if( li> pivot) li ]
    )
    concat( _link_indicator(lesser ,imin,pivot-1),
            search(pivot,l,1) ? 1 : 0 ,
            _link_indicator(greater,pivot+1,imax) ) ;

// Function: vnf_triangulate()
// Synopsis: Triangulates the faces of a VNF.
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_reverse_faces(), vnf_quantize(), vnf_merge_points(), vnf_drop_unused_points(), vnf_slice(), vnf_unify_faces() 
// Usage:
//   vnf2 = vnf_triangulate(vnf);
// Description:
//   Triangulates faces in the VNF that have more than 3 vertices.
// Arguments:
//   vnf = VNF to triangulate
// Example(3D):
//   include <BOSL2/polyhedra.scad>
//   vnf = zrot(33,regular_polyhedron_info("vnf", "dodecahedron", side=12));
//   vnf_polyhedron(vnf);
//   triangulated = vnf_triangulate(vnf);
//   color("red")vnf_wireframe(triangulated,width=.3);
function vnf_triangulate(vnf) =
    let(
        verts = vnf[0],
        faces = [for (face=vnf[1])
                    each (len(face)==3 ? [face] :
                    let( tris = polygon_triangulate(verts, face) )
                    assert( tris!=undef, "Some `vnf` face cannot be triangulated.")
                    tris ) ]
    )
    [verts, faces];



// Function: vnf_unify_faces()
// Synopsis: Remove triangulation from VNF, returning a copy with full faces
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_reverse_faces(), vnf_quantize(), vnf_merge_points(), vnf_triangulate(), vnf_slice() 
// Usage:
//   newvnf = vnf_unify_faces(vnf);
// Description:
//   When a VNF has been triangulated, the polygons that form the true faces have been chopped up into
//   triangles.  This can create problems for algorithms that operate on the VNF itself, where you might
//   want to be able to identify the true faces.  This function merges together the triangles that
//   form those true faces, turning a VNF where each true face is represented by a single entry
//   in the faces list of the VNF.  This function requires that the true faces have no internal vertices.
//   This will always be true for a triangulated VNF, but might fail for a VNF with some other
//   face partition.  If internal vertices are present, the output will include backtracking paths from
//   the boundary to all of those vertices.
// Arguments:
//   vnf = vnf whose faces you want to unify
// Example(3D,Med,NoAxes): Original prism on the left is triangulated.  On the right, the result of unifying the faces.
//   $fn=16;
//   poly = linear_sweep(hexagon(side=10),h=35);
//   vnf = vnf_unify_faces(poly);
//   vnf_wireframe(poly);
//   color([0,1,1,.70])vnf_polyhedron(poly);
//   right(25){
//     vnf_wireframe(vnf);
//     color([0,1,1,.70])vnf_polyhedron(vnf);
//   }

function vnf_unify_faces(vnf) =
   let(
       faces = vnf[1],
       edges =  [for(i=idx(faces), edge=pair(faces[i],wrap=true))
                   [[min(edge),max(edge)],i]],
       normals = [for(face=faces) polygon_normal(select(vnf[0],face))],
       facelist = count(faces), //[for(i=[1:1:len(faces)-1]) i],
       newfaces = _detri_combine_faces(edges,faces,normals,facelist,0)
   )
   [vnf[0],newfaces];


function _detri_combine_faces(edgelist,faces,normals,facelist,curface) =
    curface==len(faces)? select(faces,facelist)
  : !in_list(curface,facelist) ? _detri_combine_faces(edgelist,faces,normals,facelist,curface+1)
  :
    let(
        thisface=faces[curface],
        neighbors = [for(i=idx(thisface))
                       let(
                           edgepair = search([sort(select(thisface,i,i+1))],edgelist,0)[0],
                           choices = select(edgelist,edgepair),
                           good_choice=[for(choice=choices)
                              if (choice[1]!=curface && in_list(choice[1],facelist) && normals[choice[1]]*normals[curface]>1-EPSILON)
                                choice],
                           d=assert(len(good_choice)<=1)
                       )
                       len(good_choice)==1 ? good_choice[0][1] : -1
                    ],
        // Check for duplicates in the neighbor list so we don't add them twice
        dups = search([for(n=neighbors) if (n>=0) n], neighbors,0),
        goodind = column(dups,0),
        newface = [for(i=idx(thisface))
                    each
                     !in_list(i,goodind) ? [thisface[i]]
                    :
                     let(
                         ind = search(select(thisface,i,i+1), faces[neighbors[i]])
                     )
                     select(faces[neighbors[i]],ind[0],ind[1]-1)
                   ],
        usedfaces = [for(n=neighbors) if (n>=0) n],
        faces = list_set(faces,curface,newface),
        facelist = list_remove_values(facelist,usedfaces)
     )
     _detri_combine_faces(edgelist,faces,normals,facelist,len(usedfaces)==0?curface+1:curface);



function _vnf_sort_vertices(vnf, idx=[2,1,0]) =
    let(
        verts = vnf[0],
        faces = vnf[1],
        vidx = sortidx(verts, idx=idx),
        rvidx = sortidx(vidx),
        sorted_vnf = [
            [ for (i = vidx) verts[i] ],
            [ for (face = faces) [ for (i = face) rvidx[i] ] ],
        ]
    ) sorted_vnf;




// Function: vnf_slice()
// Synopsis: Slice the faces of a VNF along an axis.
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_reverse_faces(), vnf_quantize(), vnf_merge_points(), vnf_drop_unused_points(), vnf_triangulate()
// Usage:
//   sliced = vnf_slice(vnf, dir, cuts);
// Description:
//   Slice the faces of a VNF along a specified axis direction at a given list of cut points.
//   The cut points can appear in any order.  You can use this to refine the faces of a VNF before
//   applying a nonlinear transformation to its vertex set.
// Arguments:
//   vnf = VNF to slice
//   dir = normal direction to the slices, either "X", "Y" or "Z"
//   cuts = X, Y or Z values where cuts occur
// Example(3D):
//   include <BOSL2/polyhedra.scad>
//   vnf = regular_polyhedron_info("vnf", "dodecahedron", side=12);
//   vnf_polyhedron(vnf);
//   sliced = vnf_slice(vnf, "X", [-6,-1,10]);
//   color("red")vnf_wireframe(sliced,width=.3);
function vnf_slice(vnf,dir,cuts) =
    let(
        //  Code below seems to be unnecessary
        //cuts = [for (cut=cuts) _shift_cut_plane(vnf,dir,cut)],
        vert = vnf[0],
        faces = [for(face=vnf[1]) select(vert,face)],
        poly_list = _slice_3dpolygons(faces, dir, cuts)
    )
    vnf_merge_points(vnf_from_polygons(poly_list));


function _shift_cut_plane(vnf,dir,cut,off=0.001) =
    let(
        I = ident(3),
        dir_ind = ord(dir)-ord("X"),
        verts = vnf[0],
        on_cut = [for (x = verts * I[dir_ind]) if(approx(x,cut,eps=1e-4)) 1] != []
    ) !on_cut? cut :
    _shift_cut_plane(vnf,dir,cut+off);


function _split_polygon_at_x(poly, x) =
    let(
        xs = column(poly,0)
    ) (min(xs) >= x || max(xs) <= x)? [poly] :
    let(
        poly2 = [
            for (p = pair(poly,true)) each [
                p[0],
                if(
                    (p[0].x < x && p[1].x > x) ||
                    (p[1].x < x && p[0].x > x)
                ) let(
                    u = (x - p[0].x) / (p[1].x - p[0].x)
                ) [
                    x,  // Important for later exact match tests
                    u*(p[1].y-p[0].y)+p[0].y
                ]
            ]
        ],
        out1 = [for (p = poly2) if(p.x <= x) p],
        out2 = [for (p = poly2) if(p.x >= x) p],
        out3 = [
            if (len(out1)>=3) each split_path_at_self_crossings(out1),
            if (len(out2)>=3) each split_path_at_self_crossings(out2),
        ],
        out = [for (p=out3) if (len(p) > 2) list_unwrap(p)]
    ) out;


function _split_2dpolygons_at_each_x(polys, xs, _i=0) =
    _i>=len(xs)? polys :
    _split_2dpolygons_at_each_x(
        [
            for (poly = polys)
            each _split_polygon_at_x(poly, xs[_i])
        ], xs, _i=_i+1
    );

/// Internal Function: _slice_3dpolygons()
/// Usage:
///   splitpolys = _slice_3dpolygons(polys, dir, cuts);
/// Topics: Geometry, Polygons, Intersections
/// Description:
///   Given a list of 3D polygons, a choice of X, Y, or Z, and a cut list, `cuts`, splits all of the polygons where they cross
///   X/Y/Z at any value given in cuts.
/// Arguments:
///   polys = A list of 3D polygons to split.
///   dir_ind = slice direction, 0=X, 1=Y, or 2=Z
///   cuts = A list of scalar values for locating the cuts
function _slice_3dpolygons(polys, dir, cuts) =
    assert( [for (poly=polys) if (!is_path(poly,3)) 1] == [], "Expects list of 3D paths.")
    assert( is_vector(cuts), "The split list must be a vector.")
    assert( in_list(dir, ["X", "Y", "Z"]))
    let(
        I = ident(3),
        dir_ind = ord(dir)-ord("X")
    )
    flatten([
        for (poly = polys)
            if (polygon_area(poly)>EPSILON)   // Discard zero area polygons
            let( 
                 plane = plane_from_polygon(poly,1e-4))
            assert(plane,"Found non-coplanar face.")
            let(
                normal = point3d(plane),
                pnormal = normal - (normal*I[dir_ind])*I[dir_ind]
            )
            approx(pnormal,[0,0,0]) ? [poly]     // Polygons parallel to cut plane just pass through
          : let(
                pind = max_index(v_abs(pnormal)),  // project along this direction
                otherind = 3-pind-dir_ind,         // keep dir_ind and this direction
                keep = [I[dir_ind], I[otherind]],  // dir ind becomes the x dir
                poly2d = poly*transpose(keep),     // project to 2d, putting selected direction in the X position
                poly_list = [for(p=_split_2dpolygons_at_each_x([poly2d], cuts))
                                let(
                                    a = p*keep,    // unproject, but pind dimension data is missing
                                    ofs = outer_product((repeat(plane[3], len(a))-a*normal)/plane[pind],I[pind])
                                 )
                                 a+ofs]    // ofs computes the missing pind dimension data and adds it back in
            )
            poly_list
    ]);





// Section: Turning a VNF into geometry


// Module: vnf_polyhedron()
// Synopsis: Returns a polyhedron from a VNF or list of VNFs.
// SynTags: Geom
// Topics: VNF Manipulation
// See Also: vnf_wireframe()
// Usage:
//   vnf_polyhedron(vnf) [ATTACHMENTS];
//   vnf_polyhedron([VNF, VNF, VNF, ...]) [ATTACHMENTS];
// Description:
//   Given a VNF structure, or a list of VNF structures, creates a polyhedron from them.
// Arguments:
//   vnf = A VNF structure, or list of VNF structures.
//   convexity = Max number of times a line could intersect a wall of the shape.
//   cp = Centerpoint for determining intersection anchors or centering the shape.  Determines the base of the anchor vector.  Can be "centroid", "mean", "box" or a 3D point.  Default: "centroid"
//   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `"origin"`
//   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
//   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
//   atype = Select "hull" or "intersect" anchor type.  Default: "hull"
// Anchor Types:
//   "hull" = Anchors to the virtual convex hull of the shape.
//   "intersect" = Anchors to the surface of the shape.
// Named Anchors:
//   "origin" = Anchor at the origin, oriented UP.
module vnf_polyhedron(vnf, convexity=2, cp="centroid", anchor="origin", spin=0, orient=UP, atype="hull") {
    vnf = is_vnf_list(vnf)? vnf_join(vnf) : vnf;
    assert(in_list(atype, _ANCHOR_TYPES), "Anchor type must be \"hull\" or \"intersect\"");
    attachable(anchor,spin,orient, vnf=vnf, extent=atype=="hull", cp=cp) {
        polyhedron(vnf[0], vnf[1], convexity=convexity);
        children();
    }
}


// Module: vnf_wireframe()
// Synopsis: Creates a wireframe model from a VNF.
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_polyhedron()
// Usage:
//   vnf_wireframe(vnf, [width]);
// Description:
//   Given a VNF, creates a wire frame ball-and-stick model of the polyhedron with a cylinder for
//   each edge and a sphere at each vertex.  The width parameter specifies the width of the sticks
//   that form the wire frame and the diameter of the balls.
// Arguments:
//   vnf = A vnf structure
//   width = width of the cylinders forming the wire frame.  Default: 1
// Example:
//   $fn=32;
//   ball = sphere(r=20, $fn=6);
//   vnf_wireframe(ball,width=1);
// Example:
//   include <BOSL2/polyhedra.scad>
//   $fn=32;
//   cube_oct = regular_polyhedron_info("vnf",
//                      name="cuboctahedron", or=20);
//   vnf_wireframe(cube_oct);
// Example: The spheres at the vertex are imperfect at aligning with the cylinders, so especially at low $fn things look prety ugly.  This is normal.
//   include <BOSL2/polyhedra.scad>
//   $fn=8;
//   octahedron = regular_polyhedron_info("vnf",
//                         name="octahedron", or=20);
//   vnf_wireframe(octahedron,width=5);
module vnf_wireframe(vnf, width=1)
{
  no_children($children);
  vertex = vnf[0];
  edges = unique([for (face=vnf[1], i=idx(face))
                    sort([face[i], select(face,i+1)])
                 ]);
  attachable()
  {
    union(){
      for (e=edges) extrude_from_to(vertex[e[0]],vertex[e[1]]) circle(d=width);
      // Identify vertices actually used and draw them
      vertused = search(count(len(vertex)), flatten(edges), 1);
      for(i=idx(vertex)) if(vertused[i]!=[]) move(vertex[i]) sphere(d=width);
    }
    union();
  }
}


// Section: Operations on VNFs

// Function: vnf_volume()
// Synopsis: Returns the volume of a VNF.
// Topics: VNF Manipulation
// See Also: vnf_area(), vnf_halfspace(), vnf_bend() 
// Usage:
//   vol = vnf_volume(vnf);
// Description:
//   Returns the volume enclosed by the given manifold VNF.   The VNF must describe a valid polyhedron with consistent face direction and
//   no holes; otherwise the results are undefined.  Returns a positive volume if face direction is clockwise and a negative volume
//   if face direction is counter-clockwise.

// Divide the polyhedron into tetrahedra with the origin as one vertex and sum up the signed volume.
function vnf_volume(vnf) =
    let(verts = vnf[0])
    sum([
         for(face=vnf[1], j=[1:1:len(face)-2])
             cross(verts[face[j+1]], verts[face[j]]) * verts[face[0]]
    ])/6;


// Function: vnf_area()
// Synopsis: Returns the surface area of a VNF.
// Topics: VNF Manipulation
// See Also: vnf_volume(), vnf_halfspace(), vnf_bend() 
// Usage:
//   area = vnf_area(vnf);
// Description:
//   Returns the surface area in any VNF by adding up the area of all its faces.  The VNF need not be a manifold.
function vnf_area(vnf) =
    let(verts=vnf[0])
    sum([for(face=vnf[1]) polygon_area(select(verts,face))]);


/// Internal Function: _vnf_centroid()
/// Usage:
///   vol = _vnf_centroid(vnf);
/// Description:
///   Returns the centroid of the given manifold VNF.  The VNF must describe a valid polyhedron with consistent face direction and
///   no holes; otherwise the results are undefined.

/// Divide the solid up into tetrahedra with the origin as one vertex.
/// The centroid of a tetrahedron is the average of its vertices.
/// The centroid of the total is the volume weighted average.
function _vnf_centroid(vnf,eps=EPSILON) =
    assert(is_vnf(vnf) && len(vnf[0])!=0 && len(vnf[1])!=0,"Invalid or empty VNF given to centroid")
    let(
        verts = vnf[0],
        pos = sum([
            for(face=vnf[1], j=[1:1:len(face)-2]) let(
                v0  = verts[face[0]],
                v1  = verts[face[j]],
                v2  = verts[face[j+1]],
                vol = cross(v2,v1)*v0
            )
            [ vol, (v0+v1+v2)*vol ]
        ])
    )
    assert(!approx(pos[0],0, eps), "The vnf has self-intersections.")
    pos[1]/pos[0]/4;


// Function: projection()
// Synopsis: Returns projection or intersection of vnf with XY plane
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_halfspace()
// Usage:
//   region = projection(vnf, [cut]);
// Description:
//   When `cut=false`, which is the default, projects the input VNF
//   onto the XY plane, returning a region.  Note that as currently implemented,  this operation
//   involves the 2D union of all the projected faces and can be very
//   slow if the VNF has many faces.  Minimize the face count of the VNF for best performance. 
//   .
//   When `cut=true`, returns the intersection of the VNF with the
//   XY plane, which is again a region.  If the VNF does not intersect
//   the XY plane then returns the empty set.  This operation is
//   much faster than `cut=false`.
// Example(3D): Here's a VNF with two linked toruses and a small cube
//   vnf = vnf_join([
//            xrot(90,torus(id=15,od=24,$fn=5)),
//            right(12,torus(id=15,od=24,$fn=4)),
//            up(13,right(15,cube(3,center=true)))
//         ]);
//   vnf_polyhedron(vnf);
// Example(2D): Projection of above VNF with default behavior, `cut=false`
//   vnf = vnf_join([
//            xrot(90,torus(id=15,od=24,$fn=5)),
//            right(12,torus(id=15,od=24,$fn=4)),
//            up(13,right(15,cube(3,center=true)))
//         ]);
//   reg = projection(vnf);
//   region(reg);
// Example(3D): Tilted torus
//   vnf = xrot(35,torus(id=4,od=12,$fn=32));
//   vnf_polyhedron(vnf);
// Example(2D): Projection of tilted torus using `cut=true`
//   vnf = xrot(35,torus(id=4,od=12,$fn=32));
//   reg = projection(vnf,cut=true);
//   region(reg);

function projection(vnf,cut=false,eps=EPSILON) =
   assert(is_vnf(vnf))
   cut ?
         let(
              vnf_bdy = vnf_halfspace([0,0,1,0],vnf, boundary=true),
              ind = vnf_bdy[1],
              pts = path2d(vnf_bdy[0][0])
         )
         ind==[] ? []
                 : [for (path=ind) select(pts, path)]
  :
   let(
        pts = vnf[0],
        faces = vnf[1], 
        facets = [for(face=faces)
                     let(projface = path2d(select(pts,face)))
                     if (!approx(polygon_area(projface),0,eps=eps))
                        projface
                 ]
   )
   union(facets);



// Function: vnf_halfspace()
// Synopsis: Returns the intersection of the vnf with a half space.
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_volume(), vnf_area(), vnf_bend() 
// Usage:
//   newvnf = vnf_halfspace(plane, vnf, [closed], [boundary]);
// Description:
//   Returns the intersection of the vnf with a half space.  The half space is defined by
//   plane = [A,B,C,D], taking the side where the normal [A,B,C] points: Ax+By+Cz≥D.
//   If closed is set to false then the cut face is not included in the vnf.  This could
//   allow further extension of the vnf by join with other vnfs using {{vnf_join()}}.
//   Note that if your given VNF has holes (missing faces) or is not a complete polyhedron
//   then closed=true is may produce invalid results when it tries to construct closing faces
//   on the cut plane.  Set closed=false for such inputs.
//   .
//   If you set boundary to true then the return will be the pair [vnf,boundary] where vnf is the
//   vnf as usual (with closed=false) and boundary is a list giving each connected component of the cut
//   boundary surface.  Each entry in boundary is a list of index values that index into the vnf vertex list (vnf[0]).
//   This makes it possible to construct mating shapes, e.g. with {{skin()}} or {{vnf_vertex_array()}} that
//   can be combined using {{vnf_join()}} to make a valid polyhedron.
//   .
//   Note that the input to vnf_halfspace() does not need to be a closed, manifold polyhedron.
//   Because it adds the faces on the cut surface, you can use vnf_halfspace() to cap off an open shape if you
//   slice through a region that excludes all of the gaps in the input VNF.  
// Arguments:
//   plane = plane defining the boundary of the half space
//   vnf = VNF to cut
//   closed = if false do not return the cut face(s) in the returned VNF.  Default: true
//   boundary = if true return a pair [vnf,boundary] where boundary is a list of paths on the cut boundary indexed into the VNF vertex list.  If boundary is true, then closed is set to false.  Default: false
// Example(3D):
//   vnf = cube(10,center=true);
//   cutvnf = vnf_halfspace([-1,1,-1,0], vnf);
//   vnf_polyhedron(cutvnf);
// Example(3D):  Cut face has 2 components
//   vnf = path_sweep(circle(r=4, $fn=16),
//                    circle(r=20, $fn=64),closed=true);
//   cutvnf = vnf_halfspace([-1,1,-4,0], vnf);
//   vnf_polyhedron(cutvnf);
// Example(3D): Cut face is not simply connected
//   vnf = path_sweep(circle(r=4, $fn=16),
//                    circle(r=20, $fn=64),closed=true);
//   cutvnf = vnf_halfspace([0,0.7,-4,0], vnf);
//   vnf_polyhedron(cutvnf);
// Example(3D): Cut object has multiple components
//   function knot(a,b,t) =   // rolling knot
//        [ a * cos (3 * t) / (1 - b* sin (2 *t)),
//          a * sin( 3 * t) / (1 - b* sin (2 *t)),
//        1.8 * b * cos (2 * t) /(1 - b* sin (2 *t))];
//   a = 0.8; b = sqrt (1 - a * a);
//   ksteps = 400;
//   knot_path = [for (i=[0:ksteps-1]) 50 * knot(a,b,(i/ksteps)*360)];
//   ushape = [[-10, 0],[-10, 10],[ -7, 10],[ -7, 2],[  7, 2],[  7, 7],[ 10, 7],[ 10, 0]];
//   knot=path_sweep(ushape, knot_path, closed=true, method="incremental");
//   cut_knot = vnf_halfspace([1,0,0,0], knot);
//   vnf_polyhedron(cut_knot);
// Example(VPR=[80,0,15]): Cut a sphere with an arbitrary plane
//   vnf1=sphere(r=50, style="icosa", $fn=16);
//   vnf2=vnf_halfspace([.8,1,-1.5,0], vnf1);
//   vnf_polyhedron(vnf2);
// Example(VPR=[80,0,15]): Cut it again, but with closed=false to leave an open boundary. 
//   vnf1=sphere(r=50, style="icosa", $fn=16);
//   vnf2=vnf_halfspace([.8,1,-1.5,0], vnf1);
//   vnf3=vnf_halfspace([0,0,-1,0], vnf2, closed=false);
//   vnf_polyhedron(vnf3);
// Example(VPR=[80,0,15]): Use {vnf_join()} to combine with a mating vnf, in this case a reflection of the part we made. 
//   vnf1=sphere(r=50, style="icosa", $fn=16);
//   vnf2=vnf_halfspace([.8,1,-1.5,0], vnf1);
//   vnf3=vnf_halfspace([0,0,-1,0], vnf2, closed=false);
//   vnf4=vnf_join([vnf3, zflip(vnf3,1)]);
//   vnf_polyhedron(vnf4);
// Example: When the input VNF is a surface with a boundary, if you use the default setting closed=true, then vnf_halfspace() tries to construct closing faces from the edges created by the cut.  These faces may be invalid, for example if the cut points are collinear.  In this example the constructed face is a valid face.
//   include <BOSL2/beziers.scad>
//   patch=[
//          [[10,-10,0],[1,-1,0],[-1,-1,0],[-10,-10,0]],
//          [[10,-10,20],[1,-1,20],[-1,-1,20],[-10,-10,20]]
//         ];
//   vnf=bezier_vnf(patch);
//   vnfcut = vnf_halfspace([-.8,0,-1,-14],vnf);
//   vnf_polyhedron(vnfcut);
// Example: Setting closed to false eliminates this (possibly invalid) face:
//   include <BOSL2/beziers.scad>
//   patch=[
//          [[10,-10,0],[1,-1,0],[-1,-1,0],[-10,-10,0]],
//          [[10,-10,20],[1,-1,20],[-1,-1,20],[-10,-10,20]]
//         ];
//   vnf=bezier_vnf(patch);
//   vnfcut = vnf_halfspace([-.8,0,-1,-14],vnf,closed=false);
//   vnf_polyhedron(vnfcut);
// Example: Here is a VNF that has holes, so it is not a valid manifold. 
//   outside = linear_sweep(circle(r=30), h=100, caps=false);
//   inside = yrot(7,linear_sweep(circle(r=10), h=120, caps=false));
//   open_vnf=vnf_join([outside, vnf_reverse_faces(inside)]);
//   vnf_polyhedron(open_vnf);
// Example: By cutting it at each end we can create closing faces, resulting in a valid manifold without holes.
//   outside = linear_sweep(circle(r=30), h=100, caps=false);
//   inside = yrot(11,linear_sweep(circle(r=10), h=120, caps=false));
//   open_vnf=vnf_join([outside, vnf_reverse_faces(inside)]);
//   vnf = vnf_halfspace([0,0,1,5], vnf_halfspace([0,.7,-1,-75], open_vnf));
//   vnf_polyhedron(vnf);
// Example: If boundary=true then the return is a list with the VNF and boundary data.  
//   vnf = path_sweep(circle(r=4, $fn=16),
//                    circle(r=20, $fn=64),closed=true);
//   cut_bnd = vnf_halfspace([-1,1,-4,0], vnf, boundary=true);*/
//   cutvnf = cut_bnd[0];
//   boundary = [for(b=cut_bnd[1]) select(cutvnf[0],b)];
//   vnf_polyhedron(cutvnf);
//   stroke(boundary,color="red");
function vnf_halfspace(plane, vnf, closed=true, boundary=false) =
    assert(_valid_plane(plane), "Invalid plane")
    assert(is_vnf(vnf), "Invalid vnf")
    let(
         inside = [for(x=vnf[0]) plane*[each x,-1] >= -EPSILON ? 1 : 0],
         vertexmap = [0,each cumsum(inside)],
         faces_edges_vertices = _vnfcut(plane, vnf[0],vertexmap,inside, vnf[1], last(vertexmap)),
         newvert = concat(bselect(vnf[0],inside), faces_edges_vertices[2])
    )
    closed==false && !boundary ? [newvert, faces_edges_vertices[0]]
  : let(
        allpaths = _assemble_paths(newvert, faces_edges_vertices[1]),
        newpaths = [for(p=allpaths) if (len(p)>=3) p
                                    else assert(approx(p[0],p[1]),"Orphan edge found when assembling cut edges.")
           ]
    )
    boundary ? [[newvert, faces_edges_vertices[0]], newpaths]
  : len(newpaths)<=1 ? [newvert, concat(faces_edges_vertices[0], newpaths)]
  : let(
           M = project_plane(plane),
           faceregion = [for(path=newpaths) path2d(apply(M,select(newvert,path)))],
           facevnf = vnf_from_region(faceregion,transform=rot_inverse(M),reverse=true)
      )
      vnf_join([[newvert, faces_edges_vertices[0]], facevnf]);

function _assemble_paths(vertices, edges, paths=[],i=0) =
     i==len(edges) ? paths :
     norm(vertices[edges[i][0]]-vertices[edges[i][1]])<EPSILON ? _assemble_paths(vertices,edges,paths,i+1) :
     let(    // Find paths that connects on left side and right side of the edges (if one exists)
         left = [for(j=idx(paths)) if (approx(vertices[last(paths[j])],vertices[edges[i][0]])) j],
         right = [for(j=idx(paths)) if (approx(vertices[edges[i][1]],vertices[paths[j][0]])) j]
     )
     assert(len(left)<=1 && len(right)<=1)
     let(
          keep_path = list_remove(paths,concat(left,right)),
          update_path = left==[] && right==[] ? edges[i]
                      : left==[] ? concat([edges[i][0]],paths[right[0]])
                      : right==[] ?  concat(paths[left[0]],[edges[i][1]])
                      : left != right ? concat(paths[left[0]], paths[right[0]])
                      : paths[left[0]]
     )
     _assemble_paths(vertices, edges, concat(keep_path, [update_path]), i+1);


function _vnfcut(plane, vertices, vertexmap, inside, faces, vertcount, newfaces=[], newedges=[], newvertices=[], i=0) =
   i==len(faces) ? [newfaces, newedges, newvertices] :
   let(
        pts_inside = select(inside,faces[i])
   )
   all(pts_inside) ? _vnfcut(plane, vertices, vertexmap, inside, faces, vertcount,
                             concat(newfaces, [select(vertexmap,faces[i])]), newedges, newvertices, i+1):
   !any(pts_inside) ? _vnfcut(plane, vertices, vertexmap,inside, faces, vertcount, newfaces, newedges, newvertices, i+1):
   let(
        first = search([[1,0]],pair(pts_inside,wrap=true),0)[0],
        second = search([[0,1]],pair(pts_inside,wrap=true),0)[0]
   )
   assert(len(first)==1 && len(second)==1, "Found concave face in VNF.  Run vnf_triangulate first to ensure convex faces.")
   let(
        newface = [each select(vertexmap,select(faces[i],second[0]+1,first[0])),vertcount, vertcount+1],
        newvert = [plane_line_intersection(plane, select(vertices,select(faces[i],first[0],first[0]+1)),eps=0),
                   plane_line_intersection(plane, select(vertices,select(faces[i],second[0],second[0]+1)),eps=0)]
   )
   true //!approx(newvert[0],newvert[1])
       ? _vnfcut(plane, vertices, vertexmap, inside, faces, vertcount+2,
                 concat(newfaces, [newface]), concat(newedges,[[vertcount+1,vertcount]]),concat(newvertices,newvert),i+1)
   :len(newface)>3
       ? _vnfcut(plane, vertices, vertexmap, inside, faces, vertcount+1,
                 concat(newfaces, [list_head(newface)]), newedges,concat(newvertices,[newvert[0]]),i+1)
   :
   _vnfcut(plane, vertices, vertexmap, inside, faces, vertcount,newfaces, newedges, newvert, i+1);




function _triangulate_planar_convex_polygons(polys) =
    polys==[]? [] :
    let(
        tris = [for (poly=polys) if (len(poly)==3) poly],
        bigs = [for (poly=polys) if (len(poly)>3) poly],
        newtris = [for (poly=bigs) select(poly,-2,0)],
        newbigs = [for (poly=bigs) select(poly,0,-2)],
        newtris2 = _triangulate_planar_convex_polygons(newbigs),
        outtris = concat(tris, newtris, newtris2)
    ) outtris;

//**
// this function may produce degenerate triangles:
//    _triangulate_planar_convex_polygons([ [for(i=[0:1]) [i,i],
//                                           [1,-1], [-1,-1],
//                                           for(i=[-1:0]) [i,i] ] ] )
//    == [[[-1, -1], [ 0,  0], [0,  0]]
//        [[-1, -1], [-1, -1], [0,  0]]
//        [[ 1, -1], [-1, -1], [0,  0]]
//        [[ 0,  0], [ 1,  1], [1, -1]] ]
//

// Function: vnf_bend()
// Synopsis: Bends a VNF around an axis.
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_volume(), vnf_area(), vnf_halfspace() 
// Usage:
//   bentvnf = vnf_bend(vnf,r|d=,[axis=]);
// Description:
//   Bend a VNF around the X, Y or Z axis, splitting up faces as necessary.  Returns the bent
//   VNF.  For bending around the Z axis the input VNF must not cross the Y=0 plane.  For bending
//   around the X or Y axes the VNF must not cross the Z=0 plane.  Note that if you wrap a VNF all the way around
//   it may intersect itself, which produces an invalid polyhedron.  It is your responsibility to
//   avoid this situation.  The 1:1
//   radius is where the curved length of the bent VNF matches the length of the original VNF.  If the
//   `r` or `d` arguments are given, then they will specify the 1:1 radius or diameter.  If they are
//   not given, then the 1:1 radius will be defined by the distance of the furthest vertex in the
//   original VNF from the Z=0 plane.  You can adjust the granularity of the bend using the standard
//   `$fa`, `$fs`, and `$fn` variables.
// Arguments:
//   vnf = The original VNF to bend.
//   r = If given, the radius where the size of the original shape is the same as in the original.
//   ---
//   d = If given, the diameter where the size of the original shape is the same as in the original.
//   axis = The axis to wrap around.  "X", "Y", or "Z".  Default: "Z"
// Example(3D):
//   vnf0 = cube([100,40,10], center=true);
//   vnf1 = up(50, p=vnf0);
//   vnf2 = down(50, p=vnf0);
//   bent1 = vnf_bend(vnf1, axis="Y");
//   bent2 = vnf_bend(vnf2, axis="Y");
//   vnf_polyhedron([bent1,bent2]);
// Example(3D):
//   vnf0 = linear_sweep(star(n=5,step=2,d=100), height=10);
//   vnf1 = up(50, p=vnf0);
//   vnf2 = down(50, p=vnf0);
//   bent1 = vnf_bend(vnf1, axis="Y");
//   bent2 = vnf_bend(vnf2, axis="Y");
//   vnf_polyhedron([bent1,bent2]);
// Example(3D):
//   rgn = union(rect([100,20]),
//               rect([20,100]));
//   vnf0 = linear_sweep(zrot(45,p=rgn), height=10);
//   vnf1 = up(50, p=vnf0);
//   vnf2 = down(50, p=vnf0);
//   bent1 = vnf_bend(vnf1, axis="Y");
//   bent2 = vnf_bend(vnf2, axis="Y");
//   vnf_polyhedron([bent1,bent2]);
// Example(3D): Bending Around X Axis.
//   rgnr = union(
//       rect([20,100]),
//       back(50, p=trapezoid(w1=40, w2=0, h=20, anchor=FRONT))
//   );
//   vnf0 = xrot(00,p=linear_sweep(rgnr, height=10));
//   vnf1 = up(50, p=vnf0);
//   #vnf_polyhedron(vnf1);
//   bent1 = vnf_bend(vnf1, axis="X");
//   vnf_polyhedron([bent1]);
// Example(3D): Bending Around Y Axis.
//   rgn = union(
//       rect([20,100]),
//       back(50, p=trapezoid(w1=40, w2=0, h=20, anchor=FRONT))
//   );
//   rgnr = zrot(-90, p=rgn);
//   vnf0 = xrot(00,p=linear_sweep(rgnr, height=10));
//   vnf1 = up(50, p=vnf0);
//   #vnf_polyhedron(vnf1);
//   bent1 = vnf_bend(vnf1, axis="Y");
//   vnf_polyhedron([bent1]);
// Example(3D): Bending Around Z Axis.
//   rgn = union(
//       rect([20,100]),
//       back(50, p=trapezoid(w1=40, w2=0, h=20, anchor=FRONT))
//   );
//   rgnr = zrot(90, p=rgn);
//   vnf0 = xrot(90,p=linear_sweep(rgnr, height=10));
//   vnf1 = fwd(50, p=vnf0);
//   #vnf_polyhedron(vnf1);
//   bent1 = vnf_bend(vnf1, axis="Z");
//   vnf_polyhedron([bent1]);
// Example(3D): Bending more than once around the cylinder
//   $fn=32;
//   vnf = apply(fwd(5)*yrot(30),cube([100,2,5],center=true));
//   bent = vnf_bend(vnf, axis="Z");
//   vnf_polyhedron(bent);
function vnf_bend(vnf,r,d,axis="Z") =
    let(
        chk_axis = assert(in_list(axis,["X","Y","Z"])),
        verts = vnf[0],
        bounds = pointlist_bounds(verts),
        bmin = bounds[0],
        bmax = bounds[1],
        dflt = axis=="Z"?
            max(abs(bmax.y), abs(bmin.y)) :
            max(abs(bmax.z), abs(bmin.z)),
        r = get_radius(r=r,d=d,dflt=dflt),
        extent = axis=="X" ? [bmin.y, bmax.y] : [bmin.x, bmax.x]
    )
    let(
        span_chk = axis=="Z"?
            assert(bmin.y > 0 || bmax.y < 0, "Entire shape MUST be completely in front of or behind y=0.") :
            assert(bmin.z > 0 || bmax.z < 0, "Entire shape MUST be completely above or below z=0."),
        steps = 1+ceil(segs(r) * (extent[1]-extent[0])/(2*PI*r)),
        step = (extent[1]-extent[0]) / steps,
        bend_at = [for(i = [1:1:steps-1]) i*step+extent[0]],
        slicedir = axis=="X"? "Y" : "X",   // slice in y dir for X axis case, and x dir otherwise
        sliced = vnf_slice(vnf, slicedir, bend_at),
        coord = axis=="X" ? [0,sign(bmax.z),0] : axis=="Y" ? [sign(bmax.z),0,0] : [sign(bmax.y),0,0],
        new_vert = [for(p=sliced[0])
                       let(a=coord*p*180/(PI*r))
                       axis=="X"? [p.x, p.z*sin(a), p.z*cos(a)] :
                       axis=="Y"? [p.z*sin(a), p.y, p.z*cos(a)] :
                       [p.y*sin(a), p.y*cos(a), p.z]]
   ) [new_vert,sliced[1]];



// Function&Module: vnf_hull()
// Synopsis: Compute convex hull of VNF or 3d path
// Usage:
//    vnf_hull = hull_vnf(vnf);
//    hull_vnf(vnf,[fast]);
// Description:
//   Given a VNF or a list of 3d points, compute the convex hull
//   and return it as a VNF.  This differs from {{hull()}} and {{hull3d_faces()}} which
//   return just the face list referenced to the input point list.  Note that the point
//   list that is returned will contain all the points that are actually used in the input
//   VNF, which may be many more points than are needed to represent the convex hull.
//   This is not usually a problem, but you can run the somewhat slow {{vnf_drop_unused_points()}}
//   function to fix this if necessary.  
// Arguments:
//   region = region or path listing points to compute the hull from.
//   fast = (module only) if input is a point list (not a VNF) use a fasterer cheat that may handle more points, but could emit warnings.  Ignored if input is a VNF.  Default: false.  
// Example(3D,Big,NoAxes,VPR=[55,0,25],VPT=[9.47096,-4.50217,8.45727],VPD=60.2654): Input is a VNF
//   ellipse = xscale(2, p=circle($fn=48, r=3));
//   pentagon = subdivide_path(pentagon(r=1), 20);
//   vnf=path_sweep(pentagon, path3d(ellipse),
//                  closed=true, twist=360*2);
//   vnfhull = vnf_hull(vnf);
//   vnf_polyhedron(vnf);
//   move([10,10])
//     vnf_polyhedron(vnfhull);
// Example(3D,Med,NoAxes,VPR=[70.4,0,110.4],VPT=[5.97456,1.26459,18.0317],VPD=126): Input is a point list
//   h=helix(l=40, turns=1, r=8);
//   color("red")move_copies(h)
//     sphere(r=0.5,$fn=12);
//   vnf_polyhedron(vnf_hull(h));
function vnf_hull(vnf) =
  assert(is_vnf(vnf) || is_path(vnf,3),"Input must be a VNF or a 3d path")
  let(
      pts = is_vnf(vnf) ? select(vnf[0],unique(flatten(vnf[1])))
                        : vnf,
      faces = hull3d_faces(pts)
  )
  [pts, faces];

module vnf_hull(vnf, fast=false)
{
  if (is_vnf(vnf)) hull()vnf_polyhedron(vnf);
  else hull_points(vnf, fast);
}  



function _sort_pairs0(arr) =
    len(arr)<=1 ? arr : 
    let(
        pivot   = arr[floor(len(arr)/2)][0],
        lesser  = [ for (y = arr) if (y[0].x  < pivot.x || (y[0].x==pivot.x && y[0].y<pivot.y)) y ],
        equal   = [ for (y = arr) if (y[0] == pivot) y ],
        greater = [ for (y = arr) if (y[0].x > pivot.x || (y[0].x==pivot.x && y[0].y>pivot.y)) y ]
    ) 
    concat( _sort_pairs0(lesser), equal, _sort_pairs0(greater) );


// Function: vnf_boundary()
// Synopsis: Returns the boundary of a VNF as an list of paths
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_halfspace(), vnf_merge_points()
// Usage:
//   boundary = vnf_boundary(vnf, [merge=], [idx=]);
// Description:
//   Returns the boundary of a VNF as a list of paths.  **The input VNF must not contain duplicate points.**  By default, vnf_boundary() calls {{vnf_merge_points()}}
//   to remove duplicate points.  Note, however, that this operation can be slow.  If you are **certain** there are no duplicate points you can
//   set `merge=false` to disable the automatic point merge and save time.  The result of running on a VNF with duplicate points is likely to
//   be incorrect or invalid; it may produce obscure errors.   
//   .
//   The output will be a list of closed 3D paths.  If the VNF has no boundary then the output is `[]`.  The boundary path(s) are
//   traversed in the same direction as the edges in the original VNF.  
//   .
//   It is sometimes desirable to have the boundary available as an index list into the VNF vertex list.  However, merging the points in the VNF changes the 
//   VNF vertex point list.  If you set `merge=false` you can also set `idx=true` to get an index list.  As noted above, you must be certain
//   that your in put VNF has no duplicate vertices, perhaps by running {{vnf_merge_points()}} yourself on it.  With `idx=true`
//   the output will be indices into the VNF vertex list, which enables you to associate the vertices on the boundary path with the original VNF.
// Arguments:
//   vnf = input vnf
//   ---
//   merge = set to false to suppress the automatic invocation of {{vnf_merge_points()}}.  Default: true
//   idx = if true, return indices into VNF vertices instead of actual 3D points.  Must set `merge=false` to enable this.  Default: false
// Example(3D,NoAxes,VPT=[7.06325,-20.8414,20.1803],VPD=292.705,VPR=[55,0,25.7]):  In this example we know that the bezier patch VNF has no duplicate vertices, so we do not need to run {{vnf_merge_points()}}.
//   include <BOSL2/beziers.scad>
//   patch = [
//        // u=0,v=0                                         u=1,v=0
//        [[-50,-50,  0], [-16,-50,  20], [ 16,-50, -20], [50,-50,  0]],
//        [[-50,-16, 20], [-16,-16,  20], [ 16,-16, -20], [50,-16, 20]],
//        [[-50, 16, 20], [-16, 16, -20], [ 16, 16,  20], [50, 16, 20]],
//        [[-50, 50,  0], [-16, 50, -20], [ 16, 50,  20], [50, 50,  0]],
//        // u=0,v=1                                         u=1,v=1
//   ];                
//   bezvnf = bezier_vnf(patch);
//   boundary = vnf_boundary(bezvnf);
//   vnf_polyhedron(bezvnf);
//   stroke(boundary,color="green");
// Example(3D,NoAxes,VPT=[-11.1252,-19.7333,8.39927],VPD=82.6686,VPR=[71.8,0,335.3]): An example with two path components on the boundary.  The output from {{vnf_halfspace()}} can contain duplicate vertices, so we must invoke {{vnf_merge_points()}}.  
//   vnf = torus(id=20,od=40,$fn=28);
//   cutvnf=vnf_halfspace([0,1,0,0],
//            vnf_halfspace([-1,.5,-2.5,-12], vnf, closed=false),
//            closed=false);
//   vnf_polyhedron(cutvnf);
//   boundary = vnf_boundary(vnf_merge_points(cutvnf));
//   stroke(boundary,color="green");
function vnf_boundary(vnf,merge=true,idx=false) =
   assert(!idx || !merge, "Cannot request indices unless marge=false and VNF contains no duplicate vertices")
   let(
       vnf = merge ? vnf_merge_points(vnf) : vnf,
       edgelist= [ for(face=vnf[1], edge=pair(face,wrap=true))
                      [edge.x<edge.y ? edge : [edge.y,edge.x],edge]
                 ],
       sortedge = _sort_pairs0(edgelist),  
       edges=  [
                if (sortedge[0][0]!=sortedge[1][0]) sortedge[0][1],
                for(i=[1:1:len(sortedge)-2])
                     if (sortedge[i][0]!=sortedge[i-1][0] && sortedge[i][0]!=sortedge[i+1][0]) sortedge[i][1],
                if (last(sortedge)[0] != sortedge[len(sortedge)-2][0]) last(sortedge)[1]
               ],
       paths = _assemble_paths(vnf[0], edges)    // could be made cleaner and maybe more robust with an _assemble_path version that 
   )                                             // uses edge vertex indices instead of actual point values
   idx ? paths : [for(path=paths) select(vnf[0],path)];


// Function: vnf_small_offset()
// Synopsis: Computes an offset surface to a VNF for small offset distances
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_sheet(), vnf_merge_points()
// Usage:
//   newvnf = vnf(vnf, delta, [merge=]);
// Description:
//   Computes a simple offset of a VNF by estimating the normal at every point based on the weighted average of surrounding polygons
//   in the mesh.  The offset distance, `delta`, must be small enough so that no self-intersection occurs, which is no issue when the
//   curvature is positive (like the outside of a sphere) but for negative curvature it means the offset distance must be smaller
//   than the smallest radius of curvature of the VNF.  If self-intersection
//   occurs, the resulting geometry will be invalid and you will get an error when you introduce a second object into the model.
//   **It is your responsibility to avoid invalid geometry!**  It cannot be detected automatically.  
//   The positive offset direction is towards the outside of the VNF, the faces that are colored yellow in the "thrown together" view.  
//   .
//   **The input VNF must not contain duplicate points.**  By default, vnf_small_offset() calls {{vnf_merge_points()}}
//   to remove duplicate points.  Note, however, that this operation can be slow.  If you are **certain** there are no duplicate points you can
//   set `merge=false` to disable the automatic point merge and save time.  The result of running on a VNF with duplicate points is likely to
//   be incorrect or invalid.
// Arguments:
//   vnf = vnf to offset
//   delta = distance of offset, positive to offset out, negative to offset in
//   ---
//   merge = set to false to suppress the automatic invocation of {{vnf_merge_points()}}.  Default: true
// Example(3D):  The original sphere is on the left and an offset sphere on the right.  
//   vnf = sphere(d=100);
//   xdistribute(spacing=125){
//     vnf_polyhedron(vnf);
//     vnf_polyhedron(vnf_small_offset(vnf,18));
//   }
// Example(3D): The polyhedron on the left is enlarged to match the size of the offset polyhedron on the right.  Note that the offset does **not** preserve coplanarity of faces.  This is because the vertices all move independently, so nothing constrains faces to remain coplanar.  
//   include <BOSL2/polyhedra.scad>
//   vnf = regular_polyhedron_info("vnf","pentagonal icositetrahedron",d=25);
//   xdistribute(spacing=300){
//     scale(11)vnf_polyhedron(vnf);
//     vnf_polyhedron(vnf_small_offset(vnf,125));
//   }
function vnf_small_offset(vnf, delta, merge=true) =
   let(
        vnf = merge ? vnf_merge_points(vnf) : vnf, 
        vertices = vnf[0],
        faces = vnf[1],
        vert_faces = group_data(
            [for (i = idx(faces), vert = faces[i]) vert],
            [for (i = idx(faces), vert = faces[i]) i]
        ),
        normals = [for(face=faces) polygon_normal(select(vertices,face))],   // Normals for each face
        offset = [for(vertex=idx(vertices))
                    let(
                        vfaces = vert_faces[vertex], // Faces that surround this vertex
                        adjacent_normals = select(normals,vfaces),
                        angles = [for(faceind=vfaces)
                                    let(
                                        thisface = faces[faceind],
                                        vind = search(vertex,thisface)[0]
                                    )
                                    vector_angle(select(vertices, select(thisface,vind-1,vind+1)))
                                 ]
                    )
                    vertices[vertex] +unit(angles*adjacent_normals)*delta
                 ]
    )
    [offset,faces];

// Function: vnf_sheet()
// Synopsis: Extends a VNF into a thin sheet by extruding normal to the VNF
// SynTags: VNF
// Topics: VNF Manipulation
// See Also: vnf_small_offset(), vnf_boundary(), vnf_merge_points()
// Usage:
//   newvnf = vnf_sheet(vnf, thickness, [style=], [merge=]);
// Description:
//   Constructs a thin sheet from a vnf by offsetting the vnf along the normal vectors estimated at
//   each vertex by averaging the normals of the adjacent faces.  This is done using {{vnf_small_offset()}.
//   The thickness value must be small enough so that no points cross each other
//   when the offset is computed, because that results in invalid geometry and will give rendering errors.
//   Rendering errors may not manifest until you add other objects to your model.  
//   **It is your responsibility to avoid invalid geometry!**
//   .
//   Once the offset to the original VNF is computed the original and offset VNF are connected by filling
//   in the boundary strip(s) between them
//   .
//   When thickness is positive, the given bezier patch is extended towards its "inside", which is the
//   side that appears purple in the "thrown together" view.  Note that this is the opposite direction
//   of {{vnf_small_offset()}}.  Extending toward the inside means that your original VNF remains unchanged
//   in the output.  You can extend the patch in the other direction
//   using a negative thickness value.   When you extend to the outside with a negative thickness, your VNF needs to have all
//   of its faces reversed to produce a valid polyhedron, so your original VNF is reversed in the output.
//   .
//   **The input VNF must not contain duplicate points.**  By default, vnf_sheet() calls {{vnf_merge_points()}}
//   to remove duplicate points.  Note, however, that this operation can be slow.  If you are **certain** there are no duplicate points you can
//   set `merge=false` to disable the automatic point merge and save time.  The result of running on a VNF with duplicate points is likely to
//   be incorrect or invalid, or it may result in cryptic errors.  
// Arguments:
//   vnf = vnf to process
//   thickness = thickness of sheet to produce; can be positive or negative
//   ---
//   style = {{vnf_vertex_array()}} style to use.  Default: "default"
//   merge = if false then do not run {{vnf_merge_points()}}.  Default: true
// Example(3D): 
//   pts = [for(x=[30:5:180]) [for(y=[-6:0.5:6])  [7*y,x, sin(x)*y^2]]];
//   vnf=vnf_vertex_array(pts);
//   vnf_polyhedron(vnf_sheet(vnf,-10));
// Example(3D): This example has multiple holes
//   pts = [for(x=[-10:2:10]) [ for(y=[-10:2:10]) [x,1.4*y,(-abs(x)^3+y^3)/250]]];
//   vnf = vnf_vertex_array(pts);
//   newface = list_remove(vnf[1], [43,42,63,88,108,109,135,134,129,155,156,164,165]);
//   newvnf = [vnf[0],newface];
//   vnf_polyhedron(vnf_sheet(newvnf,2));
// Example(3D): When applied to a sphere the sheet is constructed inward, so the object appears unchanged, but cutting it in half reveals that we have changed the sphere into a shell.  
//   vnf = sphere(d=100, $fn=28);
//   left_half()
//     vnf_polyhedron(vnf_sheet(vnf,15));

function vnf_sheet(vnf, thickness, style="default", merge=true) =
  let(
       vnf = merge ? vnf_merge_points(vnf) : vnf,
       offset = vnf_small_offset(vnf, -thickness, merge=false),
       boundary = vnf_boundary(vnf,merge=false,idx=true),
       newvnf = vnf_join([vnf,
                         vnf_reverse_faces(offset),
                         for(p=boundary) vnf_vertex_array([select(offset[0],p),select(vnf[0],p)],col_wrap=true,style=style)
                         ])
  )
  thickness < 0 ? vnf_reverse_faces(newvnf) : newvnf;



// Section: Debugging Polyhedrons

/// Internal Module: _show_vertices()
/// Usage:
///   _show_vertices(vertices, [size], [filter=])
/// Description:
///   Draws all the vertices in an array, at their 3D position, numbered by their
///   position in the vertex array.  Also draws any children of this module with
///   transparency.
/// Arguments:
///   vertices = Array of point vertices.
///   size = The size of the text used to label the vertices.  Default: 1
/// Example:
///   verts = [for (z=[-10,10], y=[-10,10], x=[-10,10]) [x,y,z]];
///   faces = [[0,1,2], [1,3,2], [0,4,5], [0,5,1], [1,5,7], [1,7,3], [3,7,6], [3,6,2], [2,6,4], [2,4,0], [4,6,7], [4,7,5]];
///   _show_vertices(vertices=verts, size=2) {
///       polyhedron(points=verts, faces=faces);
///   }
module _show_vertices(vertices, size=1, filter) {
    color("blue") {
        dups = vector_search(vertices, EPSILON, vertices);
        for (ind = dups) {
            if (is_undef(filter) || any(ind, filter)) {
                numstr = str_join([for(i=ind) str(i)],",");
                v = vertices[ind[0]];
                translate(v) {
                    rot($vpr) back(size/8){
                       linear_extrude(height=size/10, center=true, convexity=10) {
                          text(text=numstr, size=size, halign="center");
                       }
                    }
                    sphere(size/10);
                }
            }
        }
    }
}


/// Internal Module: _show_faces()
/// Usage:
///   _show_faces(vertices, faces, [size=], [filter=]);
/// Description:
///   Draws all the vertices at their 3D position, numbered in blue by their
///   position in the vertex array.  Each face will have their face number drawn
///   in red, aligned with the center of face.  All children of this module are drawn
///   with transparency.
/// Arguments:
///   vertices = Array of point vertices.
///   faces = Array of faces by vertex numbers.
///   size = The size of the text used to label the faces and vertices.  Default: 1
/// Example(EdgesMed):
///   verts = [for (z=[-10,10], y=[-10,10], x=[-10,10]) [x,y,z]];
///   faces = [[0,1,2], [1,3,2], [0,4,5], [0,5,1], [1,5,7], [1,7,3], [3,7,6], [3,6,2], [2,6,4], [2,4,0], [4,6,7], [4,7,5]];
///   _show_faces(vertices=verts, faces=faces, size=2) {
///       polyhedron(points=verts, faces=faces);
///   }
module _show_faces(vertices, faces, size=1, filter) {
    vlen = len(vertices);
    color("red") {
        for (i = [0:1:len(faces)-1]) {
            face = faces[i];
            if (face[0] < 0 || face[1] < 0 || face[2] < 0 || face[0] >= vlen || face[1] >= vlen || face[2] >= vlen) {
                echo(str("INVALID FACE: indices of face ",i," are out of bounds [0,",vlen-1,"]: face=",face));
            }
            else if (is_undef(filter) || any(face,filter)) {
                verts = select(vertices,face);
                normal = polygon_normal(verts);
                if (is_undef(normal))
                    echo(str("DEGENERATE FACE: face ",i," has no normal vector, face=", face));
                else {
                    axis = vector_axis(normal, DOWN);
                    ang = vector_angle(normal, DOWN);
                    theta = atan2(normal[1], normal[0]);
                    translate(mean(verts)) 
                      rotate(a=(180-ang), v=axis)
                      zrot(theta+90)
                      linear_extrude(height=size/10, center=true, convexity=10) {
                                text(text=str(i), size=size, halign="center");
                                text(text=str("_"), size=size, halign="center");
                      }
                }
            }
        }
    }        
}



// Module: debug_vnf()
// Synopsis: A replacement for `vnf_polyhedron()` to help with debugging.
// SynTags: VNF
// Topics: VNF Manipulation, Debugging
// See Also: vnf_validate()
// Usage:
//   debug_vnf(vnfs, [faces=], [vertices=], [opacity=], [size=], [convexity=], [filter=]);
// Description:
//   A drop-in module to replace `vnf_polyhedron()` to help debug vertices and faces.
//   Draws all the vertices at their 3D position, numbered in blue by their
//   position in the vertex array.  Each face will have its face number drawn
//   in red, aligned with the center of face.  All given faces are drawn with
//   transparency. All children of this module are drawn with transparency.
//   Works best with Thrown-Together preview mode, to see reversed faces.
//   You can set opacity to 0 if you want to supress the display of the polyhedron faces.
//   .
//   The vertex numbers are shown rotated to face you.  As you rotate your polyhedron you
//   can rerun the preview to display them oriented for viewing from a different viewpoint.
// Topics: Polyhedra, Debugging
// Arguments:
//   vnf = VNF to display
//   ---
//   faces = if true display face numbers.  Default: true
//   vertices = if true display vertex numbers.  Default: true
//   opacity = Opacity of the polyhedron faces.  Default: 0.5
//   convexity = The max number of walls a ray can pass through the given polygon paths.
//   size = The size of the text used to label the faces and vertices.  Default: 1
//   filter = If given a function literal of signature `function(i)`, will only show labels for vertices and faces that have a vertex index that gets a true result from that function.  Default: no filter.
// Example(EdgesMed):
//   verts = [for (z=[-10,10], a=[0:120:359.9]) [10*cos(a),10*sin(a),z]];
//   faces = [[0,1,2], [5,4,3], [0,3,4], [0,4,1], [1,4,5], [1,5,2], [2,5,3], [2,3,0]];
//   debug_vnf([verts,faces], size=2);
module debug_vnf(vnf, faces=true, vertices=true, opacity=0.5, size=1, convexity=6, filter ) {
    no_children($children);
    if (faces)
      _show_faces(vertices=vnf[0], faces=vnf[1], size=size, filter=filter);
    if (vertices)
      _show_vertices(vertices=vnf[0], size=size, filter=filter);
    if (opacity > 0)
      color([0.2, 1.0, 0, opacity])
        vnf_polyhedron(vnf,convexity=convexity);
}


// Module: vnf_validate()
// Synopsis: Echos non-manifold VNF errors to the console.
// SynTags: VNF
// Topics: VNF Manipulation, Debugging
// See Also: debug_vnf()
// 
// Usage: 
//   vnf_validate(vnf, [size], [show_warns=], [check_isects=], [opacity=], [adjacent=], [label_verts=], [label_faces=], [wireframe=]);
// Description:
//   When called as a module, echoes the non-manifold errors to the console, and color hilites the
//   bad edges and vertices, overlaid on a transparent gray polyhedron of the VNF.
//   .
//   Currently checks for these problems:
//   .
//   Type    | Color    | Code         | Message
//   ------- | -------- | ------------ | ---------------------------------
//   WARNING | Yellow   | BIG_FACE     | Face has more than 3 vertices, and may confuse CGAL.
//   WARNING | Blue     | NULL_FACE    | Face has zero area.
//   ERROR   | Cyan     | NONPLANAR    | Face vertices are not coplanar.
//   ERROR   | Brown    | DUP_FACE     | Multiple instances of the same face.
//   ERROR   | Orange   | MULTCONN     | Multiply Connected Geometry. Too many faces attached at Edge.
//   ERROR   | Violet   | REVERSAL     | Faces reverse across edge.
//   ERROR   | Red      | T_JUNCTION   | Vertex is mid-edge on another Face.
//   ERROR   | Brown    | FACE_ISECT   | Faces intersect.
//   ERROR   | Magenta  | HOLE_EDGE    | Edge bounds Hole.
//   .
//   Still to implement:
//   - Overlapping coplanar faces.
// Arguments:
//   vnf = The VNF to validate.
//   size = The width of the lines and diameter of points used to highlight edges and vertices.  Module only.  Default: 1
//   ---
//   show_warns = If true show warnings for non-triangular faces.  Default: true
//   check_isects = If true, performs slow checks for intersecting faces.  Default: false
//   opacity = The opacity level to show the polyhedron itself with.    Default: 0.67
//   label_verts = If true, shows labels at each vertex that show the vertex number.    Default: false
//   label_faces = If true, shows labels at the center of each face that show the face number.    Default: false
//   wireframe = If true, shows edges more clearly so you can see them in Thrown Together mode.    Default: false
//   adjacent = If true, only display faces adjacent to a vertex listed in the errors.    Default: false
// Example(3D,Edges): BIG_FACE Warnings; Faces with More Than 3 Vertices.  CGAL often will fail to accept that a face is planar after a rotation, if it has more than 3 vertices.
//   vnf = skin([
//       path3d(regular_ngon(n=3, d=100),0),
//       path3d(regular_ngon(n=5, d=100),100)
//   ], slices=0, caps=true, method="tangent");
//   vnf_validate(vnf);
// Example(3D,Edges): NONPLANAR Errors; Face Vertices are Not Coplanar
//   a = [  0,  0,-50];
//   b = [-50,-50, 50];
//   c = [-50, 50, 50];
//   d = [ 50, 50, 60];
//   e = [ 50,-50, 50];
//   vnf = vnf_from_polygons([
//       [a, b, e], [a, c, b], [a, d, c], [a, e, d], [b, c, d, e]
//   ],fast=true);
//   vnf_validate(vnf);
// Example(3D,Edges): MULTCONN Errors; More Than Two Faces Attached to the Same Edge.  This confuses CGAL, and can lead to failed renders.
//   vnf = vnf_triangulate(linear_sweep(union(square(50), square(50,anchor=BACK+RIGHT)), height=50));
//   vnf_validate(vnf);
// Example(3D,Edges): REVERSAL Errors; Faces Reversed Across Edge
//   vnf1 = skin([
//       path3d(square(100,center=true),0),
//       path3d(square(100,center=true),100),
//   ], slices=0, caps=false);
//   vnf = vnf_join([vnf1, vnf_from_polygons([
//       [[-50,-50,  0], [ 50, 50,  0], [-50, 50,  0]],
//       [[-50,-50,  0], [ 50,-50,  0], [ 50, 50,  0]],
//       [[-50,-50,100], [-50, 50,100], [ 50, 50,100]],
//       [[-50,-50,100], [ 50,-50,100], [ 50, 50,100]],
//   ])]);
//   vnf_validate(vnf);
// Example(3D,Edges): T_JUNCTION Errors; Vertex is Mid-Edge on Another Face.
//   vnf = [
//       [
//           each path3d(square(100,center=true),0),
//           each path3d(square(100,center=true),100),
//           [0,-50,100],
//       ], [
//          [0,2,1], [0,3,2], [0,8,4], [0,1,8], [1,5,8],
//          [0,4,3], [4,7,3], [1,2,5], [2,6,5], [3,7,6],
//          [3,6,2], [4,5,6], [4,6,7],
//       ]
//   ];
//   vnf_validate(vnf);
// Example(3D,Edges): FACE_ISECT Errors; Faces Intersect
//   vnf = vnf_join([
//       linear_sweep(square(100,center=true), height=100),
//       move([75,35,30],p=linear_sweep(square(100,center=true), height=100))
//   ]);
//   vnf_validate(vnf,size=2,check_isects=true);
// Example(3D,Edges): HOLE_EDGE Errors; Edges Adjacent to Holes.
//   vnf = skin([
//       path3d(regular_ngon(n=4, d=100),0),
//       path3d(regular_ngon(n=5, d=100),100)
//   ], slices=0, caps=false);
//   vnf_validate(vnf,size=2);


//   Returns a list of non-manifold errors with the given VNF.
//   Each error has the format `[ERR_OR_WARN,CODE,MESG,POINTS,COLOR]`.
function _vnf_validate(vnf, show_warns=true, check_isects=false) =
    assert(is_vnf(vnf), "Invalid VNF")
    let(
        varr = vnf[0],
        faces = vnf[1],
        lvarr = len(varr),
        edges = sort([
            for (face=faces, edge=pair(face,true))
            edge[0]<edge[1]? edge : [edge[1],edge[0]]
        ]),
        dfaces = [
            for (face=faces) let(
                face=deduplicate_indexed(varr,face,closed=true)
            ) if(len(face)>=3)
            face
        ],
        face_areas = [
            for (face = faces)
            len(face) < 3? 0 :
            polygon_area([for (k=face) varr[k]])
        ],
        edgecnts = unique_count(edges),
        uniq_edges = edgecnts[0],
        issues = []
    )
    let(
        big_faces = !show_warns? [] : [
            for (face = faces)
            if (len(face) > 3)
            _vnf_validate_err("BIG_FACE", face)
        ],
        null_faces = !show_warns? [] : [
            for (i = idx(faces)) let(
                face = faces[i],
                area = face_areas[i]
            )
            if (is_num(area) && abs(area) < EPSILON)
            _vnf_validate_err("NULL_FACE", face)
        ],
        issues = concat(big_faces, null_faces)
    )
    let(
        bad_indices = [
            for (face = faces, idx = face)
            if (idx < 0 || idx >= lvarr)
            _vnf_validate_err("BAD_INDEX", [idx])
        ],
        issues = concat(issues, bad_indices)
    ) bad_indices? issues :
    let(
        repeated_faces = [
            for (i=idx(dfaces), j=idx(dfaces))
            if (i!=j) let(
                face1 = dfaces[i],
                face2 = dfaces[j]
            ) if (min(face1) == min(face2)) let(
                min1 = min_index(face1),
                min2 = min_index(face2)
            ) if (min1 == min2) let(
                sface1 = list_rotate(face1,min1),
                sface2 = list_rotate(face2,min2)
            ) if (sface1 == sface2)
            _vnf_validate_err("DUP_FACE", sface1)
        ],
        issues = concat(issues, repeated_faces)
    ) repeated_faces? issues :
    let(
        multconn_edges = unique([
            for (i = idx(uniq_edges))
            if (edgecnts[1][i]>2)
            _vnf_validate_err("MULTCONN", uniq_edges[i])
        ]),
        issues = concat(issues, multconn_edges)
    ) multconn_edges? issues :
    let(
        reversals = unique([
            for(i = idx(dfaces), j = idx(dfaces)) if(i != j)
            for(edge1 = pair(faces[i],true))
            for(edge2 = pair(faces[j],true))
            if(edge1 == edge2)  // Valid adjacent faces will never have the same vertex ordering.
            if(_edge_not_reported(edge1, varr, multconn_edges))
            _vnf_validate_err("REVERSAL", edge1)
        ]),
        issues = concat(issues, reversals)
    ) reversals? issues :
    let(
        t_juncts = unique([
            for (v=idx(varr), edge=uniq_edges) let(
                ia = edge[0],
                ib = v,
                ic = edge[1]
            )
            if (ia!=ib && ib!=ic && ia!=ic) let(
                a = varr[ia],
                b = varr[ib],
                c = varr[ic]
            )
            if (!approx(a,b) && !approx(b,c) && !approx(a,c)) let(
                pt = line_closest_point([a,c],b,SEGMENT)
            )
            if (approx(pt,b))
            _vnf_validate_err("T_JUNCTION", [ib])
        ]),
        issues = concat(issues, t_juncts)
    ) t_juncts? issues :
    let(
        isect_faces = !check_isects? [] : unique([
            for (i = [0:1:len(faces)-2])
              let(
                  f1 = faces[i],
                  poly1   = select(varr, faces[i]),
                  plane1  = plane3pt(poly1[0], poly1[1], poly1[2]),
                  normal1 = [plane1[0], plane1[1], plane1[2]]
              )
              for (j = [i+1:1:len(faces)-1])
                let(
                  f2 = faces[j],
                  poly2 = select(varr, f2),
                  val = poly2 * normal1
                )
                // The next test skips f2 if it lies entirely on one side of the plane of poly1
                if( min(val)<=plane1[3] && max(val)>=plane1[3] )
                  let(
                      plane2  = plane_from_polygon(poly2),
                      normal2 = [plane2[0], plane2[1], plane2[2]],
                      val = poly1 * normal2
                  )
                  // Skip if f1 lies entirely on one side of the plane defined by poly2
                  if( min(val)<=plane2[3] && max(val)>=plane2[3] )
                    let(
                        shared_edges = [
                                        for (edge1 = pair(f1, true), edge2 = pair(f2, true))
                                           if (edge1 == [edge2[1], edge2[0]]) 1
                                       ]
                    )
                    if (shared_edges==[])
                       let(
                           line = plane_intersection(plane1, plane2)
                       )
                       if (is_def(line))
                          let(
                              isects = polygon_line_intersection(poly1, line)
                          )
                          if (is_def(isects))
                            for (isect = isects)
                              if (len(isect) > 1)
                                let(
                                    isects2 = polygon_line_intersection(poly2, isect, bounded=true)
                                )
                                if (is_def(isects2))
                                  for (seg = isects2) 
                                    if (len(seg)>1 && seg[0] != seg[1]) _vnf_validate_err("FACE_ISECT", seg)
        ]),
        issues = concat(issues, isect_faces)
    ) isect_faces? issues :
    let(
        hole_edges = unique([
            for (i=idx(uniq_edges))
            if (edgecnts[1][i]<2)
            if (_pts_not_reported(uniq_edges[i], varr, t_juncts))
            if (_pts_not_reported(uniq_edges[i], varr, isect_faces))
            _vnf_validate_err("HOLE_EDGE", uniq_edges[i])
        ]),
        issues = concat(issues, hole_edges)
    ) hole_edges? issues :
    let(
        nonplanars = unique([
            for (i = idx(faces))
               if (is_undef(face_areas[i])) 
                  _vnf_validate_err("NONPLANAR", faces[i])
        ]),
        issues = concat(issues, nonplanars)
    ) issues;


_vnf_validate_errs = [
    ["BIG_FACE",    "WARNING", "cyan",    "Face has more than 3 vertices, and may confuse CGAL"],
    ["NULL_FACE",   "WARNING", "blue",    "Face has zero area."],
    ["BAD_INDEX",   "ERROR",   "cyan",    "Invalid face vertex index."],
    ["NONPLANAR",   "ERROR",   "yellow",  "Face vertices are not coplanar"],
    ["DUP_FACE",    "ERROR",   "brown",   "Multiple instances of the same face."],
    ["MULTCONN",    "ERROR",   "orange",  "Multiply Connected Geometry. Too many faces attached at Edge"],
    ["REVERSAL",    "ERROR",   "violet",  "Faces Reverse Across Edge"],
    ["T_JUNCTION",  "ERROR",   "magenta", "Vertex is mid-edge on another Face"],
    ["FACE_ISECT",  "ERROR",   "brown",   "Faces intersect"],
    ["HOLE_EDGE",   "ERROR",   "red",     "Edge bounds Hole"]
];


function _vnf_validate_err(name, extra) =
    let(
        info = [for (x = _vnf_validate_errs) if (x[0] == name) x][0]
    ) concat(info, [extra]);


function _pts_not_reported(pts, varr, reports) =
    [
        for (i = pts, report = reports, pt = report[3])
        if (varr[i] == pt) 1
    ] == [];


function _edge_not_reported(edge, varr, reports) =
    let(
        edge = sort([for (i=edge) varr[i]])
    ) [
        for (report = reports) let(
            pts = sort(report[3])
        ) if (len(pts)==2 && edge == pts) 1
    ] == [];


module vnf_validate(vnf, size=1, show_warns=true, check_isects=false, opacity=0.67, adjacent=false, label_verts=false, label_faces=false, wireframe=false) {
    no_children($children);
    vcount = len(vnf[0]);
    fcount = len(vnf[1]);
    vnf = vnf_merge_points(vnf);
    faults = _vnf_validate(
        vnf, show_warns=show_warns,
        check_isects=check_isects
    );
    verts = vnf[0];
    vnf_changed = len(verts)!=vcount || len(vnf[1])!=fcount;
    if (!faults) {
        echo("VNF appears valid.");
    }
    if (vnf_changed) echo("VNF changed when merging points; unable to display indices");
    for (fault = faults) {
        err = fault[0];
        typ = fault[1];
        clr = fault[2];
        msg = fault[3];
        idxs = fault[4];
        pts = err=="FACE_ISECT" ? idxs : [for (i=idxs) if(is_finite(i) && i>=0 && i<len(verts)) verts[i]];
        if (vnf_changed || err=="FACE_ISECT")
          echo(str(typ, " ", err, " (", clr ,"): ", msg, " at ", pts));
        else
          echo(str(typ, " ", err, " (", clr ,"): ", msg, " at ", pts, " indices: ", idxs));
        color(clr) {
            if (is_vector(pts[0])) {
                if (len(pts)==2) {
                    stroke(pts, width=size, endcaps="butt", $fn=8);
                } else if (len(pts)>2) {
                    stroke(pts, width=size, closed=true, $fn=8);
                    polyhedron(pts,[[for (i=idx(pts)) i]]);
                } else {
                    move_copies(pts) sphere(d=size*3, $fn=18);
                }
            }
        }
    }
    badverts = unique([for (fault=faults) each fault[4]]);
    badverts2 = unique([for (j=idx(verts), i=badverts) if (i!=j && verts[i]==verts[j]) j]);
    all_badverts = unique(concat(badverts, badverts2));
    adjacent = !faults? false : adjacent;
    filter_fn = !adjacent? undef : function(i) in_list(i,all_badverts);
    adj_vnf = !adjacent? vnf : [
        verts, [for (face=vnf[1]) if (any(face,filter_fn)) face]
    ];
    if (wireframe) {
        vnf_wireframe(adj_vnf, width=size*0.25);
    }
    if (label_verts) {
        debug_vnf(adj_vnf, size=size*3, opacity=0, faces=false, vertices=true, filter=filter_fn);
    }
    if (label_faces) {
        debug_vnf(vnf, size=size*3, opacity=0, faces=true, vertices=false, filter=filter_fn);
    }
    if (opacity > 0) {
        color([0.5,1,0.5,opacity]) vnf_polyhedron(adj_vnf);
    }
}


function _vnf_find_edge_faces(vnf,edge) =
  let(
      faces = vnf[1],
      goodind = [for(i=idx(faces))
                    let(result=search(edge,faces[i]))
                    if (result*0==[0,0] && 
                          (abs(result[0]-result[1])==1
                           || (min(result)==0 && max(result)==len(faces[i])-1)))
                       i
                ]
  )
  goodind;


function _vnf_find_corner_faces(vnf,corner) =
  let(
      faces = vnf[1]
  )
  [for(i=idx(faces))
     let(result=search([corner],faces[i])[0])
     if (result!=[])
        i];



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