tiles.go

package genworldvoronoi

import (
	"fmt"
	"image"
	"image/color"
	"log"
	"math"
	"sort"

	"github.com/Flokey82/genbiome"
	"github.com/Flokey82/genworldvoronoi/bio"
	"github.com/Flokey82/genworldvoronoi/geo"
	"github.com/Flokey82/genworldvoronoi/various"
	"github.com/Flokey82/geoquad"
	"github.com/Flokey82/go_gens/gameconstants"
	"github.com/Flokey82/go_gens/vectors"
	"github.com/davvo/mercator"
	"github.com/llgcode/draw2d/draw2dimg"
	"github.com/mazznoer/colorgrad"

	geojson "github.com/paulmach/go.geojson"
)

// Display modes for tile rendering.
const (
	DisplayModeDefault          = 0
	DisplayModeOceanPressure    = 1
	DisplayModeMoisture         = 2
	DisplayModeRainfall         = 3
	DisplayModeFlux             = 4
	DisplayModeCompression      = 5
	DisplayModeEarthquake       = 6
	DisplayModeVolcano          = 7
	DisplayModeRockSlide        = 8
	DisplayModeFlood            = 9
	DisplayModeErosion          = 10
	DisplayModeErosion2         = 11
	DisplayModeSteepness        = 12
	DisplayModeSlope            = 13
	DisplayModeCityStates       = 14
	DisplayModeEmpires          = 15
	DisplayModeCultures         = 16
	DisplayModeReligions        = 17
	DisplayModeSpecies          = 18
	DisplayModePlates           = 19
	DisplayModeElevation        = 20
	DisplayModeAirTemperature   = 21
	DisplayModeOceanTemperature = 22
	DisplayModeInsolation       = 23
	DisplayModePopulation       = 24
	DisplayModeSuitability      = 25
	DisplayModeSoilExhaustion   = 26
	DisplayModeTribes           = 27
)

// GetTile returns the image of the tile at the given coordinates and zoom level.
func (m *Map) GetTile(x, y, zoom, displayMode, vectorMode int, drawRivers, drawTradeRoutes, drawLakes, drawShadows, aspectShading bool) image.Image {
	// NOTE:
	//
	// For now we don't use the LOD logic until we have figured out how to handle triangles.
	// While regions simply have n*step as their ID, triangles aren't stable as they are
	// generated from the voronoi diagram from scratch. This means their order changes...
	// Which also means we can't easily re-use the pre-computed triangle values from the
	// primary mesh.
	//
	// mesh, step := m.getCoarseForZoom(zoom)
	mesh := m.SphereMesh
	step := 1

	var colorFunc func(int, float64) color.Color
	switch displayMode {
	case DisplayModeCityStates, DisplayModeEmpires, DisplayModeCultures, DisplayModeReligions, DisplayModeSpecies, DisplayModePlates:
		colorGrad := colorgrad.Rainbow()
		terrToColor := make(map[int]int)
		var territory []int
		var terrLen int
		if displayMode == DisplayModeCityStates {
			terr := m.CityStates
			terrLen = len(terr)
			for i, c := range terr {
				terrToColor[c.ID] = i
			}
			territory = m.RegionToCityState
		} else if displayMode == DisplayModeEmpires {
			terr := m.Empires
			terrLen = len(terr)
			for i, c := range terr {
				terrToColor[c.ID] = i
			}
			territory = m.RegionToEmpire
		} else if displayMode == DisplayModeCultures {
			terr := m.Cultures
			terrLen = len(terr)
			for i, c := range terr {
				terrToColor[c.ID] = i
			}
			territory = m.RegionToCulture
		} else if displayMode == DisplayModeReligions {
			terr := m.Religions
			terrLen = len(terr)
			for i, c := range terr {
				terrToColor[c.ID] = i
			}
			territory = m.RegionToReligion
		} else if displayMode == DisplayModeSpecies {
			terr := m.Species
			terrLen = len(terr)
			for i, c := range terr {
				terrToColor[c.Origin] = i
			}
			territory = m.SpeciesRegions
		} else {
			terr := m.PlateRegs
			terrLen = len(terr)
			for i, c := range terr {
				terrToColor[c] = i
			}
			territory = m.RegionToPlate
		}

		min, max := minMax(m.Elevation)
		_, maxMois := minMax(m.Moisture)
		cols := colorGrad.Colors(uint(terrLen) + 1)
		colorFunc = func(i int, n float64) color.Color {
			// Calculate the color of the region.
			elev := m.Elevation[i]
			val := (elev - min) / (max - min)

			// If we have a territory, return the color of the territory.
			if territory[i] != -1 {
				terrID := terrToColor[territory[i]]
				return genColor(cols[terrID], math.Pow(val, 1/n))
			}

			// Return blue for water (oceans and lakes).
			if elev <= 0 || (m.Waterpool[i] > 0 && drawLakes) {
				return genBlue(val)
			}

			// Return the biome color for land.
			rLat := m.LatLon[i][0]
			valElev := elev / max
			valMois := m.Moisture[i] / maxMois
			return geo.GetWhittakerModBiomeColor(rLat, valElev, valMois, math.Pow(val, 1/n))
		}
	case DisplayModeElevation, DisplayModeAirTemperature, DisplayModeOceanTemperature: // Temperatures and elevation.
		// Create a blue to red color gradient.
		colorGrad := colorgrad.NewGradient()
		colorGrad.Colors(
			color.RGBA{0, 0, 255, 255},
			color.RGBA{0, 255, 255, 255},
			color.RGBA{0, 255, 0, 255},
			color.RGBA{255, 255, 0, 255},
			color.RGBA{255, 0, 0, 255},
		)
		cb, err := colorGrad.Build()
		if err != nil {
			log.Fatal(err)
		}

		if displayMode == DisplayModeElevation { // Elevation.
			_, max := minMax(m.Elevation)

			// Create the color function.
			colorFunc = func(i int, n float64) color.Color {
				// Calculate the color of the region.
				val := m.Elevation[i] / max
				return genColor(cb.At(val), math.Pow(val, 1/n))
			}
		} else if displayMode == DisplayModeAirTemperature { // Air temperature.
			temp := m.AirTemperature
			minTemp, maxTemp := minMax(temp)

			// Create the color function.
			colorFunc = func(i int, n float64) color.Color {
				// Calculate the color of the region.
				val := (temp[i] - minTemp) / (maxTemp - minTemp)
				return genColor(cb.At(val), math.Pow(val, 1/n))
			}
		} else if displayMode == DisplayModeOceanTemperature { // Ocean temperature.
			temp := m.OceanTemperature
			minTemp, maxTemp := minMax(temp)

			// Create the color function.
			colorFunc = func(i int, n float64) color.Color {
				// Calculate the color of the region.
				val := (temp[i] - minTemp) / (maxTemp - minTemp)
				return genColor(cb.At(val), math.Pow(val, 1/n))
			}
		}
	default:
		vals := m.Elevation
		if displayMode == DisplayModeOceanPressure {
			vals = m.CalcCurrentPressure(m.RegionToOceanVec)
		} else if displayMode == DisplayModeMoisture {
			vals = m.Moisture
		} else if displayMode == DisplayModeRainfall {
			vals = m.Rainfall
		} else if displayMode == DisplayModeFlux {
			vals = m.Flux
		} else if displayMode == DisplayModeCompression {
			vals = m.PropagateCompression(m.RegionCompression)
		} else if displayMode == DisplayModeEarthquake {
			vals = m.GetEarthquakeChance()
		} else if displayMode == DisplayModeVolcano {
			vals = m.GetVolcanoEruptionChance()
		} else if displayMode == DisplayModeRockSlide {
			vals = m.GetRockSlideAvalancheChance()
		} else if displayMode == DisplayModeFlood {
			vals = m.GetFloodChance()
		} else if displayMode == DisplayModeErosion {
			vals = m.GetErosionRate()
		} else if displayMode == DisplayModeErosion2 {
			vals = m.GetErosionRate2()
		} else if displayMode == DisplayModeSteepness {
			vals = m.GetSteepness()
		} else if displayMode == DisplayModeSlope {
			vals = m.GetSlope()
		} else if displayMode == DisplayModeInsolation {
			vals = m.Geo.AvgInsolation
		} else if displayMode == DisplayModePopulation { // Population.
			vals = make([]float64, m.NumRegions)
			for i, pop := range m.Population {
				if pop <= 0 {
					vals[i] = 0
				} else {
					vals[i] = float64(pop)
				}
			}
		} else if displayMode == DisplayModeSuitability {
			vals = make([]float64, m.NumRegions)
			for i, sus := range m.Suitability {
				vals[i] = sus
			}
		} else if displayMode == DisplayModeSoilExhaustion {
			vals = make([]float64, m.NumRegions)
			for i, sus := range m.SoilExhaustion {
				vals[i] = sus
			}
		} else if displayMode == DisplayModeTribes { // Tribes.
			vals = make([]float64, m.NumRegions)
			for _, tribe := range m.Tribes {
				vals[tribe.RegionID] = float64(tribe.Population)
				if tribe.Path != nil {
					for _, p := range tribe.Path.Steps {
						vals[p] = float64(tribe.Population) * 2
					}
				}
			}
		}

		// Calculate the min and max elevation.
		_, max := minMax(m.Elevation)
		_, maxMois := minMax(m.Moisture)
		minVal, maxVal := minMax(vals)
		colorFunc = func(i int, n float64) color.Color {
			// Calculate the color of the region.
			elev := m.Elevation[i]
			val := (vals[i] - minVal) / (maxVal - minVal)

			// Return blue for water (oceans and lakes).
			if elev <= 0 || (m.Waterpool[i] > 0 && drawLakes) {
				return genBlue(val)
			}

			// Return the biome color for land.
			rLat := m.LatLon[i][0]
			valElev := elev / max
			valMois := m.Moisture[i] / maxMois
			return geo.GetWhittakerModBiomeColor(rLat, valElev, valMois, math.Pow(val, 1/n))
		}
	}

	// Wrap the tile coordinates.
	x, y = wrapTileCoordinates(x, y, zoom)

	// Calculate an approximation of the distance between regions.
	distRegion := math.Sqrt(4 * math.Pi / float64(m.NumRegions))

	// Convert into degrees.
	distRegionDeg := distRegion * 180 / math.Pi

	// Calculate the bounds of the tile.
	tbb := newTileBoundingBox(x, y, zoom)
	la1, lo1, la2, lo2 := tbb.toLatLon()
	latLonMargin := math.Max(20/float64(zoom), distRegionDeg*10)

	// Calculate the bounds with margin for the tile.
	la1Margin := la1 - latLonMargin //math.Max(-90, math.Min(90, la1-latLonMargin))
	la2Margin := la2 + latLonMargin //math.Max(-90, math.Min(90, la2+latLonMargin))
	lo1Margin := lo1 - latLonMargin
	lo2Margin := lo2 + latLonMargin

	// Check if we are within the tile with a small margin, taking
	// into account that we might have wrapped around the world.
	isLatLonInBounds := func(lat, lon float64) bool {
		if lat < la1Margin || lat > la2Margin || lon < lo1Margin || lon > lo2Margin {
			// Check if the tile and the region we are looking at is adjecent to +/- 180 degrees and
			// NOTE: This could be improved by checking if one of the corners of the region is within the tile.
			if lo1 > -175 && lo2 < 175 || lon < 175 && lon > -175 {
				return false
			}
		}
		return true
	}

	// Since our mercator conversion gives us absolute pixel coordinates, we need to
	// remove the offset of the tile we are rendering from the path coordinates.
	dx, _ := latLonToPixels(la1, lo1, zoom)
	_, dy2 := latLonToPixels(la2, lo2, zoom)

	// Create a new image to draw the tile on.
	dest := image.NewRGBA(image.Rect(0, 0, tileSize, tileSize))
	gc := draw2dimg.NewGraphicContext(dest)

	// Get all regions that are within the tile bounds.
	var inQuadTreeRegs []int
	qds := mesh.RegQuadTree.FindPointsInRect(geoquad.Rect{
		MinLat: la1Margin,
		MaxLat: la2Margin,
		MinLon: lo1Margin,
		MaxLon: lo2Margin,
	})
	for _, qd := range qds {
		inQuadTreeRegs = append(inQuadTreeRegs, qd.Data.(int))
	}

	// Draw the regions.
	out_t := make([]int, 0, 6)
	gc.SetLineWidth(1)

	for _, i := range inQuadTreeRegs {
		// rLat := m.LatLon[i][0]
		rLon := mesh.LatLon[i][1]
		// Draw the path that outlines the region.
		var path [][2]float64
		for _, j := range mesh.R_circulate_t(out_t, i) {
			tLat := mesh.TriLatLon[j][0]
			tLon := mesh.TriLatLon[j][1]

			// Check if we have wrapped around the world.
			if tLon-rLon > 120 {
				tLon -= 360
			} else if tLon-rLon < -120 {
				tLon += 360
			}

			// Calculate the coordinates of the path point.
			x, y := latLonToPixels(tLat, tLon, zoom)
			path = append(path, [2]float64{(x - dx), (y - dy2)})
		}

		// Now check if the region we are looking at has wrapped around the world /
		// +- 180 degrees. If so, we need to adjust the points in the path.
		if lo1 < -175 && rLon > 175 {
			for i := range path {
				path[i][0] -= float64(sizeFromZoom(zoom))
			}
		} else if lo2 > 175 && rLon < -175 {
			for i := range path {
				path[i][0] += float64(sizeFromZoom(zoom))
			}
		}

		// Calculate the color of the region.
		col := colorFunc(i*step, 1.0)

		// If the path is empty, we can skip it.
		if len(path) == 0 {
			continue
		}

		// Draw the path.
		gc.SetStrokeColor(col)
		gc.SetFillColor(col)
		gc.BeginPath()
		for i, p := range path {
			if i == 0 {
				gc.MoveTo(p[0], p[1])
			} else {
				gc.LineTo(p[0], p[1])
			}
		}
		gc.Close()
		gc.FillStroke()
	}

	if drawShadows {
		// Get all triangles that are within the tile bounds.
		var inQuadTreeTris []int
		qds = m.TriQuadTree.FindPointsInRect(geoquad.Rect{
			MinLat: la1Margin,
			MaxLat: la2Margin,
			MinLon: lo1Margin,
			MaxLon: lo2Margin,
		})
		for _, qd := range qds {
			inQuadTreeTris = append(inQuadTreeTris, qd.Data.(int))
		}

		// Set the global light direction (upper left when looking at the map)
		lightDir := vectors.Vec3{X: -1.0, Y: 1.0, Z: 1.0}.Normalize()

		// Set our initial line width.
		gc.SetLineWidth(1)

	Loop:
		for _, i := range inQuadTreeTris {
			// Hacky way to filter paths/triangles that wrap around the entire SVG.
			triLat := m.TriLatLon[i][0]
			triLon := m.TriLatLon[i][1]

			// Check if we are within the tile with a small margin, taking
			// into account that we might have wrapped around the world.
			if !isLatLonInBounds(triLat, triLon) {
				continue
			}

			// Draw the path that outlines the region.
			var path [][2]float64
			for _, j := range m.T_circulate_r(out_t, i) {
				rLat := m.LatLon[j][0]
				rLon := m.LatLon[j][1]

				// Check if we the region is across the +/- 180 degrees longitude line
				// compared to the triangle.
				// In this case, the longitude is almost 360 degrees off, which means
				// we need to adjust the longitude.
				if rLon-triLon > 110 {
					rLon -= 360
				} else if rLon-triLon < -110 {
					rLon += 360
				}

				// Calculate the coordinates of the path point.
				x, y := latLonToPixels(rLat, rLon, zoom)
				p := [2]float64{(x - dx), (y - dy2)}

				// Check if we are way outside the tile.
				if p[0] < -1000 || p[0] > 1000 || p[1] < -1000 || p[1] > 1000 {
					continue Loop
				}
				path = append(path, p)
			}

			// Now check if the region we are looking at has wrapped around the world /
			// +- 180 degrees. If so, we need to adjust the points in the path.
			if lo1 < -175 && triLon > 175 {
				for i := range path {
					path[i][0] -= float64(sizeFromZoom(zoom))
				}
			} else if lo2 > 175 && triLon < -175 {
				for i := range path {
					path[i][0] += float64(sizeFromZoom(zoom))
				}
			}

			// Get the 3 regions of the triangle.
			regions := m.T_circulate_r(out_t, i)

			// Get the normal of the triangle.
			normal := m.RegTriNormal(i, regions)

			// Now take the dot product of the slope and our global light
			// direction to get the amount of light on the triangle.
			diffuseShading := math.Max(0, vectors.Dot3(normal, lightDir))

			// Brightness is the amount of light on the triangle.
			var brightness float64

			// If we have aspect shading enabled, calculate the aspect shading
			// and mix it with the diffuse shading.
			if aspectShading {
				// Calculate the slope angle from the normal vector as
				// a value between 0 and 1.
				slope := math.Abs(math.Acos(normal.Y) / (math.Pi / 2))

				// Get the azimuth of the light source vector3.
				azimuth := math.Atan2(lightDir.Z, lightDir.X)

				// Get the aspect of the triangle based on the normal vector.
				aspect := math.Atan2(normal.Z, normal.X)

				// Now calculate the aspect based shading.
				aspectShading := math.Max(0, math.Cos(azimuth-aspect))

				// Calculate the brightness of the triangle and mix
				// brightness and shade based on the slope angle.
				brightness = math.Min(1, math.Max(0, diffuseShading*(1-slope)+aspectShading*slope))
			} else {
				// Calculate the brightness of the triangle.
				brightness = diffuseShading
			}

			// We have already the coordinates of all 3 regions, so we can just use them.

			// We need to calculate the midpoint for each side of the triangle if one or two
			// of the regions are below the sea level.
			// We do this by calculating the intersection of the triangle segment
			// with the sea level plane and then calculate the new midpoint for the triangle.
			var midSides [3][2]float64

			// Calculate the midpoints of each side of the triangle,
			// given a possible intercept with the sea level plane.
			// We need to do this for each side of the triangle where
			// one of the regions is below the sea level.
			var regsBelowSeaLevel [3]bool
			var countBelowSeaLevel int
			for j := 0; j < 3; j++ {
				// Get the 2 regions of the triangle segment.
				r1 := regions[j]
				r2 := regions[(j+1)%3]
				// Get the 2 points of the triangle segment.
				x1, y1, z1 := path[j][0], path[j][1], m.Elevation[r1]
				x2, y2, z2 := path[(j+1)%3][0], path[(j+1)%3][1], m.Elevation[r2]

				if z1 <= 0 {
					regsBelowSeaLevel[j] = true
					countBelowSeaLevel++
				}

				// Check if we have a triangle segment that crosses the sea level plane.
				if (z1 <= 0) != (z2 <= 0) {
					// Calculate the intersection of the triangle segment with the sea level plane
					// (when z becomes zero).
					// We do this by calculating the ratio of the distance from the first point
					// to the intersection point to the distance between the first and second point.
					// We then use this ratio to calculate the coordinates of the intersection point.
					ratio := math.Abs(z1) / (math.Abs(z1) + math.Abs(z2))
					x := x1 + (x2-x1)*ratio
					y := y1 + (y2-y1)*ratio

					// Set the midpoint of the triangle segment to the intersection point.
					midSides[j][0] = x
					midSides[j][1] = y
				} else {
					// If the triangle segment does not cross the sea level plane,
					// we can just use the midpoint of the triangle segment.
					midSides[j][0] = (x1 + x2) / 2
					midSides[j][1] = (y1 + y2) / 2
				}
			}

			// Calculate the new midpoint of the triangle, which needs to be
			// at sea level if one or two of the regions are below the sea level.
			// - If one or two regions ar below the sea level, the midpoint of the triangle should
			// be set at the midpoint between the two midpoints of the triangle segments that
			// cross the sea level plane.
			// - If all regions are above the sea level, the midpoint of the triangle should
			// be set at the midpoint of the triangle.
			var midTriangle [2]float64

			// If only one region is below the sea level, the midpoint of the triangle should
			// be set at the midpoint between the two midpoints of the triangle segments that
			// cross the sea level plane.
			if countBelowSeaLevel == 1 {
				for j := 0; j < 3; j++ {
					if regsBelowSeaLevel[j] {
						midTriangle[0] = (midSides[(j+0)%3][0] + midSides[(j+2)%3][0]) / 2
						midTriangle[1] = (midSides[(j+0)%3][1] + midSides[(j+2)%3][1]) / 2
						break
					}
				}
			} else if countBelowSeaLevel == 2 {
				// If two regions are below the sea level, the midpoint of the triangle should
				// be set at the midpoint between the two midpoints of the triangle segments that
				// cross the sea level plane.
				for j := 0; j < 3; j++ {
					// If the current region is above the sea level, we can use the midpoint
					// of the triangle segments, which are the current segment and the previous segment.
					if !regsBelowSeaLevel[j] {
						midTriangle[0] = (midSides[(j+0)%3][0] + midSides[(j+2)%3][0]) / 2
						midTriangle[1] = (midSides[(j+0)%3][1] + midSides[(j+2)%3][1]) / 2
						break
					}
				}
			} else {
				// If all regions are above the sea level, the midpoint of the triangle should
				// be set at the midpoint of the triangle.
				midTriangle[0] = (path[0][0] + path[1][0] + path[2][0]) / 3
				midTriangle[1] = (path[0][1] + path[1][1] + path[2][1]) / 3
			}

			// Now we can draw the triangle using the adjusted midpoints.
			for j := 0; j < 3; j++ {
				// Set the color of the triangle shard.
				col := colorFunc(regions[j], brightness)
				gc.SetStrokeColor(col)
				gc.SetFillColor(col)

				// Get the 4 points of the triangle shard
				// One region is the original point of the triangle,
				// the other two are the midpoints of the triangle segment,
				// and the midpoint of the triangle.
				x1, y1 := path[j][0], path[j][1]
				x2, y2 := midSides[(j+0)%3][0], midSides[(j+0)%3][1]
				x3, y3 := midTriangle[0], midTriangle[1]
				x4, y4 := midSides[(j+2)%3][0], midSides[(j+2)%3][1]

				// Draw the triangle shard.
				gc.BeginPath()
				// First, we move to the first point of the triangle shard,
				gc.MoveTo(x1, y1)
				// then we draw a line to the midpoint of the first line segment,
				gc.LineTo(x2, y2)
				// then we draw a line to the midpoint of the triangle,
				gc.LineTo(x3, y3)
				// then we draw a line to the midpoint of the third line segment,
				gc.LineTo(x4, y4)
				// and finally we close the path.
				gc.Close()
				gc.FillStroke()
			}
		}
	}

	// Draw all the wind vectors on top.
	if vectorMode > 0 {
		vects := m.RegionToWindVec
		if vectorMode == 2 {
			vects = m.RegionToWindVecLocal
		} else if vectorMode == 3 {
			vects = m.RegionToOceanVec
		}
		// Set the color and line width of the wind vectors.
		gc.SetStrokeColor(color.NRGBA{0, 0, 0, 255})
		gc.SetLineWidth(1)
		for _, i := range inQuadTreeRegs {
			rLat := m.LatLon[i][0]
			rLon := m.LatLon[i][1]

			// Now draw the wind vector for the region.
			// NOTE: I'm not 100% sure if this is correct, but it seems to work.
			wLat, wLon := various.VectorToLatLong(various.Normalize2(vects[i]))
			windVec := various.Normalize2([2]float64{wLat, wLon})

			// Calculate the coordinates of the center of the region.
			x, y := latLonToPixels(rLat, rLon, zoom)
			x -= dx
			y -= dy2

			// Calculate the length of the wind vector.
			length := various.Len2(windVec)

			// Calculate the angle of the wind vector.
			angle := math.Atan2(windVec[1], windVec[0])

			// Calculate the coordinates of the end of the wind vector.
			// Since we are on a computer screen, we need to flip the y-axis.
			x2 := x + math.Cos(angle)*length*50
			y2 := y - math.Sin(angle)*length*50

			// Draw the wind vector.
			gc.BeginPath()
			gc.MoveTo(x, y)
			gc.LineTo(x2, y2)
			gc.Stroke()

			// Draw the arrow head.
			gc.BeginPath()
			gc.MoveTo(x2, y2)
			gc.LineTo(x2-math.Cos(angle+math.Pi/6)*5, y2+math.Sin(angle+math.Pi/6)*5)
			gc.Stroke()

			gc.BeginPath()
			gc.MoveTo(x2, y2)
			gc.LineTo(x2-math.Cos(angle-math.Pi/6)*5, y2+math.Sin(angle-math.Pi/6)*5)
			gc.Stroke()
		}
	}

	// Now we do something completely inefficient and
	// fetch all the rivers and filter them by the tile.
	// We should filter this stuff before we generate the rivers.
	if drawRivers {
		rivers := m.GetRiversInLatLonBB(0.001/float64(int(1)<<zoom), la1Margin, lo1Margin, la2Margin, lo2Margin)
		_, maxFlux := minMax(m.Flux)

		// Set our stroke color to a nice river blue.
		gc.SetStrokeColor(color.NRGBA{0, 0, 255, 255})

		for _, river := range rivers {
			// Set the initial line width.
			gc.SetLineWidth(1)
			gc.BeginPath()

			// Move to the first point.
			rLat, rLon := m.LatLon[river[0]][0], m.LatLon[river[0]][1]
			x, y := latLonToPixels(rLat, rLon, zoom)
			gc.MoveTo(x-dx, y-dy2)
			for i, p := range river[1:] {
				// Set the line width based on the flux of the river, averaged with the previous flux.
				gc.SetLineWidth(4 * math.Sqrt((m.Flux[p]+m.Flux[river[i]])/(2*maxFlux)))

				// Set the line width based on the flux of the river.
				rLat, rLon = m.LatLon[p][0], m.LatLon[p][1]

				// Now compare the longitude to the previous longitude.
				// If we have crossed the +- 180 degree boundary, we need to
				// draw to a fake point at the same latitude but on the same side of the world.
				if diff := rLon - m.LatLon[river[i]][1]; math.Abs(diff) > 110 {
					rLonFake := rLon - 360
					if diff < 0 {
						rLonFake = rLon + 360
					}
					// Draw to the fake point.
					x, y := latLonToPixels(rLat, rLonFake, zoom)
					gc.LineTo(x-dx, y-dy2)
					gc.Stroke()

					// Move to the real point and start a new path.
					x, y = latLonToPixels(rLat, rLon, zoom)
					gc.BeginPath()
					gc.MoveTo(x-dx, y-dy2)
				}

				x, y := latLonToPixels(rLat, rLon, zoom)
				x -= dx
				y -= dy2

				// If both points are in a pool or below sea level, we end the path,
				// move to the new point and start a new path.
				// TODO: Calculate intercept of the river with the sea level.
				if (m.Elevation[p] <= 0 || m.Waterpool[p] > 0 && drawLakes) &&
					(m.Elevation[river[i]] <= 0 || m.Waterpool[river[i]] > 0 && drawLakes) {
					// Draw from the last position to the midpoint.
					// This will cause the river to end at the sea level.
					gc.Stroke()
					gc.Close()

					// Move to the new point and start a new path.
					gc.BeginPath()
					gc.MoveTo(x, y)
				} else if m.Elevation[p] <= 0 || (m.Waterpool[p] > 0 && drawLakes) {
					// If we are below sea level, interpolate the point with the previous point.
					// Draw from the last position to the midpoint.
					// This will cause the river to end at the sea level.
					lx, ly := gc.LastPoint()
					gc.LineTo((x+lx)/2, (y+ly)/2)

					// Move to the new point.
					gc.MoveTo(x, y)
				} else if m.Elevation[river[i]] <= 0 || (m.Waterpool[river[i]] > 0 && drawLakes) {
					// If the previous point was below sea level, interpolate the point with the next point.
					// This will cause the river to start at the sea level.
					lx, ly := gc.LastPoint()
					gc.MoveTo((x+lx)/2, (y+ly)/2)

					// Draw to the new point.
					gc.LineTo(x, y)
				} else {
					gc.LineTo(x, y)
				}
				// TODO: Use steepness to determine the amplitude of meandering.
				// The less steep the river is, the more it meanders.
				// lx, ly := gc.LastPoint()
				// gc.CubicCurveTo((x+2*lx)/3, ly, lx, (ly+2*y)/3, x, y)
			}
			gc.Stroke()
			gc.Close()
		}
	}

	if drawTradeRoutes {
		// Get all the trade routes.
		traderoutes := m.getTradeRoutesInLatLonBB(la1Margin, lo1Margin, la2Margin, lo2Margin)

		// Set our stroke color to a nice traderoute red.
		gc.SetStrokeColor(color.NRGBA{255, 0, 0, 255})

		for _, traderoute := range traderoutes {
			// Set the initial line width.
			gc.SetLineWidth(1)
			gc.BeginPath()
			for i, p := range traderoute {
				rLat, rLon := m.LatLon[p][0], m.LatLon[p][1]
				x, y := latLonToPixels(rLat, rLon, zoom)
				if i == 0 {
					// Move to the first point.
					gc.MoveTo(x-dx, y-dy2)
				} else {
					// Now compare the longitude to the previous longitude.
					// If we have crossed the +- 180 degree boundary, we need to
					// draw to a fake point at the same latitude but on the same side of the world.
					if diff := rLon - m.LatLon[traderoute[i-1]][1]; math.Abs(diff) > 110 {
						rLonFake := rLon - 360
						if diff < 0 {
							rLonFake = rLon + 360
						}
						// Draw to the fake point.
						x, y := latLonToPixels(rLat, rLonFake, zoom)
						gc.LineTo(x-dx, y-dy2)
						gc.Stroke()

						// Move to the real point and start a new path.
						x, y = latLonToPixels(rLat, rLon, zoom)
						gc.BeginPath()
						gc.MoveTo(x-dx, y-dy2)
					}
					// Draw to the next point.
					gc.LineTo(x-dx, y-dy2)
				}
			}
			gc.Stroke()
			gc.Close()
		}
	}

	return dest
}

func wrapTileCoordinates(x, y, zoom int) (int, int) {
	// Wrap the tile coordinates.
	x = x % (1 << uint(zoom))
	if x < 0 {
		x += 1 << uint(zoom)
	}
	y = y % (1 << uint(zoom))
	if y < 0 {
		y += 1 << uint(zoom)
	}
	return x, y
}

func wrapLatLon(la, lo float64) (float64, float64) {
	// Wrap the lat lon coordinates.
	la = math.Mod(la, 180)
	if la < -90 {
		la += 180
	} else if la > 90 {
		la -= 180
	}
	lo = math.Mod(lo, 360)
	if lo < -180 {
		lo += 360
	} else if lo > 180 {
		lo -= 360
	}
	return la, lo
}

func limitLatLon(la, lo float64) (float64, float64) {
	// Limit the lat lon coordinates.
	if la < -90 {
		la = -90
	} else if la > 90 {
		la = 90
	}
	if lo < -180 {
		lo = -180
	} else if lo > 180 {
		lo = 180
	}
	return la, lo
}

type latLonBounds struct {
	la1, lo1, la2, lo2 float64
}

// InBounds checks if the given lat lon coordinates are within the bounds.
// NOTE: We need to take in account that the bounds might have wrapped around the world.
func (b latLonBounds) InBounds(la, lo float64) bool {
	if b.la1 < b.la2 {
		// We wrapped around north or south.
		if la > b.la1 && la < b.la2 {
			return false
		}
	} else {
		if la > b.la1 || la < b.la2 {
			return false
		}
	}
	if b.lo1 > b.lo2 {
		// We wrapped around east or west.
		if lo < b.lo1 && lo > b.lo2 {
			return false
		}
	} else {
		if lo < b.lo1 || lo > b.lo2 {
			return false
		}
	}
	return true
}

func wrapLatitude(la float64) float64 {
	// Wrap the latitude.
	la = math.Mod(la, 180)
	if la < -90 {
		la += 180
	} else if la > 90 {
		la -= 180
	}
	return la
}

func wrapLongitude(lo float64) float64 {
	// Wrap the longitude.
	lo = math.Mod(lo, 360)
	if lo < -180 {
		lo += 360
	} else if lo > 180 {
		lo -= 360
	}
	return lo
}

func limitLatitude(la float64) float64 {
	// Limit the latitude.
	if la < -90 {
		la = -90
	} else if la > 90 {
		la = 90
	}
	return la
}

func limitLongitude(lo float64) float64 {
	// Limit the longitude.
	if lo < -180 {
		lo = -180
	} else if lo > 180 {
		lo = 180
	}
	return lo
}

// GetCitiesInTile returns all cities within the given tile coordinates and zoom level.
func (m *Map) GetCitiesInTile(x, y, zoom int) []*City {
	// Wrap the tile coordinates.
	x, y = wrapTileCoordinates(x, y, zoom)

	// Calculate the bounds of the tile.
	tbb := newTileBoundingBox(x, y, zoom)
	la1, lo1, la2, lo2 := tbb.toLatLon()

	// Wrap the lat lon coordinates.
	la1, lo1 = wrapLatLon(la1, lo1)
	la2, lo2 = wrapLatLon(la2, lo2)

	// Get the cities within the tile.
	var cities []*City
	for _, c := range m.Cities {
		cLatLon := m.LatLon[c.ID]
		// Check if we are within the tile with a small margin.
		if la1 < cLatLon[0] && cLatLon[0] < la2 && lo1 < cLatLon[1] && cLatLon[1] < lo2 {
			continue
		}
		cities = append(cities, c)
	}
	return cities
}

// GetGeoJSONCities returns all cities as GeoJSON within the given bounds and zoom level.
func (m *Map) GetGeoJSONCities(la1, lo1, la2, lo2 float64, zoom int) ([]byte, error) {
	geoJSON := geojson.NewFeatureCollection()
	// Wrap the latitude only if we see less than 180 degrees, otherwise just limit it.
	if math.Abs(la1-la2) < 180 {
		la1 = wrapLatitude(la1)
		la2 = wrapLatitude(la2)
	} else {
		la1 = limitLatitude(la1)
		la2 = limitLatitude(la2)
	}
	// Wrap the longitude only if we see less than 360 degrees.
	if math.Abs(lo1-lo2) < 360 {
		lo1 = wrapLongitude(lo1)
		lo2 = wrapLongitude(lo2)
	} else {
		lo1 = limitLongitude(lo1)
		lo2 = limitLongitude(lo2)
	}
	lbb := latLonBounds{la1, lo1, la2, lo2}

	// Get the last settled year.
	_, maxSettled := minMax64(m.Settled)
	distRegion := math.Sqrt(4 * math.Pi / float64(m.NumRegions))

	biomeFunc := m.GetRegWhittakerModBiomeFunc()
	_, maxElev := minMax(m.Elevation)
	_, maxMois := minMax(m.Moisture)

	regPropertyFunc := m.GetRegPropertyFunc()

	// Depending on the zoom level we want to show more or less cities.
	sortedCities := make([]*City, len(m.Cities))
	copy(sortedCities, m.Cities)

	// Sort the cities by population (descending).
	sort.Slice(sortedCities, func(i, j int) bool {
		return sortedCities[i].Population > sortedCities[j].Population
	})

	showNumCities := 0

	// At zoom level 9 we want to show all cities.
	if zoom >= 9 {
		showNumCities = len(sortedCities)
	} else {
		// Show per tile and zoom level:
		showNumCities = 10 * (1 << uint(zoom))
	}

	log.Println("Showing", showNumCities, "cities at zoom level", zoom)

	// Loop through all the cities and check if they are within the tile.
	// TODO: Just show the largest cities for lower zoom levels.
	for idx, c := range sortedCities {
		if idx >= showNumCities {
			break
		}

		cLat := m.LatLon[c.ID][0]
		cLon := m.LatLon[c.ID][1]

		// Check if we are within the tile with a small margin.
		if !lbb.InBounds(cLat, cLon) {
			continue
		}

		// Add the city to the GeoJSON as a feature.
		f := geojson.NewPointFeature([]float64{cLon, cLat})
		f.SetProperty("id", c.ID)
		f.SetProperty("name", c.Name)
		f.SetProperty("type", c.Type)
		f.SetProperty("culture", fmt.Sprintf("%s (%s)", c.Culture.Name, c.Culture.Type))
		if r := m.GetReligion(c.ID); r != nil {
			f.SetProperty("religion", r.Name)
			f.SetProperty("deity", r.Deity.FullName())
		}
		if e := m.GetEmpire(c.ID); e != nil {
			f.SetProperty("empire", e.Name)
		}
		if cs := m.GetCityState(c.ID); cs != nil {
			f.SetProperty("citystate", cs.Capital.Name)
		}
		f.SetProperty("population", c.Population)
		f.SetProperty("popgrowth", c.PopulationGrowthRate())
		f.SetProperty("maxpop", c.MaxPopulation)
		f.SetProperty("maxpoplimit", c.MaxPopulationLimit())
		f.SetProperty("settled", maxSettled-c.Founded)
		temperature := m.GetRegTemperature(c.ID, maxElev)
		precip := geo.MaxPrecipitation * m.Moisture[c.ID] / maxMois
		elev := geo.MaxAltitudeFactor * m.Elevation[c.ID] / maxElev
		f.SetProperty("biome", genbiome.WhittakerModBiomeToString(biomeFunc(c.ID))+
			fmt.Sprintf(" (%.1f°C, %.1fdm, %.1fm)", temperature, precip, elev))
		f.SetProperty("coordinates", fmt.Sprintf("lat %.2f, lon %.2f", cLat, cLon))
		f.SetProperty("attractiveness", c.Attractiveness)
		f.SetProperty("economic", c.PotentialEconomic)
		f.SetProperty("agriculture", c.PotentialAgricultural)
		f.SetProperty("trade", c.PotentialTrade)
		f.SetProperty("resources", c.PotentialResources)
		f.SetProperty("radius", (c.radius()+2*distRegion)*gameconstants.EarthCircumference/(2*math.Pi))
		f.SetProperty("tradepartners", c.TradePartners)
		f.SetProperty("flavortext", m.generateCityFlavorText(c, regPropertyFunc(c.ID)))
		var sName string
		if m.SpeciesRegions[c.ID] >= 0 {
			var s *bio.Species
			for _, sp := range m.Species {
				if sp.Origin == m.SpeciesRegions[c.ID] {
					s = sp
					break
				}
			}
			if s != nil {
				sName = s.Name
				if sName == "" {
					sName = s.String()
				}
			}
		}
		f.SetProperty("species", sName)
		var msgs []string
		hist := m.History.GetEvents(c.ID, ObjectTypeCity)

		// Only show the last 10 events.
		numEvents := len(hist)
		if numEvents > 10 {
			numEvents = 10
		}
		for _, event := range hist[len(hist)-numEvents:] {
			msgs = append(msgs, fmt.Sprintf("%d: %s", event.Year, event.String()))
		}
		f.SetProperty("history", msgs)

		// Generate the list of local resources.
		var resources []string
		// Metals.
		for i := 0; i < geo.ResMaxMetals; i++ {
			if m.Metals[c.ID]&(1<<i) != 0 {
				resources = append(resources, geo.MetalToString(i))
			}
		}
		// Gems.
		for i := 0; i < geo.ResMaxGems; i++ {
			if m.Gems[c.ID]&(1<<i) != 0 {
				resources = append(resources, geo.GemToString(i))
			}
		}
		// Stones.
		for i := 0; i < geo.ResMaxStones; i++ {
			if m.Stones[c.ID]&(1<<i) != 0 {
				resources = append(resources, geo.StoneToString(i))
			}
		}
		// Woods.
		for i := 0; i < geo.ResMaxWoods; i++ {
			if m.Wood[c.ID]&(1<<i) != 0 {
				resources = append(resources, geo.WoodToString(i))
			}
		}
		// Various.
		for i := 0; i < geo.ResMaxVarious; i++ {
			if m.Various[c.ID]&(1<<i) != 0 {
				resources = append(resources, geo.VariousToString(i))
			}
		}
		f.SetProperty("reslist", resources)

		geoJSON.AddFeature(f)
	}

	log.Printf("%d out of %d cities in tile", len(geoJSON.Features), len(m.Cities))

	// Now encode the GeoJSON.
	geoJSONBytes, err := geoJSON.MarshalJSON()
	if err != nil {
		return nil, err
	}
	return geoJSONBytes, nil
}

// GetGeoJSONBorders returns all borders as GeoJSON within the given bounds and zoom level.
func (m *Map) GetGeoJSONBorders(la1, lo1, la2, lo2 float64, zoom, displayMode int) ([]byte, error) {
	geoJSON := geojson.NewFeatureCollection()
	var borders [][]int
	switch displayMode {
	case 1:
		borders = m.getCustomBorders(m.RegionToCityState)
	case 2:
		borders = m.getCustomBorders(m.RegionToCulture)
	case 3:
		borders = m.getCustomBorders(m.RegionToPlate)
	case 4:
		borders = m.getCustomBorders(m.BiomeRegions)
	case 5:
		// Nothing.
	default:
		borders = m.getCustomBorders(m.RegionToEmpire)
	}

	// Get all borders and add them to the GeoJSON.
	// Right now we ignore the bounds and zoom level.
	for i, border := range borders {
		// Now get the coordinates for each point of the border.
		var borderLatLons [][]float64
		for _, p := range border {
			// Get the lat lon coordinates of the point.
			la := m.TriLatLon[p][0]
			lo := m.TriLatLon[p][1]

			// Check if we have crossed the 180 degree longitude line.
			// If so, we stop here, add the border to the GeoJSON and start a new one.
			if len(borderLatLons) > 0 && math.Abs(borderLatLons[len(borderLatLons)-1][0]-lo) > 180 {
				// Add the border to the GeoJSON as a feature.
				f := geojson.NewLineStringFeature(borderLatLons)
				f.ID = i
				geoJSON.AddFeature(f)

				// Start a new border.
				borderLatLons = nil
			}
			borderLatLons = append(borderLatLons, []float64{lo, la})
		}
		// Add the border to the GeoJSON as a feature.
		f := geojson.NewLineStringFeature(borderLatLons)
		f.ID = i
		geoJSON.AddFeature(f)
	}
	// Now encode the GeoJSON.
	geoJSONBytes, err := geoJSON.MarshalJSON()
	if err != nil {
		return nil, err
	}
	return geoJSONBytes, nil
}

const tileSize = 256

// sizeFromZoom returns the expected size of the world for the mercato projection used below.
func sizeFromZoom(zoom int) int {
	return int(math.Pow(2.0, float64(zoom)) * float64(tileSize))
}

func latLonToPixels(lat, lon float64, zoom int) (float64, float64) {
	return mercator.LatLonToPixels(-lat, lon, zoom)
}

// tileBoundingBox represents a bounding box in pixels for a tile.
type tileBoundingBox struct {
	x1, y1 float64
	x2, y2 float64
	zoom   int
	*merc
}

// toLatLon returns the lat lon coordinates of the north-west and
// south-east corners of the bounding box.
func (t *tileBoundingBox) toLatLon() (lat1, lon1, lat2, lon2 float64) {
	lat1, lon1 = t.PixelsToLatLon(t.x1, t.y1, t.zoom)
	lat2, lon2 = t.PixelsToLatLon(t.x2, t.y2, t.zoom)
	return
}

// newTileBoundingBox returns a new tile bounding box for the given tile coordinates
// and zoom level.
func newTileBoundingBox(tx, ty, zoom int) tileBoundingBox {
	return tileBoundingBox{
		x1:   float64(tx * 256),
		y1:   float64(ty * 256),
		x2:   float64((tx + 1) * 256),
		y2:   float64((ty + 1) * 256),
		zoom: zoom,
		merc: merc256,
	}
}

func newTileBoundingBox32(tx, ty, zoom int) tileBoundingBox {
	return tileBoundingBox{
		x1:   float64(tx * 32),
		y1:   float64(ty * 32),
		x2:   float64((tx + 1) * 32),
		y2:   float64((ty + 1) * 32),
		zoom: zoom,
		merc: merc32,
	}
}

var merc32 = newMerc(32)
var merc256 = newMerc(256)

type merc struct {
	tileSize          float64
	initialResolution float64
	originShift       float64
}

func newMerc(tileSize float64) *merc {
	return &merc{
		tileSize:          tileSize,
		initialResolution: 2 * math.Pi * 6378137 / tileSize,
		originShift:       2 * math.Pi * 6378137 / 2,
	}
}

// Resolution calculates the resolution (meters/pixel) for given zoom level (measured at Equator)
func (m *merc) Resolution(zoom int) float64 {
	return m.initialResolution / math.Pow(2, float64(zoom))
}

// PixelsToMeters converts pixel coordinates in given zoom level of pyramid to EPSG:900913
func (m *merc) PixelsToMeters(px, py float64, zoom int) (float64, float64) {
	res := m.Resolution(zoom)
	x := px*res - m.originShift
	y := py*res - m.originShift
	return x, y
}

// PixelsToLatLon converts pixel coordinates in given zoom level to lat/lon in WGS84 Datum
func (m *merc) PixelsToLatLon(px, py float64, zoom int) (float64, float64) {
	x, y := m.PixelsToMeters(px, py, zoom)
	return m.MetersToLatLon(x, y)
}

// MetersToLatLon converts XY point from Spherical Mercator EPSG:900913 to lat/lon in WGS84 Datum
func (m *merc) MetersToLatLon(x, y float64) (float64, float64) {
	lon := (x / m.originShift) * 180
	lat := (y / m.originShift) * 180
	lat = 180 / math.Pi * (2*math.Atan(math.Exp(lat*math.Pi/180)) - math.Pi/2)
	return lat, lon
}