gpu: sw: Discard pixels based on the texture alpha test

fleroviux · Dec 10, 2023 · d683d22 · d683d22
1 parent be1133f
commit d683d22
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Showing 4 changed files with 21 additions and 5 deletions.
diff --git a/src/dual/include/dual/nds/video_unit/gpu/gpu.hpp b/src/dual/include/dual/nds/video_unit/gpu/gpu.hpp
@@ -68,14 +68,20 @@ namespace dual::nds {
         m_cmd_processor.Write_GXSTAT(value, mask);
       }
 
+      void Write_ALPHA_TEST_REF(u32 value, u32 mask) {
+        const u32 write_mask = 0x1Fu & mask;
+
+        m_io.alpha_test_ref = (value & write_mask) | (m_io.alpha_test_ref & ~write_mask);
+      }
+
       void Write_CLEAR_COLOR(u32 value, u32 mask) {
         const u32 write_mask = 0x3F1FFFFFu & mask;
 
         m_io.clear_color.word = (value & write_mask) | (m_io.clear_color.word & ~write_mask);
       }
 
-      void Write_CLEAR_DEPTH(u16 value, u16 mask) {
-        const u16 write_mask = 0x7FFFu & mask;
+      void Write_CLEAR_DEPTH(u32 value, u32 mask) {
+        const u32 write_mask = 0x7FFFu & mask;
 
         m_io.clear_depth = (value & write_mask) | (m_io.clear_depth & ~write_mask);
       }

diff --git a/src/dual/include/dual/nds/video_unit/gpu/registers.hpp b/src/dual/include/dual/nds/video_unit/gpu/registers.hpp
@@ -59,8 +59,9 @@ namespace dual::nds::gpu {
   struct IO {
     DISP3DCNT disp3dcnt;
     GXSTAT gxstat;
+    u32 alpha_test_ref{};
     CLEAR_COLOR clear_color;
-    u16 clear_depth{};
+    u32 clear_depth{};
   };
 
 } // namespace dual::nds::gpu
diff --git a/src/dual/src/nds/arm9/io.cpp b/src/dual/src/nds/arm9/io.cpp
@@ -377,8 +377,9 @@ namespace dual::nds::arm9 {
       case REG(0x04000304): hw.video_unit.Write_POWCNT((u16)value, (u16)mask); break;
 
       // GPU3D
+      case REG(0x04000340): if(gpu.GetRenderEnginePowerOn()) gpu.Write_ALPHA_TEST_REF(value, mask); break;
       case REG(0x04000350): if(gpu.GetRenderEnginePowerOn()) gpu.Write_CLEAR_COLOR(value, mask); break;
-      case REG(0x04000354): if(gpu.GetRenderEnginePowerOn()) gpu.Write_CLEAR_DEPTH((u16)value, (u16)mask); break;
+      case REG(0x04000354): if(gpu.GetRenderEnginePowerOn()) gpu.Write_CLEAR_DEPTH(value, mask); break;
       case REG(0x04000400): // GXFIFO (mirrored from 04000400 to 0400043C)
       case REG(0x04000404):
       case REG(0x04000408):

diff --git a/src/dual/src/nds/video_unit/gpu/renderer/software/rasterizer.cpp b/src/dual/src/nds/video_unit/gpu/renderer/software/rasterizer.cpp
@@ -212,6 +212,12 @@ namespace dual::nds::gpu {
     const int x_min = line.x[0];
     const int x_max = line.x[1];
 
+    int alpha_test_threshold = 0u;
+
+    if(m_io.disp3dcnt.enable_alpha_test) {
+      alpha_test_threshold = m_io.alpha_test_ref << 1 | m_io.alpha_test_ref >> 4;
+    }
+
     for(int x = x0; x <= x1; x++) {
       line_interp.Setup(line.w_16[0], line.w_16[1], x, x_min, x_max);
 
@@ -237,7 +243,9 @@ namespace dual::nds::gpu {
       if(m_io.disp3dcnt.enable_texture_mapping && polygon.texture_params.format != TextureParams::Format::Disabled) {
         const Color4 texel = SampleTexture(polygon.texture_params, polygon.palette_base, uv);
 
-        // @todo: alpha test
+        if(texel.A().Raw() <= alpha_test_threshold) {
+          continue;
+        }
 
         switch((Polygon::Mode)polygon.attributes.polygon_mode) {
           case Polygon::Mode::Modulation: {