Treat alpha as 0 if alpha is 1 for blending

This removes the white box in fortune street again, without causing Mario Kart Wii to regress.
dolphin-emu · Jan 28, 2022 · 2194fa9 · 2194fa9
1 parent 2d67fe2
commit 2194fa9
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diff --git a/Source/Core/VideoBackends/Software/Tev.cpp b/Source/Core/VideoBackends/Software/Tev.cpp
@@ -714,6 +714,19 @@ void Tev::Draw()
   if (!TevAlphaTest(output[ALP_C]))
     return;
 
+  // Hardware testing indicates that an alpha of 1 can pass an alpha test,
+  // but doesn't do anything in blending
+  // This situation is important for Mario Kart Wii's menus (they will render incorrectly if the
+  // alpha test for the FMV in the background fails, since they depend on depth for drawing a yellow
+  // border) and Fortune Street's gameplay (where a rectangle with an alpha value of 1 is drawn over
+  // the center of the screen several times, but those rectangles shouldn't be visible).
+  // Blending seems to result in no changes to the output with an alpha of 1, even if the input
+  // color is white.
+  // TODO: Investigate this further: we might be handling blending incorrectly in general (though
+  // there might not be any good way of changing blending behavior)
+  if (output[ALP_C] == 1)
+    output[ALP_C] = 0;
+
   // z texture
   if (bpmem.ztex2.op != ZTexOp::Disabled)
   {

diff --git a/Source/Core/VideoCommon/PixelShaderGen.cpp b/Source/Core/VideoCommon/PixelShaderGen.cpp
@@ -1234,6 +1234,18 @@ ShaderCode GeneratePixelShaderCode(APIType api_type, const ShaderHostConfig& hos
                    use_dual_source || use_shader_blend);
   }
 
+  // This situation is important for Mario Kart Wii's menus (they will render incorrectly if the
+  // alpha test for the FMV in the background fails, since they depend on depth for drawing a yellow
+  // border) and Fortune Street's gameplay (where a rectangle with an alpha value of 1 is drawn over
+  // the center of the screen several times, but those rectangles shouldn't be visible).
+  // Blending seems to result in no changes to the output with an alpha of 1, even if the input
+  // color is white.
+  // TODO: Investigate this further: we might be handling blending incorrectly in general (though
+  // there might not be any good way of changing blending behavior)
+  out.Write("\t// Hardware testing indicates that an alpha of 1 can pass an alpha test,\n"
+            "\t// but doesn't do anything in blending\n"
+            "\tif (prev.a == 1) prev.a = 0;\n");
+
   if (uid_data->zfreeze)
   {
     out.SetConstantsUsed(C_ZSLOPE, C_ZSLOPE);

diff --git a/Source/Core/VideoCommon/UberShaderPixel.cpp b/Source/Core/VideoCommon/UberShaderPixel.cpp
@@ -1032,6 +1032,9 @@ ShaderCode GenPixelShader(APIType api_type, const ShaderHostConfig& host_config,
             "  }}\n"
             "\n");
 
+  out.Write("  // Hardware testing indicates that an alpha of 1 can pass an alpha test,\n"
+            "  // but doesn't do anything in blending\n"
+            "  if (TevResult.a == 1) TevResult.a = 0;\n");
   // =========
   // Dithering
   // =========