GPU: Implement dithering on software backend

src/core/gpu.h

@@ -243,6 +243,15 @@ protected:
              (static_cast<u16>(c_) << 15);
     }
 
+    void SetRGB24Dithered(u32 x, u32 y, u8 r8, u8 g8, u8 b8, bool c_ = false)
+    {
+      const s32 offset = DITHER_MATRIX[y & 3][x & 3];
+      r8 = static_cast<u8>(std::clamp<s32>(static_cast<s32>(ZeroExtend32(r8)) + offset, 0, 255));
+      g8 = static_cast<u8>(std::clamp<s32>(static_cast<s32>(ZeroExtend32(g8)) + offset, 0, 255));
+      b8 = static_cast<u8>(std::clamp<s32>(static_cast<s32>(ZeroExtend32(b8)) + offset, 0, 255));
+      SetRGB24(r8, g8, b8, c_);
+    }
+
     u32 ToRGB24() const
     {
       const u32 r_ = ZeroExtend32(r.GetValue());

src/core/gpu_sw.cpp

@@ -357,7 +357,8 @@ void GPU_SW::DrawTriangle(RenderCommand rc, const SWVertex* v0, const SWVertex*
         const u8 texcoord_x = Interpolate(v0->texcoord_x, v1->texcoord_x, v2->texcoord_x, b0, b1, b2, ws);
         const u8 texcoord_y = Interpolate(v0->texcoord_y, v1->texcoord_y, v2->texcoord_y, b0, b1, b2, ws);
 
-        ShadePixel(rc, static_cast<u32>(x), static_cast<u32>(y), r, g, b, texcoord_x, texcoord_y);
+        ShadePixel(rc, static_cast<u32>(x), static_cast<u32>(y), r, g, b, texcoord_x, texcoord_y,
+                   rc.IsDitheringEnabled() && m_GPUSTAT.dither_enable);
       }
 
       row_w0 += a12;
@@ -395,13 +396,13 @@ void GPU_SW::DrawRectangle(RenderCommand rc, s32 origin_x, s32 origin_y, u32 wid
 
       const u8 texcoord_x = Truncate8(ZeroExtend32(origin_texcoord_x) + offset_x);
 
-      ShadePixel(rc, static_cast<u32>(x), static_cast<u32>(y), r, g, b, texcoord_x, texcoord_y);
+      ShadePixel(rc, static_cast<u32>(x), static_cast<u32>(y), r, g, b, texcoord_x, texcoord_y, false);
     }
   }
 }
 
 void GPU_SW::ShadePixel(RenderCommand rc, u32 x, u32 y, u8 color_r, u8 color_g, u8 color_b, u8 texcoord_x,
-                        u8 texcoord_y)
+                        u8 texcoord_y, bool dithering)
 {
   VRAMPixel color;
   bool transparent = true;
@@ -461,15 +462,21 @@ void GPU_SW::ShadePixel(RenderCommand rc, u32 x, u32 y, u8 color_r, u8 color_g,
     }
     else
     {
-      color.SetRGB24(
-        Truncate8(std::min<u16>((ZeroExtend16(texture_color.GetR8()) * ZeroExtend16(color_r)) >> 7, 0xFF)),
-        Truncate8(std::min<u16>((ZeroExtend16(texture_color.GetG8()) * ZeroExtend16(color_g)) >> 7, 0xFF)),
-        Truncate8(std::min<u16>((ZeroExtend16(texture_color.GetB8()) * ZeroExtend16(color_b)) >> 7, 0xFF)));
+      const u8 r = Truncate8(std::min<u16>((ZeroExtend16(texture_color.GetR8()) * ZeroExtend16(color_r)) >> 7, 0xFF));
+      const u8 g = Truncate8(std::min<u16>((ZeroExtend16(texture_color.GetG8()) * ZeroExtend16(color_g)) >> 7, 0xFF));
+      const u8 b = Truncate8(std::min<u16>((ZeroExtend16(texture_color.GetB8()) * ZeroExtend16(color_b)) >> 7, 0xFF));
+      if (dithering)
+        color.SetRGB24Dithered(x, y, r, g, b);
+      else
+        color.SetRGB24(r, g, b);
     }
   }
   else
   {
-    color.SetRGB24(color_r, color_g, color_b);
+    if (dithering)
+      color.SetRGB24Dithered(x, y, color_r, color_g, color_b);
+    else
+      color.SetRGB24(color_r, color_g, color_b);
   }
 
   if (rc.transparency_enable && transparent)

src/core/gpu_sw.h

@@ -58,7 +58,7 @@ protected:
 
   static bool IsClockwiseWinding(const SWVertex* v0, const SWVertex* v1, const SWVertex* v2);
 
-  void ShadePixel(RenderCommand rc, u32 x, u32 y, u8 color_r, u8 color_g, u8 color_b, u8 texcoord_x, u8 texcoord_y);
+  void ShadePixel(RenderCommand rc, u32 x, u32 y, u8 color_r, u8 color_g, u8 color_b, u8 texcoord_x, u8 texcoord_y, bool dithering);
   void DrawTriangle(RenderCommand rc, const SWVertex* v0, const SWVertex* v1, const SWVertex* v2);
   void DrawRectangle(RenderCommand rc, s32 origin_x, s32 origin_y, u32 width, u32 height, u8 r, u8 g, u8 b,
                      u8 origin_texcoord_x, u8 origin_texcoord_y);