GPU: 24-bit display depth support

src/core/gpu_hw.cpp

@@ -378,6 +378,68 @@ void main()
   return ss.str();
 }
 
+std::string GPU_HW::GenerateRGB24DecodeFragmentShader()
+{
+  std::stringstream ss;
+  GenerateShaderHeader(ss);
+
+  ss << R"(
+in vec2 v_tex0;
+out vec4 o_col0;
+
+uniform sampler2D samp0;
+uniform ivec2 u_base_coords;
+
+void main()
+{
+  // compute offset in dwords from the start of the 24-bit values
+  ivec2 base = ivec2(u_base_coords.x, u_base_coords.y + int(gl_FragCoord.y));
+  int xoff = int(gl_FragCoord.x);
+  int dword_index = (xoff / 2) + (xoff / 4);
+
+  // sample two adjacent dwords, or four 16-bit values as the 24-bit value will lie somewhere between these
+  uint s0 = RGBA8ToRGBA5551(texelFetch(samp0, ivec2(base.x + dword_index * 2 + 0, base.y), 0));
+  uint s1 = RGBA8ToRGBA5551(texelFetch(samp0, ivec2(base.x + dword_index * 2 + 1, base.y), 0));
+  uint s2 = RGBA8ToRGBA5551(texelFetch(samp0, ivec2(base.x + (dword_index + 1) * 2 + 0, base.y), 0));
+  uint s3 = RGBA8ToRGBA5551(texelFetch(samp0, ivec2(base.x + (dword_index + 1) * 2 + 1, base.y), 0));
+
+  // select the bit for this pixel depending on its offset in the 4-pixel block
+  uint r, g, b;
+  switch (xoff & 3)
+  {
+    case 0:
+      r = s0 & 0xFFu;
+      g = s0 >> 8;
+      b = s1 & 0xFFu;
+      break;
+
+    case 1:
+      r = s1 >> 8;
+      g = s2 & 0xFFu;
+      b = s2 >> 8;
+      break;
+
+    case 2:
+      r = s1 & 0xFFu;
+      g = s1 >> 8;
+      b = s2 & 0xFFu;
+      break;
+
+    case 3:
+      r = s2 >> 8;
+      g = s3 & 0xFFu;
+      b = s3 >> 8;
+      break;
+  }
+
+  // and normalize
+  o_col0 = vec4(float(r) / 255.0, float(g) / 255.0, float(b) / 255, 1.0);
+}
+)";
+
+  return ss.str();
+}
+
 GPU_HW::HWRenderBatch::Primitive GPU_HW::GetPrimitiveForCommand(RenderCommand rc)
 {
   if (rc.primitive == Primitive::Line)