implement overlay transparency for FillRect

data/shaders/fillrect.frag

@@ -1,13 +1,23 @@
 #version 330
 
-uniform ivec4 uClip;
-uniform int   uFlags;
-uniform vec4  uColour[2];
+uniform ivec2       uScreenSize;
+uniform ivec4       uClip;
+uniform int         uFlags;
+uniform vec4        uColour[2];
+uniform sampler2D   uSourceFramebuffer;
+uniform ivec4       uBounds;
 
 in vec2 fPosition;
 
 layout (location = 0) out vec4 oColour;
 
+float getluma(vec3 colour)
+{
+    return (colour.r * 0.2126) +
+           (colour.g * 0.7152) +
+           (colour.b * 0.0722);
+}
+
 void main()
 {
     if (fPosition.x < uClip.x || fPosition.x > uClip.z ||
@@ -16,13 +26,36 @@ void main()
         discard;
     }
 
+    vec4 targetColour;
     int posSum = int(fPosition.x) + int(fPosition.y);
     if ((posSum % 2) == 0)
     {
-        oColour = uColour[0];
+        targetColour = uColour[0];
     }
     else
     {
-        oColour = uColour[1];
+        targetColour = uColour[1];
+    }
+
+    if ((uFlags & 1) != 0)
+    {
+        vec2 textureCoordinates = (fPosition / vec2(uScreenSize)) * vec2(1, -1);
+        vec4 sourceColour = texture(uSourceFramebuffer, textureCoordinates);
+
+        float luma = getluma(sourceColour.rgb);
+        sourceColour = vec4(vec3(luma), 1);
+
+        if (luma < 0.5)
+        {
+            oColour = 2.0 * sourceColour * targetColour;
+        }
+        else
+        {
+            oColour = 1.0 - 2.0 * (1.0 - sourceColour) * (1.0 - targetColour);
+        }
+    }
+    else
+    {
+        oColour = targetColour;
     }
 }

src/drawing/engines/opengl/FillRectShader.cpp

@@ -17,6 +17,7 @@
 #ifndef DISABLE_OPENGL
 
 #include "FillRectShader.h"
+#include "OpenGLFramebuffer.h"
 
 FillRectShader::FillRectShader() : OpenGLShaderProgram("fillrect")
 {
@@ -32,6 +33,9 @@ FillRectShader::FillRectShader() : OpenGLShaderProgram("fillrect")
     glBindVertexArray(_vao);
     glEnableVertexAttribArray(vIndex);
     glVertexAttribIPointer(vIndex, 1, GL_INT, 0, 0);
+
+    Use();
+    SetFlags(0);
 }
 
 FillRectShader::~FillRectShader()
@@ -79,6 +83,13 @@ void FillRectShader::SetColour(int index, vec4f colour)
     glUniform4f(uColour[index], colour.r, colour.g, colour.b, colour.a);
 }
 
+void FillRectShader::SetSourceFramebuffer(GLuint texture)
+{
+    glActiveTexture(GL_TEXTURE0);
+    glBindTexture(GL_TEXTURE_2D, texture);
+    glUniform1i(uSourceFramebuffer, 0);
+}
+
 void FillRectShader::Draw(sint32 left, sint32 top, sint32 right, sint32 bottom)
 {
     SetBounds(left, top, right, bottom);

src/drawing/engines/opengl/FillRectShader.h

@@ -19,6 +19,8 @@
 #include "GLSLTypes.h"
 #include "OpenGLShaderProgram.h"
 
+class OpenGLFramebuffer;
+
 class FillRectShader : public OpenGLShaderProgram
 {
 private:
@@ -27,6 +29,7 @@ private:
     GLuint uBounds;
     GLuint uFlags;
     GLuint uColour[2];
+    GLuint uSourceFramebuffer;
 
     GLuint vIndex;
 
@@ -42,6 +45,7 @@ public:
     void SetBounds(sint32 left, sint32 top, sint32 right, sint32 bottom);
     void SetFlags(uint32 flags);
     void SetColour(int index, vec4f colour);
+    void SetSourceFramebuffer(GLuint texture);
 
     void Draw(sint32 left, sint32 top, sint32 right, sint32 bottom);
 

src/drawing/engines/opengl/OpenGLDrawingEngine.cpp

@@ -281,7 +281,7 @@ public:
         for (int i = 0; i < 256; i++)
         {
             SDL_Color colour = palette[i];
-            colour.a = 255;
+            colour.a = i == 0 ? 0 : 255;
 
             Palette[i] = colour;
             GLPalette[i] = { colour.r / 255.0f,
@@ -371,6 +371,12 @@ public:
         return &_bitsDPI;
     }
 
+    GLuint SwapCopyReturningSourceTexture()
+    {
+        _swapFramebuffer->SwapCopy();
+        return _swapFramebuffer->GetSourceTexture();
+    }
+
 private:
     void ConfigureBits(uint32 width, uint32 height, uint32 pitch)
     {
@@ -486,6 +492,9 @@ void OpenGLDrawingContext::FillRect(uint32 colour, sint32 left, sint32 top, sint
     if (colour & 0x1000000)
     {
         paletteColour[1].a = 0;
+
+        _fillRectShader->Use();
+        _fillRectShader->SetFlags(0);
     }
     else if (colour & 0x2000000)
     {
@@ -498,11 +507,20 @@ void OpenGLDrawingContext::FillRect(uint32 colour, sint32 left, sint32 top, sint
         paletteColour[0].r = transformColour.r;
         paletteColour[0].g = transformColour.g;
         paletteColour[0].b = transformColour.b;
-        paletteColour[0].a = 0.5f;
+        paletteColour[0].a = 1;
         paletteColour[1] = paletteColour[0];
+
+        GLuint srcTexture =  _engine->SwapCopyReturningSourceTexture();
+        _fillRectShader->Use();
+        _fillRectShader->SetFlags(1);
+        _fillRectShader->SetSourceFramebuffer(srcTexture);
+    }
+    else
+    {
+        _fillRectShader->Use();
+        _fillRectShader->SetFlags(0);
     }
 
-    _fillRectShader->Use();
     _fillRectShader->SetScreenSize(gScreenWidth, gScreenHeight);
     _fillRectShader->SetColour(0, paletteColour[0]);
     _fillRectShader->SetColour(1, paletteColour[1]);
@@ -726,7 +744,7 @@ void * OpenGLDrawingContext::GetImageAsARGB(uint32 image, uint32 tertiaryColour,
             *dst++ = colour.r;
             *dst++ = colour.g;
             *dst++ = colour.b;
-            *dst++ = 255;
+            *dst++ = colour.a;
         }
     }