Improve accelerated supersampling 2x antialiasing

Avoid fractional pixel offsets by forcing integer division of m_screenSize.
Shift rounded lookAtPoint onto OpenGL pixel center to ensure consistent rounding to int.
The pixel 0..1 @ 1x needs to fill the area 0..2 @ 2x so its center moves by 0.5 * (pixel size @ 1x).

common/gal/graphics_abstraction_layer.cpp

@@ -189,7 +189,8 @@ void GAL::ComputeWorldScreenMatrix()
 
     MATRIX3x3D translation;
     translation.SetIdentity();
-    translation.SetTranslation( 0.5 * VECTOR2D( m_screenSize ) );
+    // We're deliberately dividing integers to avoid fractional pixel offsets.
+    translation.SetTranslation( VECTOR2D( m_screenSize.x/2, m_screenSize.y/2 ) );
 
     MATRIX3x3D rotate;
     rotate.SetIdentity();

common/gal/opengl/gl_builtin_shaders.cpp

@@ -83,6 +83,7 @@ uniform float worldPixelSize;
 uniform vec2 screenPixelSize;
 uniform float pixelSizeMultiplier;
 uniform float minLinePixelWidth;
+uniform vec2 antialiasingOffset;
 
 
 float roundr( float f, float r )
@@ -239,6 +240,7 @@ void main()
 
     }
 
+    gl_Position.xy += antialiasingOffset;
 }
 
 )SHADER_SOURCE";

common/gal/opengl/opengl_compositor.cpp

@@ -439,3 +439,13 @@ int OPENGL_COMPOSITOR::GetAntialiasSupersamplingFactor() const
     default: return 1;
     }
 }
+
+VECTOR2D OPENGL_COMPOSITOR::GetAntialiasRenderingOffset() const
+{
+    switch( m_currentAntialiasingMode )
+    {
+    case OPENGL_ANTIALIASING_MODE::SUPERSAMPLING_X2: return VECTOR2D( 0.5, -0.5 );
+    case OPENGL_ANTIALIASING_MODE::SUPERSAMPLING_X4: return VECTOR2D( 0.25, -0.25 );
+    default: return VECTOR2D( 0, 0 );
+    }
+}

common/gal/opengl/opengl_gal.cpp

@@ -281,6 +281,7 @@ OPENGL_GAL::OPENGL_GAL( GAL_DISPLAY_OPTIONS& aDisplayOptions, wxWindow* aParent,
     ufm_worldPixelSize = 1;
     ufm_screenPixelSize = 1;
     ufm_pixelSizeMultiplier = 1;
+    ufm_antialiasingOffset = 1;
 }
 
 
@@ -528,6 +529,7 @@ void OPENGL_GAL::beginDrawing()
         ufm_worldPixelSize = m_shader->AddParameter( "worldPixelSize" );
         ufm_screenPixelSize = m_shader->AddParameter( "screenPixelSize" );
         ufm_pixelSizeMultiplier = m_shader->AddParameter( "pixelSizeMultiplier" );
+        ufm_antialiasingOffset = m_shader->AddParameter( "antialiasingOffset" );
 
         m_shader->Use();
         m_shader->SetParameter( ufm_fontTexture, (int) FONT_TEXTURE_UNIT );
@@ -541,9 +543,14 @@ void OPENGL_GAL::beginDrawing()
     m_shader->Use();
     m_shader->SetParameter( ufm_worldPixelSize,
                             (float) ( getWorldPixelSize() / GetScaleFactor() ) );
-    m_shader->SetParameter( ufm_screenPixelSize, getScreenPixelSize() );
+    const VECTOR2D& screenPixelSize = getScreenPixelSize();
+    m_shader->SetParameter( ufm_screenPixelSize, screenPixelSize );
     double pixelSizeMultiplier = m_compositor->GetAntialiasSupersamplingFactor();
     m_shader->SetParameter( ufm_pixelSizeMultiplier, (float) pixelSizeMultiplier );
+    VECTOR2D renderingOffset = m_compositor->GetAntialiasRenderingOffset();
+    renderingOffset.x *= screenPixelSize.x;
+    renderingOffset.y *= screenPixelSize.y;
+    m_shader->SetParameter( ufm_antialiasingOffset, renderingOffset );
     m_shader->Deactivate();
 
     // Something betreen BeginDrawing and EndDrawing seems to depend on
@@ -2321,18 +2328,20 @@ void OPENGL_GAL::EnableDepthTest( bool aEnabled )
 }
 
 
-static double roundr( double f, double r )
+inline double round_to_half_pixel( double f, double r )
 {
-    return floor( f / r + 0.5 ) * r;
+    return ( ceil( f / r ) - 0.5 ) * r;
 }
 
-
 void OPENGL_GAL::ComputeWorldScreenMatrix()
 {
+    computeWorldScale();
     auto pixelSize = m_worldScale;
 
-    m_lookAtPoint.x = roundr( m_lookAtPoint.x, pixelSize );
-    m_lookAtPoint.y = roundr( m_lookAtPoint.y, pixelSize );
+    // we need -m_lookAtPoint == -k * pixelSize + 0.5 * pixelSize for OpenGL
+    // meaning m_lookAtPoint = (k-0.5)*pixelSize with integer k
+    m_lookAtPoint.x = round_to_half_pixel( m_lookAtPoint.x, pixelSize );
+    m_lookAtPoint.y = round_to_half_pixel( m_lookAtPoint.y, pixelSize );
 
     GAL::ComputeWorldScreenMatrix();
 }

include/gal/opengl/opengl_compositor.h

@@ -94,6 +94,7 @@ public:
     OPENGL_ANTIALIASING_MODE GetAntialiasingMode() const;
 
     int GetAntialiasSupersamplingFactor() const;
+    VECTOR2D GetAntialiasRenderingOffset() const;
 
 protected:
     /// Binds a specific Framebuffer Object.

include/gal/opengl/opengl_gal.h

@@ -336,6 +336,7 @@ private:
     GLint                   ufm_worldPixelSize;
     GLint                   ufm_screenPixelSize;
     GLint                   ufm_pixelSizeMultiplier;
+    GLint                   ufm_antialiasingOffset;
 
     wxCursor                m_currentwxCursor;          ///< wxCursor showing the current native cursor