when drawing a transformed bitmap in opengl backend, scale it better

The plain scaling is rather rough, and in fact drawing a scaled bitmap
the normal way gives much better results (because OutputDevice pre-scales
the bitmap before it's drawn). This one may be a bit slow perhaps,
but hopefully nobody there's no code that'd extensively use bitmap
drawing with custom transformations (wishful thinking).

Change-Id: I83e05307adfaeac0ed0757f1a0b2603f64caf8f8

vcl/opengl/areaScaleFastFragmentShader.glsl

@@ -18,6 +18,12 @@ uniform float ratio; // = 1.0/(xscale*yscale)
 
 varying vec2 tex_coord;
 
+// This mode makes the scaling work like maskedTextureFragmentShader.glsl
+// (instead of like plain textureVertexShader.glsl).
+#ifdef MASKED
+uniform sampler2D mask;
+#endif
+
 /*
  Just make the resulting color the average of all the source pixels
  (which is an area (xscale)x(yscale) ).
@@ -30,7 +36,14 @@ void main(void)
     {
         for( int x = 0; x < xscale; ++x )
         {
+#ifndef MASKED
             sum += texture2D( sampler, tex_coord.st + offset );
+#else
+            vec4 texel;
+            texel = texture2D( sampler, tex_coord.st + offset );
+            texel.a = 1.0 - texture2D( mask, tex_coord.st + offset ).r;
+            sum += texel;
+#endif
             offset.x += xstep;
         }
         offset.y += ystep;

vcl/opengl/areaScaleFragmentShader.glsl

@@ -27,6 +27,12 @@ uniform float ydestconvert;
 
 varying vec2 tex_coord;
 
+// This mode makes the scaling work like maskedTextureFragmentShader.glsl
+// (instead of like plain textureVertexShader.glsl).
+#ifdef MASKED
+uniform sampler2D mask;
+#endif
+
 void main(void)
 {
     // Convert to pixel coordinates again.
@@ -126,7 +132,14 @@ void main(void)
         for( int x = xstart; x <= xend; ++x, ++xpos )
         {
             vec2 offset = vec2( x * xsrcconvert, y * ysrcconvert );
+#ifndef MASKED
             tmp += texture2D( sampler, offset ) * xratio[ xpos ];
+#else
+            vec4 texel;
+            texel = texture2D( sampler, offset );
+            texel.a = 1.0 - texture2D( mask, offset ).r;
+            tmp += texel * xratio[ xpos ];
+#endif
         }
         sum += tmp * yratio[ ypos ];
     }

vcl/opengl/gdiimpl.cxx

@@ -894,9 +894,34 @@ void OpenGLSalGraphicsImpl::DrawTransformedTexture(
         (float) rTexture.GetWidth(), 0, (float) rTexture.GetWidth(), (float) rTexture.GetHeight() };
     GLfloat aTexCoord[8];
 
+    // If downscaling at a higher scale ratio, use the area scaling algorithm rather
+    // than plain OpenGL's scaling, for better results.
+    // See OpenGLSalBitmap::ImplScaleArea().
+    double ixscale = rTexture.GetWidth() / fabs( rX.getX() - rNull.getX());
+    double iyscale = rTexture.GetHeight() / fabs( rY.getY() - rNull.getY());
+    bool areaScaling = false;
+    bool fastAreaScaling = false;
+    OUString textureFragmentShader;
+    if( ixscale >= 2 && iyscale >= 2 ) // Downscaling to 50% or less? (inverted scale ratios)
+    {
+        areaScaling = true;
+        fastAreaScaling = ( ixscale == int( ixscale ) && iyscale == int( iyscale ));
+        // The generic case has arrays only up to 100 ratio downscaling, which is hopefully enough
+        // in practice, but protect against buffer overflows in case such an extreme case happens
+        // (and in such case the precision of the generic algorithm probably doesn't matter anyway).
+        if( ixscale > 100 || iyscale > 100 )
+            fastAreaScaling = true;
+        if( fastAreaScaling )
+            textureFragmentShader = "areaScaleFastFragmentShader";
+        else
+            textureFragmentShader = "areaScaleFragmentShader";
+    }
+
     if( rMask )
     {
-        if( !UseProgram( "transformedTextureVertexShader", "maskedTextureFragmentShader" ) )
+        if( !UseProgram( "transformedTextureVertexShader",
+                textureFragmentShader.isEmpty() ? "maskedTextureFragmentShader" : textureFragmentShader,
+                "#define MASKED" ) )
             return;
         mpProgram->SetTexture( "mask", rMask );
         rMask.SetFilter( GL_LINEAR );
@@ -904,10 +929,40 @@ void OpenGLSalGraphicsImpl::DrawTransformedTexture(
     }
     else
     {
-        if( !UseProgram( "transformedTextureVertexShader", "textureFragmentShader" ) )
+        if( !UseProgram( "transformedTextureVertexShader",
+                textureFragmentShader.isEmpty() ? "textureFragmentShader" : textureFragmentShader ) )
             return;
     }
 
+    if( areaScaling )
+    {
+        // From OpenGLSalBitmap::ImplScaleArea().
+        if( fastAreaScaling )
+        {
+            int mnWidth = rTexture.GetWidth();
+            int mnHeight = rTexture.GetHeight();
+            mpProgram->SetUniform1i( "xscale", ixscale );
+            mpProgram->SetUniform1i( "yscale", iyscale );
+            mpProgram->SetUniform1f( "xstep", 1.0 / mnWidth );
+            mpProgram->SetUniform1f( "ystep", 1.0 / mnHeight );
+            mpProgram->SetUniform1f( "ratio", 1.0 / ( ixscale * iyscale ));
+        }
+        else
+        {
+            int mnWidth = rTexture.GetWidth();
+            int mnHeight = rTexture.GetHeight();
+            mpProgram->SetUniform1f( "xscale", ixscale );
+            mpProgram->SetUniform1f( "yscale", iyscale );
+            mpProgram->SetUniform1i( "swidth", mnWidth );
+            mpProgram->SetUniform1i( "sheight", mnHeight );
+            // For converting between <0,mnWidth-1> and <0.0,1.0> coordinate systems.
+            mpProgram->SetUniform1f( "xsrcconvert", 1.0 / ( mnWidth - 1 ));
+            mpProgram->SetUniform1f( "ysrcconvert", 1.0 / ( mnHeight - 1 ));
+            mpProgram->SetUniform1f( "xdestconvert", 1.0 * (( mnWidth / ixscale ) - 1 ));
+            mpProgram->SetUniform1f( "ydestconvert", 1.0 * (( mnHeight / iyscale ) - 1 ));
+        }
+    }
+
     mpProgram->SetUniform2f( "viewport", GetWidth(), GetHeight() );
     mpProgram->SetTransform( "transform", rTexture, rNull, rX, rY );
     rTexture.GetWholeCoord( aTexCoord );

vcl/opengl/scale.cxx

@@ -225,6 +225,7 @@ bool OpenGLSalBitmap::ImplScaleArea( double rScaleX, double rScaleY )
     OpenGLTexture aScratchTex = OpenGLTexture( nNewWidth, nNewHeight );
     OpenGLFramebuffer* pFramebuffer = mpContext->AcquireFramebuffer( aScratchTex );
 
+    // NOTE: This setup is also done in OpenGLSalGraphicsImpl::DrawTransformedTexture().
     if( fast )
     {
         pProgram->SetUniform1i( "xscale", ixscale );