From 1ad41e99bd0426f203b23fa5bbbaf070402d1fad Mon Sep 17 00:00:00 2001
From: Matej Knopp <matej.knopp@gmail.com>
Date: Sun, 28 Dec 2014 01:11:09 +0100
Subject: [PATCH 2/2] add qSetSubpixelBlendBackgroundColor

Signed-off-by: Matej Knopp <matej.knopp@gmail.com>
---
 src/gui/painting/qdrawhelper.cpp | 85 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 85 insertions(+)

diff --git a/src/gui/painting/qdrawhelper.cpp b/src/gui/painting/qdrawhelper.cpp
index 9c1b2e7..8b47965 100644
--- a/src/gui/painting/qdrawhelper.cpp
+++ b/src/gui/painting/qdrawhelper.cpp
@@ -6171,10 +6171,95 @@ static void qt_alphamapblit_quint16(QRasterBuffer *rasterBuffer,
     }
 }
 
+static quint32 qSubPixelBlendBackgroundColor = 0;
+
+// Allows specifying target background collor for subpixel blends to empty surface
+void Q_GUI_EXPORT qSetSubpixelBlendBackgroundColor(quint32 color)
+{
+    qSubPixelBlendBackgroundColor = color;
+}
+
 static inline void rgbBlendPixel(quint32 *dst, int coverage, int sr, int sg, int sb, const uchar *gamma, const uchar *invgamma)
 {
     // Do a gray alphablend...
     int da = qAlpha(*dst);
+
+    if (da == 0 && qSubPixelBlendBackgroundColor != 0)
+    {
+        int dr = qRed(qSubPixelBlendBackgroundColor);
+        int dg = qGreen(qSubPixelBlendBackgroundColor);
+        int db = qBlue(qSubPixelBlendBackgroundColor);
+        
+        int mr = qRed(coverage);
+        int mg = qGreen(coverage);
+        int mb = qBlue(coverage);
+        
+#if 0
+        int a = qGray(coverage);
+#else
+        // qGray is probably the proper way to do this, but the code below seems to give better results
+        int a = qMax(mr, mg);
+        a = qMax(a, mb);
+#endif
+        dr = gamma[dr];
+        dg = gamma[dg];
+        db = gamma[db];
+        
+        int nr = qt_div_255((sr - dr) * mr) + dr;
+        int ng = qt_div_255((sg - dg) * mg) + dg;
+        int nb = qt_div_255((sb - db) * mb) + db;
+        
+        nr = invgamma[nr];
+        ng = invgamma[ng];
+        nb = invgamma[nb];
+        
+        *dst = qRgba(qt_div_255(nr * a), qt_div_255(ng * a), qt_div_255(nb * a), a);
+        return;
+    }
+    else if (da < 255 && qSubPixelBlendBackgroundColor != 0)
+    {
+        // Blend destination color to "fake" destination color
+        int da = qAlpha(*dst);
+        int ida = 255 - da;
+        
+        int dr = qRed(*dst) + qt_div_255(qRed(qSubPixelBlendBackgroundColor) * ida);
+        int dg = qGreen(*dst) + qt_div_255(qGreen(qSubPixelBlendBackgroundColor) * ida);
+        int db = qBlue(*dst) + qt_div_255(qBlue(qSubPixelBlendBackgroundColor) * ida);
+
+        // Do subpixel blending
+        
+        int mr = qRed(coverage);
+        int mg = qGreen(coverage);
+        int mb = qBlue(coverage);
+        
+#if 0
+        int a = qGray(coverage);
+#else
+        // qGray is probably the proper way to do this, but the code below seems to give better results
+        int a = qMax(mr, mg);
+        a = qMax(a, mb);
+#endif
+        dr = gamma[dr];
+        dg = gamma[dg];
+        db = gamma[db];
+        
+        int nr = qt_div_255((sr - dr) * mr) + dr;
+        int ng = qt_div_255((sg - dg) * mg) + dg;
+        int nb = qt_div_255((sb - db) * mb) + db;
+        
+        nr = invgamma[nr];
+        ng = invgamma[ng];
+        nb = invgamma[nb];
+        
+        // Blend the result to actual destination
+        int ia = 255 - a;
+        *dst = qRgba(qt_div_255(nr * a) + qt_div_255(qRed(*dst) * ia),
+                     qt_div_255(ng * a) + qt_div_255(qGreen(*dst) * ia),
+                     qt_div_255(nb * a) + qt_div_255(qBlue(*dst) * ia),
+                     da + qt_div_255(a * ida));
+        return;
+    }
+    
     int dr = qRed(*dst);
     int dg = qGreen(*dst);
     int db = qBlue(*dst);
-- 
1.9.3 (Apple Git-50)