Fix gamma corrected source color in GL

GL_EXT_framebuffer_sRGB expects linear RGB as input color, thus we
need to convert sRGB based input color into linear RGB first before
using glBlendColor with it.

Task-number: QTBUG-20574
Reviewed-by: Samuel Rødal

1 files changed, 30 insertions, 18 deletions

diff --git a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
index a961366176..dbbb07cf57 100644
--- a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
+++ b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
@@ -1580,6 +1580,11 @@ static bool fontSmoothingApproximately(qreal target)
 }
 #endif
 
+static inline qreal qt_sRGB_to_linear_RGB(qreal f)
+{
+    return f > 0.04045 ? qPow((f + 0.055) / 1.055, 2.4) : f / 12.92;
+}
+
 // #define QT_OPENGL_DRAWCACHEDGLYPHS_INDEX_ARRAY_VBO
 
 void QGL2PaintEngineExPrivate::drawCachedGlyphs(QFontEngineGlyphCache::Type glyphType,
@@ -1739,12 +1744,34 @@ void QGL2PaintEngineExPrivate::drawCachedGlyphs(QFontEngineGlyphCache::Type glyp
     }
 
     QBrush pensBrush = q->state()->pen.brush();
-    setBrush(pensBrush);
 
-    if (glyphType == QFontEngineGlyphCache::Raster_RGBMask) {
+    bool srgbFrameBufferEnabled = false;
+    if (pensBrush.style() == Qt::SolidPattern &&
+        (ctx->d_ptr->extension_flags & QGLExtensions::SRGBFrameBuffer)) {
+#if defined(Q_WS_MAC)
+        if (glyphType == QFontEngineGlyphCache::Raster_RGBMask)
+#elif defined(Q_WS_WIN)
+        if (glyphType != QFontEngineGlyphCache::Raster_RGBMask || fontSmoothingApproximately(2.1))
+#else
+        if (false)
+#endif
+        {
+            QColor c = pensBrush.color();
+            qreal red = qt_sRGB_to_linear_RGB(c.redF());
+            qreal green = qt_sRGB_to_linear_RGB(c.greenF());
+            qreal blue = qt_sRGB_to_linear_RGB(c.blueF());
+            c = QColor::fromRgbF(red, green, blue, c.alphaF());
+            pensBrush.setColor(c);
+
+            glEnable(FRAMEBUFFER_SRGB_EXT);
+            srgbFrameBufferEnabled = true;
+        }
+    }
 
-        // Subpixel antialiasing without gamma correction
+    setBrush(pensBrush);
 
+    if (glyphType == QFontEngineGlyphCache::Raster_RGBMask) {
+        // Subpixel antialiasing with gamma correction
         QPainter::CompositionMode compMode = q->state()->composition_mode;
         Q_ASSERT(compMode == QPainter::CompositionMode_Source
             || compMode == QPainter::CompositionMode_SourceOver);
@@ -1851,21 +1878,6 @@ void QGL2PaintEngineExPrivate::drawCachedGlyphs(QFontEngineGlyphCache::Type glyp
         }
     }
 
-    bool srgbFrameBufferEnabled = false;
-    if (ctx->d_ptr->extension_flags & QGLExtensions::SRGBFrameBuffer) {
-#if defined(Q_WS_MAC)
-        if (glyphType == QFontEngineGlyphCache::Raster_RGBMask)
-#elif defined(Q_WS_WIN)
-        if (glyphType != QFontEngineGlyphCache::Raster_RGBMask || fontSmoothingApproximately(2.1))
-#else
-        if (false)
-#endif
-        {
-            glEnable(FRAMEBUFFER_SRGB_EXT);
-            srgbFrameBufferEnabled = true;
-        }
-    }
-
 #if defined(QT_OPENGL_DRAWCACHEDGLYPHS_INDEX_ARRAY_VBO)
     glDrawElements(GL_TRIANGLE_STRIP, 6 * numGlyphs, GL_UNSIGNED_SHORT, 0);
     glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);