Support gamma correction of text on GL

If the SRGB framebuffer extension in GL is available, we can support
gamma correction of text with a gamma of 2.1. On Mac this is
sufficient for gamma correcting subpixel antialiased text. Gray
antialiasing should not be gamma corrected on Mac.

On Windows, the user can potentially set the gamma value to anything
between 1.0 and 2.2 (or something like that). We support anything
that resembles 1.0 closely enough by pushing the text out without
any correction (like before). We also support anything that resembles
2.1 (the gamma hardcoded in GL's SRGB extension) by turning on the
extension before blending the text. In between the two, we'll use
gray antialiasing to avoid differing too much from the raster engine
(which is our reference in this.)

For gray antialiasing on Windows, we use a constant gamma of 2.3 which
has been determined by experimentation. Since this is close enough to
2.1 we do gamma correction with SRGB extension.

The distance limit of 0.2 is determined by some experimentation.

Reviewed-by: Samuel
(cherry picked from commit 79ba7cceca5e4029876ace2121edd25b08ae14ce)

4 files changed, 46 insertions, 3 deletions

diff --git a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
index 18c684ff1b..673f614c0d 100644
--- a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
+++ b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
@@ -1548,6 +1548,14 @@ namespace {
 
 }
 
+#if defined(Q_WS_WIN)
+static bool fontSmoothingApproximately(qreal target)
+{
+    extern Q_GUI_EXPORT qreal qt_fontsmoothing_gamma; // qapplication_win.cpp
+    return (qAbs(qt_fontsmoothing_gamma - target) < 0.2);
+}
+#endif
+
 // #define QT_OPENGL_DRAWCACHEDGLYPHS_INDEX_ARRAY_VBO
 
 void QGL2PaintEngineExPrivate::drawCachedGlyphs(QFontEngineGlyphCache::Type glyphType,
@@ -1786,7 +1794,6 @@ void QGL2PaintEngineExPrivate::drawCachedGlyphs(QFontEngineGlyphCache::Type glyp
         shaderManager->setMaskType(QGLEngineShaderManager::PixelMask);
         prepareForDraw(false); // Text always causes src pixels to be transparent
     }
-    //### TODO: Gamma correction
 
     QGLTextureGlyphCache::FilterMode filterMode = (s->matrix.type() > QTransform::TxTranslate)?QGLTextureGlyphCache::Linear:QGLTextureGlyphCache::Nearest;
     if (lastMaskTextureUsed != cache->texture() || cache->filterMode() != filterMode) {
@@ -1809,12 +1816,31 @@ 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);
 #else
     glDrawElements(GL_TRIANGLE_STRIP, 6 * numGlyphs, GL_UNSIGNED_SHORT, elementIndices.data());
 #endif
+
+    if (srgbFrameBufferEnabled)
+        glDisable(FRAMEBUFFER_SRGB_EXT);
+
 }
 
 void QGL2PaintEngineEx::drawPixmapFragments(const QPainter::PixmapFragment *fragments, int fragmentCount, const QPixmap &pixmap,
@@ -1986,7 +2012,8 @@ bool QGL2PaintEngineEx::begin(QPaintDevice *pdev)
 
 #if !defined(QT_OPENGL_ES_2)
 #if defined(Q_WS_WIN)
-    if (qt_cleartype_enabled)
+    if (qt_cleartype_enabled
+        && (fontSmoothingApproximately(1.0) || fontSmoothingApproximately(2.1)))
 #endif
 #if defined(Q_WS_MAC)
     if (qt_applefontsmoothing_enabled)
diff --git a/src/opengl/qgl.cpp b/src/opengl/qgl.cpp
index 7ce35f8d1c..4cb976f3c2 100644
--- a/src/opengl/qgl.cpp
+++ b/src/opengl/qgl.cpp
@@ -5521,6 +5521,13 @@ QGLExtensions::Extensions QGLExtensions::currentContextExtensions()
     if (extensions.match("GL_EXT_bgra"))
         glExtensions |= BGRATextureFormat;
 
+    {
+        GLboolean srgbCapableFramebuffers;
+        glGetBooleanv(FRAMEBUFFER_SRGB_CAPABLE_EXT, &srgbCapableFramebuffers);
+        if (srgbCapableFramebuffers)
+            glExtensions |= SRGBFrameBuffer;
+    }
+
     return glExtensions;
 }
 
diff --git a/src/opengl/qgl_p.h b/src/opengl/qgl_p.h
index 6a503fab90..46683c742e 100644
--- a/src/opengl/qgl_p.h
+++ b/src/opengl/qgl_p.h
@@ -288,7 +288,8 @@ public:
         PVRTCTextureCompression = 0x00020000,
         FragmentShader          = 0x00040000,
         ElementIndexUint        = 0x00080000,
-        Depth24                 = 0x00100000
+        Depth24                 = 0x00100000,
+        SRGBFrameBuffer         = 0x00200000
     };
     Q_DECLARE_FLAGS(Extensions, Extension)
 
diff --git a/src/opengl/qglextensions_p.h b/src/opengl/qglextensions_p.h
index 529c7a1b4e..ac80ce8b96 100644
--- a/src/opengl/qglextensions_p.h
+++ b/src/opengl/qglextensions_p.h
@@ -477,6 +477,14 @@ struct QGLExtensionFuncs
 
 // OpenGL constants
 
+#ifndef FRAMEBUFFER_SRGB_CAPABLE_EXT
+#define FRAMEBUFFER_SRGB_CAPABLE_EXT 0x8DBA
+#endif
+
+#ifndef FRAMEBUFFER_SRGB_EXT
+#define FRAMEBUFFER_SRGB_EXT 0x8DB9
+#endif
+
 #ifndef GL_ARRAY_BUFFER
 #define GL_ARRAY_BUFFER                   0x8892
 #endif