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)

src/gui/kernel/qapplication_win.cpp

@@ -237,6 +237,7 @@ static void resolveAygLibs()
 #  define FE_FONTSMOOTHINGCLEARTYPE 0x0002
 #endif
 
+Q_GUI_EXPORT qreal qt_fontsmoothing_gamma;
 Q_GUI_EXPORT bool qt_cleartype_enabled;
 Q_GUI_EXPORT bool qt_win_owndc_required; // CS_OWNDC is required if we use the GL graphicssystem as default
 
@@ -653,8 +654,18 @@ static void qt_win_read_cleartype_settings()
     if (SystemParametersInfo(SPI_GETFONTSMOOTHINGTYPE, 0, &result, 0))
         qt_cleartype_enabled = (result == FE_FONTSMOOTHINGCLEARTYPE);
 #endif
-}
 
+    int winSmooth;
+    if (SystemParametersInfo(SPI_GETFONTSMOOTHINGCONTRAST, 0, &winSmooth, 0)) {
+        qt_fontsmoothing_gamma = winSmooth / qreal(1000.0);
+    } else {
+        qt_fontsmoothing_gamma = 1.0;
+    }
+
+    // Safeguard ourselves against corrupt registry values...
+    if (qt_fontsmoothing_gamma > 5 || qt_fontsmoothing_gamma < 1)
+        qt_fontsmoothing_gamma = qreal(1.4);
+}
 
 static void qt_set_windows_resources()
 {

src/gui/painting/qdrawhelper.cpp

@@ -7198,14 +7198,8 @@ void qt_build_pow_tables() {
 #endif
 
 #ifdef Q_WS_WIN
-    int winSmooth;
-    if (SystemParametersInfo(0x200C /* SPI_GETFONTSMOOTHINGCONTRAST */, 0, &winSmooth, 0))
-        smoothing = winSmooth / qreal(1000.0);
-
-    // Safeguard ourselves against corrupt registry values...
-    if (smoothing > 5 || smoothing < 1)
-        smoothing = qreal(1.4);
-
+    extern qreal qt_fontsmoothing_gamma; // qapplication_win.cpp
+    smoothing = qt_fontsmoothing_gamma;
 #endif
 
 #ifdef Q_WS_X11

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)

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;
 }
 

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)
 

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