Support sRGB for text with QQuickWidget

Otherwise we get visually different results with QQuickWindow
and QQuickWidget on OS X.

Task-number: QTBUG-42861
Change-Id: Icbf6f6e980129f5de73a88e7be7bef4f592e875e
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

1 files changed, 21 insertions, 0 deletions

diff --git a/src/quickwidgets/qquickwidget.cpp b/src/quickwidgets/qquickwidget.cpp
index 8b7a42de57..38dde806ed 100644
--- a/src/quickwidgets/qquickwidget.cpp
+++ b/src/quickwidgets/qquickwidget.cpp
@@ -755,6 +755,16 @@ void QQuickWidget::createFramebufferObject()
     format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
     format.setSamples(samples);
 
+    // The default framebuffer for normal windows have sRGB support on OS X which leads to the Qt Quick text item
+    // utilizing sRGB blending. To get identical results with QQuickWidget we have to have our framebuffer backed
+    // by an sRGB texture.
+#ifdef Q_OS_OSX
+    if (context->hasExtension("GL_ARB_framebuffer_sRGB")
+            && context->hasExtension("GL_EXT_texture_sRGB")
+            && context->hasExtension("GL_EXT_texture_sRGB_decode"))
+        format.setInternalTextureFormat(GL_SRGB8_ALPHA8_EXT);
+#endif
+
     const QSize fboSize = size() * devicePixelRatio();
 
     // Could be a simple hide - show, in which case the previous fbo is just fine.
@@ -763,6 +773,17 @@ void QQuickWidget::createFramebufferObject()
         d->fbo = new QOpenGLFramebufferObject(fboSize, format);
     }
 
+    // When compositing in the backingstore, sampling the sRGB texture would perform an
+    // sRGB-linear conversion which is not what we want when the final framebuffer (the window's)
+    // is sRGB too. Disable the conversion.
+#ifdef Q_OS_OSX
+    if (format.internalTextureFormat() == GL_SRGB8_ALPHA8_EXT) {
+        QOpenGLFunctions *funcs = context->functions();
+        funcs->glBindTexture(GL_TEXTURE_2D, d->fbo->texture());
+        funcs->glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_SRGB_DECODE_EXT, GL_SKIP_DECODE_EXT);
+    }
+#endif
+
     // Even though this is just an offscreen window we should set the position on it, as it might be
     // useful for an item to know the actual position of the scene.
     // Note: The position will be update when we get a move event (see: updatePosition()).