Compose renderToTexture widgets with sRGB when needed

Just requesting an sRGB texture for a QOpenGLWidget does not lead to
anything useful when it comes to the end result, the content will just get
darkened. For proper operation the target window's default framebuffer
must be sRGB capable and linearization during blending must be enabled.

Task-number: QTBUG-50987
Change-Id: Ibad0657c29a720590fa22c84c4bc303302de6dc0
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

1 files changed, 31 insertions, 3 deletions

diff --git a/src/gui/painting/qplatformbackingstore.cpp b/src/gui/painting/qplatformbackingstore.cpp
index b8bbdefa37..e006fad437 100644
--- a/src/gui/painting/qplatformbackingstore.cpp
+++ b/src/gui/painting/qplatformbackingstore.cpp
@@ -70,6 +70,13 @@
 #define GL_UNSIGNED_INT_2_10_10_10_REV    0x8368
 #endif
 
+#ifndef GL_FRAMEBUFFER_SRB
+#define GL_FRAMEBUFFER_SRGB 0x8DB9
+#endif
+#ifndef GL_FRAMEBUFFER_SRGB_CAPABLE
+#define GL_FRAMEBUFFER_SRGB_CAPABLE 0x8DBA
+#endif
+
 QT_BEGIN_NAMESPACE
 
 class QPlatformBackingStorePrivate
@@ -269,7 +276,7 @@ static inline QRect toBottomLeftRect(const QRect &topLeftRect, int windowHeight)
 }
 
 static void blitTextureForWidget(const QPlatformTextureList *textures, int idx, QWindow *window, const QRect &deviceWindowRect,
-                                 QOpenGLTextureBlitter *blitter, const QPoint &offset)
+                                 QOpenGLTextureBlitter *blitter, const QPoint &offset, bool canUseSrgb)
 {
     const QRect clipRect = textures->clipRect(idx);
     if (clipRect.isEmpty())
@@ -289,7 +296,15 @@ static void blitTextureForWidget(const QPlatformTextureList *textures, int idx,
                                                                      deviceRect(rectInWindow, window).size(),
                                                                      QOpenGLTextureBlitter::OriginBottomLeft);
 
+    QOpenGLFunctions *funcs = QOpenGLContext::currentContext()->functions();
+    const bool srgb = textures->flags(idx).testFlag(QPlatformTextureList::TextureIsSrgb);
+    if (srgb && canUseSrgb)
+        funcs->glEnable(GL_FRAMEBUFFER_SRGB);
+
     blitter->blit(textures->textureId(idx), target, source);
+
+    if (srgb && canUseSrgb)
+        funcs->glDisable(GL_FRAMEBUFFER_SRGB);
 }
 
 /*!
@@ -334,10 +349,23 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
 
     const QRect deviceWindowRect = deviceRect(QRect(QPoint(), window->size()), window);
 
+    bool canUseSrgb = false;
+    // If there are any sRGB textures in the list, check if the destination
+    // framebuffer is sRGB capable.
+    for (int i = 0; i < textures->count(); ++i) {
+        if (textures->flags(i).testFlag(QPlatformTextureList::TextureIsSrgb)) {
+            GLint cap = 0;
+            funcs->glGetIntegerv(GL_FRAMEBUFFER_SRGB_CAPABLE, &cap);
+            if (cap)
+                canUseSrgb = true;
+            break;
+        }
+    }
+
     // Textures for renderToTexture widgets.
     for (int i = 0; i < textures->count(); ++i) {
         if (!textures->flags(i).testFlag(QPlatformTextureList::StacksOnTop))
-            blitTextureForWidget(textures, i, window, deviceWindowRect, d_ptr->blitter, offset);
+            blitTextureForWidget(textures, i, window, deviceWindowRect, d_ptr->blitter, offset, canUseSrgb);
     }
 
     // Backingstore texture with the normal widgets.
@@ -406,7 +434,7 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
     // Textures for renderToTexture widgets that have WA_AlwaysStackOnTop set.
     for (int i = 0; i < textures->count(); ++i) {
         if (textures->flags(i).testFlag(QPlatformTextureList::StacksOnTop))
-            blitTextureForWidget(textures, i, window, deviceWindowRect, d_ptr->blitter, offset);
+            blitTextureForWidget(textures, i, window, deviceWindowRect, d_ptr->blitter, offset, canUseSrgb);
     }
 
     funcs->glDisable(GL_BLEND);