Allow render-to-texture widgets to tell if they want premul blending

Instead of assuming they do not (like in >= 5.12.3) or they do (like
in < 5.12.3). QOpenGLWidget and QQuickWidget will likely want the
opposite. So allow specifying this with a QPlatformTextureList flag,
similarly to how we do it for sRGB for QOpenGLWidget.

Task-number: QTBUG-77471
Change-Id: I594ca919c8eca190fa70c6aa84f46f456fcd80e1
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

2 files changed, 16 insertions, 7 deletions

diff --git a/src/gui/painting/qplatformbackingstore.cpp b/src/gui/painting/qplatformbackingstore.cpp
index c71d82546a..601dc97be1 100644
--- a/src/gui/painting/qplatformbackingstore.cpp
+++ b/src/gui/painting/qplatformbackingstore.cpp
@@ -446,14 +446,22 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
             d_ptr->blitter->setRedBlueSwizzle(false);
     }
 
-    // There is no way to tell if the OpenGL-rendered content is premultiplied or not.
-    // For compatibility, assume that it is not, and use normal alpha blend always.
-    if (d_ptr->premultiplied)
-        funcs->glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE);
-
     // Textures for renderToTexture widgets that have WA_AlwaysStackOnTop set.
+    bool blendIsPremultiplied = d_ptr->premultiplied;
     for (int i = 0; i < textures->count(); ++i) {
-        if (textures->flags(i).testFlag(QPlatformTextureList::StacksOnTop))
+        const QPlatformTextureList::Flags flags = textures->flags(i);
+        if (flags.testFlag(QPlatformTextureList::NeedsPremultipliedAlphaBlending)) {
+            if (!blendIsPremultiplied) {
+                funcs->glBlendFuncSeparate(GL_ONE, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE);
+                blendIsPremultiplied = true;
+            }
+        } else {
+            if (blendIsPremultiplied) {
+                funcs->glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE);
+                blendIsPremultiplied = false;
+            }
+        }
+        if (flags.testFlag(QPlatformTextureList::StacksOnTop))
             blitTextureForWidget(textures, i, window, deviceWindowRect, d_ptr->blitter, offset, canUseSrgb);
     }
 
diff --git a/src/gui/painting/qplatformbackingstore.h b/src/gui/painting/qplatformbackingstore.h
index 414d2bf0de..4f08b0092f 100644
--- a/src/gui/painting/qplatformbackingstore.h
+++ b/src/gui/painting/qplatformbackingstore.h
@@ -81,7 +81,8 @@ class Q_GUI_EXPORT QPlatformTextureList : public QObject
 public:
     enum Flag {
         StacksOnTop = 0x01,
-        TextureIsSrgb = 0x02
+        TextureIsSrgb = 0x02,
+        NeedsPremultipliedAlphaBlending = 0x04
     };
     Q_DECLARE_FLAGS(Flags, Flag)