Not getting correct blending when using QOpenGLWidget and a translucent window

[Guy Gizmo]

I'm trying to create a translucent window with a QGraphicsScene using OpenGL as its backend, using Qt 6.2.3 in macOS 13.

I've got images rendering into it, but transparency is not working correctly. The window is indeed translucent, and shapes that I add to the QGraphicsScene with an alpha value of less than one are partially transparent, but rather than doing a simple alpha blend, it does a blend that seems equivalent to Photoshop's "screen" blending, where dark colors take on the color of the layer being blended into it, and light colors are unaffected.

I've tried setting the correct blending function using glBlendFunc in both the paintGL method of my QOpenGLWidget and in my QGraphicView drawBackground method, but it doesn't have any effect.

To give an illustration of the sort of wrong blending I'm getting, here's what happens if open my translucent window over a browser window, and I call glClear in my drawBackground method with a clear color of (1.0, 0.0, 0.0, 0.0), a color which ought to be 100% transparent but instead has this effect:

What do I need to do to just get regular alpha blending?

[Guy Gizmo]

I've sorted it out: I had a feeling this might have to do with premultiplied alpha, and it turns out that the macOS compositor requires all windows to premultiply their alpha before compositing. All I needed to do was change my calls to glColor4f so that the red, green, and blue values were multiplied by the alpha value. I'll also need to make sure that any textures I use are premultiplied.

[Guy Gizmo]

I've done a bit more digging into this, and I've found that this issue happens without using QGraphicsView, so that aspect of this is irrelevant. I can reproduce the issue by just using a QOpenGLWidget in a translucent window.

Any drawing I perform using QPainter in the widget's paintGL method will have correct transparency and blend properly with the windows beneath it. However, any drawing I do between calls to QPainter::beginNativePainting() and QPainter::endNativePainting() with pure GL calls will have this transparency issue. I'm still not sure why, though!

[Guy Gizmo]

I've sorted it out: I had a feeling this might have to do with premultiplied alpha, and it turns out that the macOS compositor requires all windows to premultiply their alpha before compositing. All I needed to do was change my calls to glColor4f so that the red, green, and blue values were multiplied by the alpha value. I'll also need to make sure that any textures I use are premultiplied.