Priority: P3: Somewhat important
Affects Version/s: 5.13.1
Fix Version/s: None
Component/s: GUI: OpenGL
Environment:MacBook Pro (15-inch, 2018)
Radeon Pro 560X 4 GB
Intel UHD Graphics 630 1536 MB
referring to this article, i see someone else has also run into this, but did not get a satisfactory answer.
I am implementing what on macOS Carbon is known as a "transparency layer", that is, you begin a transparency context (basically create a transparent QImage the size of the canvas you're drawing on), do your drawing (with transparency), finish your context, then blit the contents of that context as a whole with a specified transparency (this is to achieve "fade away" via animation) (done by first iterating over all pixels and multiplying the alpha channel by the desired transparency, then blitting)
When i use QWidget, my stroked paths (text) look creamy smooth along the edges, as if everything is in fact being handled as straight alpha: see screen shot QWidget.
However, if i switch to using QOpenGLWidget, my stroked paths appear to have a crusty border, as if they were drawn with premul-over-black alpha: see screen shot QOpenGLWidget
No other code changes except swapping QOpenGLWidget for QWidget.
I feel sure this is a bug. Note if i attempt to manually unmultiply, things look much worse, so i don't think it's PRECISELY a premul thing. (and i know my manual unmultiply works because unit tests)
(Ignore for now the fact that it's white-on-white, i know that, i'm NOT drawing the black text behind the stroke to more clearly illustrate the halo around the strokes)