Note: This might be a KWin bug (or design decision), but I'm filing it here just in case.
One possible cause is that KDE uses premultiplied alpha, while Godot doesn't. The background sky is not black, so even if it's fully transparent, its color is added instead of mixed.
This is what I see on the window (compositing is enabled). I can partially see the background sky:
Same window, but overlaid on a pure black background:
If I use KDE's screenshot tool to take a screenshot of the window, I get this:
The image outputted by MovieWriter (i.e. what Godot "sees") looks like this:
Steps to reproduce
Run the attached project with movie maker mode enabled (click the "movie reel" icon in the top-right corner before running the project).
Compare what you see on the window and the PNG images generated in the output/ folder.
Godot version
4.0.beta2
System information
Fedora 36, Vulkan Forward Plus, AMD Radeon RX 6900 XT, KDE + KWin
Issue description
Note: This might be a KWin bug (or design decision), but I'm filing it here just in case. One possible cause is that KDE uses premultiplied alpha, while Godot doesn't. The background sky is not black, so even if it's fully transparent, its color is added instead of mixed.
This is what I see on the window (compositing is enabled). I can partially see the background sky:
Same window, but overlaid on a pure black background:
If I use KDE's screenshot tool to take a screenshot of the window, I get this:
The image outputted by MovieWriter (i.e. what Godot "sees") looks like this:
Steps to reproduce
output/
folder.Minimal reproduction project
test_movie_maker_png_transparent.zip