TL;DR
When I stack multiple layers of components on top of each other using JLayeredPane, alpha composition does not work. Wherever there are multiple pixels on top of each other, each with an alpha color, only the topmost gets displayed, ignoring the alpha colors of the layers below.
Basic setup
I have a JLayeredPane with several layers. Each layer displays a grid of components with their own paint method. All components have setOpaque(false).
Positive observation
Drawing images containing fully transparent colors in the different layers works as expected: For each pixel, only the color value of the topmost non-transparent layer is shown.
Negative observation
When combining several layers where there is a semi-transparent pixel on each layer for a given coordinate, i.e. a color with an alpha value of 50%, only the topmost pixel seems to be shown.
A top-layer alpha pixel does combine with a lower-layer opaque-colored pixel however. If the top-layer pixel is semi-transparent and a pixel on a layer below is fully opaque (alpha = 0%), then both pixels combine correctly.
I made an image full of 50% alpha blue, which I show on the top layer. In the layer below, there is an image full of 50% alpha green. Those two should combine to cyan, but the JLayeredPane only displays blue in these places, the top-level color.
The same effect occurs when I use a custom paint method with g.fillRect and a semi-transparent color, instead of the images.
How can I make sure that my JLayeredPane combines the colors of multiple semi-transparent layers correctly?
Sorry for the confusion, I was stupid. I took a screenshot and analyzed the colors and found out that there is no issue at all.
My intuitive color "arithmetics" was wrong: If you put a 50% blue over a 50% green, the result is not cyan. You don't get half green, half blue.
Instead, what you get is 50% blue and 25% green. Because the green layer is at 50% alpha already, and it passes through another alpha of 50%. Those two need to get multiplied.