Because opacity in surfaces created with createGraphics() is binary, text
characters appear as opaque rectangles. This will be fixed in a future release.

I'm interested in this bug to get fixed, not for text but for drawing low
opacity lines in a created graphics. Any idea where should I start looking
in the code to try to get this fixed?
thanks

It's in the rendering code for PTriangle and PLine for P2D and P3D. The
issue is how blending works on top of an alpha channel, but it affects
dozens of places in the code and has to be done consistently. Basically,
need to set some rules for how drawing happens on a transparent surface
versus an opaque surface, and have that be consistent with expectations
when people overlay that image on another drawing area.

That would be great if you can take a look into it, and I'm happy to help
however I can. It's just that it's a piece that touches a lot of rendering
code (almost anywhere that you have a pixels[xxx] = xxx), so it has to be
done wholesale.