Yes. Just don't draw any background on the PGraphics. It will remain transparent.

Sorry, my example was not the best one. I know that pGraphics is initially filled with transparent pixels. My problem is that I need to DRAW with transparent pixels. For example use ellipses, boxes, texts, lines as eraser shapes on an existing pGraphics. The most simple example is a moon like shape on transparent background. First I draw a filled circle, then I should "bite out" a part of it by removing a shifted circle.

Yes, this mask() is suitable for my problem, but I dont understand, why is not there a much easier and faster method for this. The pixel array is accessible by the code, even tha alpha channel. Then it would be good a blendMode or something similar that can overwrite alpha values in the buffer only, when using drawing functions. Is it hard to implement?

Most people will probably find Processing's transparency behavior more intuitive. But actually I believe pure opengl transparency behavior is as you decribe. In the sense that using transparent colors will make the stuff that was already drawn also transparent. So perhaps you can use direct opengl calls or the GLGraphics library to do this.

You could also use a shader to do this probably. But note that several PGraphics+transparency+shader issues were fixed recently so then I would advise to use the latest build from svn instead of 2.0b7.