At least there is some progress, some time ago P2D graphics weren't supporting transparency...
Looks like a bug, although a quite puzzling one. I made a more complete test sketch:

1. void setup()
2. {
3. size(500, 500);
4. PImage pi = loadImage("G:/Images/toucan.png");
5. PGraphics pg = createGraphics(width, height, P2D);
6. pg.beginDraw();
7. pg.fill(#FF0000);
8. pg.ellipse(0, 0, width, height);
9. pg.fill(#FFFF00);
10. pg.ellipse(width, height, width * 1.5, height * 1.5);
11. pg.textureMode(NORMALIZED);
12. pg.noStroke();
13. pg.noFill();
14. pg.beginShape();
15. pg.texture(pi);
16. pg.vertex(50,   50,   0,   0);
17. pg.vertex(400,  50, 162,   0);
18. pg.vertex(400, 400, 162, 150);
19. pg.vertex(50,  400,   0, 150);
20. pg.endShape();
21. pg.endDraw();
22. pg.save("H:/Temp/test.png");
23. exit();
24. }

Toucan is the well known PNG with transparency (semi-transparent shadows). I can see the transparency over the ellipses, but there is a black rectangle behind the ellipses themselves, despite being drawn after them...

yep, it looks like as soon as there is a png with transparency over the transparent background, the background becomes black... i have no idea how to fix this (or even find a workaround)

Reported as Issue 723

I'm back at this because the Issue 723 phi.lho has now a "WontFix" status: looks like P2D won't be there anymore in Processing 2.0(!!!)
In the meantime, i really need to export images with textures imported from png files, and to keep the transparent backround from the textures. Is there a way to do it with opengl?

If I test it with this code:

1. PGraphics pg = createGraphics(width, height, OPENGL);
2. pg.beginDraw();
3. pg.endDraw();
   
4. pg.save(frameCount+".png");

it returns wglGetExtensionStringEXT not available...

I just tried to use GLGraphicsOffScreen from GLGraphics and it works + it's quite faster.