As you certainly know, P2D and P3D are now OpenGL. And I saw lot of people wondering about transparency in OpenGL. I don't recall a solution, but I might have missed it.

This issue is due to how the pixels array is composited into the offscreen buffer when using multisampling. If you initialize your pg object w/out smooth, then the transparency will be ok. Just committed the fix to the repo, so the next release won't have this problem. For the time being, a workaround is to call pg.background(0, 0) right after pg.beginDraw() (or use pg with noSmooth).

Thanks for your efforts andres. Good to see this problem will be fixed.

Unfortunately both your workarounds cause other issues themselves. noSmooth() solves the immediate issue, but this result is unusable because any subsequent call to beginDraw will clear it again. Using background(0, 0) does not allow accurate creation of pixels because of a mirror issue (already filed on googlecode). See below for details and examples.

noSmooth()
Using noSmooth will reset the PGraphics to fully transparent on each begin-endDraw call even when no background call is made. See the example code below. Without noSmooth() the drawing is cumulative (which is the expected behavior). With noSmooth() you only see one ellipse, because apparantly the PGraphics is cleared.

1. PGraphics pg;
2.  
3. void setup() {
4.   size(500, 500, P2D);
5.   pg = createGraphics(width/2, height/2, P2D);
6.   pg.beginDraw();
7.   pg.noSmooth(); // comment out to see 'normal' cumulative drawing
   
8.   pg.endDraw();
9.   noFill();
10. }
11.  
12. void draw() {
13.   pg.beginDraw();
14.   pg.ellipse((float)mouseX/width*pg.width, (float)mouseY/height*pg.height, 50, 50);
15.   pg.endDraw();
16.   background(0, 255, 0);
17.   image(pg, width/4, height/4);
18.   rect(width/4, height/4, width/2, height/2);
19. }

Background(0, 0)
Once background is used with full transparency on a PGraphics, mirror pixels will be created in the upper half of the PGraphics with the given greyscale color. I have already filed an issue for this here. Here is another example in which the top half is erroneously black.

1. PGraphics pg;
2.  
3. void setup() {
4.   size(500, 500, P2D);
5.   pg = createGraphics(width/2, height/2, P2D);
6.   noFill();
7. }
8.  
9. void draw() {
10.   setTransparent();
11.   background(0, 255, 0);
12.   image(pg, width/4, height/4);
13.   rect(width/4, height/4, width/2, height/2);
14. }
15.  
16. void setTransparent() {
17.   color transparent = color(0, 0);
18.   color red = color(255, 0, 0);
19.  
20.   pg.beginDraw();
21.   pg.background(0, 0);
22.   pg.loadPixels();
23.   for (int y=0; y<pg.height; y++) {
24.     for (int x=0; x<pg.width; x++) {
25.       if (y>pg.height/2) {
26.         pg.pixels[x+y*pg.width] = red;
27.       }
28.     }
29.   }
30.   pg.updatePixels();
31.   pg.endDraw();
32. }

Thank you for yet another fix andres!

For the above noSmooth example I am seeing different behavior between my own computers (Radeon 4800 = not cumulative vs NVIDIA GTX 570M = cumulative). No errors/warnings, just different behavior. From now on I will make sure to test on both before reporting issues.

The problem with cumulative drawing and the Radeon happens on OSX or Windows?

On Windows XP x32 SP3 with Radeon HD 4800 using the latest available drivers AMD Catalyst Display Driver 12.6 (release date: 24/07/2012).

andres, it seems a lot of the problems were fixed with your recent alterations to the source code. I just downloaded and compiled the latest svn-build 0216. And then I re-tested the three code examples from this thread with my WinXP+Radeon system. Good news. All three of them work correctly now!

Example 1
Setting transparent pixels works now under all smooth() and noSmooth() settings. Subsequent calls to begin-endDraw also leave both the transparant and opaque pixels intact.

Example 2
The drawing is cumulative when no background() call is made under all smooth() settings and for both opaque and transparent backgrounds.

Example 3
As you mentioned the mirror pixel issue has been solved, so this example now draws correctly.