A few things:

Thank you very much for the thorough explanation and example, makes a lot more sense to me now.

It's not so much PImage, but the fact that my method wasn't picking up correctly the semi-transparency in the original image. So it was only copying the fully transparent areas, where if one where to do it visually correct it would have to check the alpha of each pixel and copy that from the original to the final image. But it was just a basic example that could be further improved upon given specific needs.

However to prevent some hassle you could also turn it around. Just start with the original image and then copy the transparency from the mask in a customMask function. Now there is only some minor crudeness around the outer edges. Like the comment above, to solve this you would just have to finetune the checking/copying process further.

Code Example

1. PGraphics drawing, mask;
2.  
3. void setup() {
4.   size(400, 400);
5.   drawing = createGraphics(width, height, JAVA2D);
6.   mask = createGraphics(width, height, JAVA2D);
7.   drawImage();
8.   drawMask();
9.   customMask();
10.   imageMode(CENTER);
11. }
12.  
13. void drawImage() {
14.   drawing.beginDraw();
15.   drawing.smooth();
16.   drawing.noStroke();
17.   for(int y = 0; y < height; y+= int(height/30)) {
18.     for(int x = 0; x < width; x+= int(width/30)) {
19.       drawing.ellipse(x+5, y+5, 10, 10);
20.     }
21.   }
22.   drawing.endDraw();
23. }
24.  
25. void drawMask() {
26.   mask.beginDraw();
27.   mask.smooth();
28.   mask.noStroke();
29.   mask.fill(255);
30.   mask.triangle(width/2, 30, width-30, height-30, 30, height-30);
31.   mask.endDraw();
32. }
33.  
34. void customMask() {
35.   color t = color(0,0);
36.   for (int i=0; i<drawing.pixels.length; i++) {
37.     if (mask.pixels[i] == t) {
38.       drawing.pixels[i] = t;
39.     }
40.   }
41. }
42.  
43. void draw() {
44.   background(0, 0, 255);
45.   image(drawing, mouseX, mouseY);
46. }

Another way would be to use texture mapping instead of a mask.

Code Example

1. PImage drawing;
2.  
3. void setup() {
4.   size(400, 400, P2D);
5.   textureMode(IMAGE);
6.   noStroke();
7. }
8.  
9. void draw() {
10.   background(0, 0, 255);
11.   drawing = drawImage();
12.   translate(mouseX, mouseY);
13.   beginShape();
14.   texture(drawing);
15.   vertex(0, -height/2+30, width/2, 30);
16.   vertex(width/2-30, height/2-30, width-30, height-30);
17.   vertex(-width/2+30, height/2-30, 30, height-30);
18.   endShape();
19. }
20.  
21. PImage drawImage() {
22.   PGraphics drawing = createGraphics(width, height, JAVA2D);
23.   drawing.beginDraw();
24.   drawing.colorMode(HSB, 360, 100, 100);
25.   drawing.smooth();
26.   drawing.noStroke();
27.   for(int y = 0; y < height; y+= int(height/30)) {
28.     for(int x = 0; x < width; x+= int(width/30)) {
29.       drawing.fill((x+y+frameCount*3)%360, 100, 100);
30.       drawing.ellipse(x+5, y+5, 10, 10);
31.     }
32.   }
33.   drawing.endDraw();
34.   return drawing;
35. }

Thanks again