Hi,

I'm not an expert, but I've seen a lot of people asking questions about masks. I've not found so many simple informations about the subject. I made this small sketch to show beginners how they can use PGraphics as masks.
Mask applet

1. /**
    * Alpha Mask with PGraphics
    *
    * "img.jpg" is the background image that I load in backgroundimage."mask.jpg" is the grayscale image
    * used as a mask. It's loaded in maskimage. Then maskimage is drawn to a PGraphics object and this
    * PGraphics is used as the mask. (I couldn't use a PImage as a mask if it was not the applet's size.)
    * They must be in this sketch's data folder. Left click to change the mask and right click to
    * invert it...
    */
   
   PImage maskimage;
   PImage backgroundimage;
   PGraphics pg;
   String maskname="mask";
   int i=0;
   boolean inver=false;
   
   void setup() {
     size(800, 600);
     backgroundimage = loadImage("img.jpg");
     maskimage = loadImage(maskname+"0.jpg");
     backgroundimage.loadPixels();
     pg = createGraphics(800, 600, P2D); 
     imageMode(CENTER);
   }
   
   void draw() {
     background(0); 
     pg.beginDraw();
     pg.background(0);
     pg.fill(255);
     pg.image(maskimage, mouseX-maskimage.width/2, mouseY-maskimage.height/2);
   
     if (inver) {
       pg.filter(INVERT);
     }
     pg.endDraw();
     backgroundimage.mask(pg);
     image(backgroundimage, width/2, height/2);
   }
   
   void mousePressed() {
     if (mouseButton == RIGHT) {
       inver=!inver;
     }
   }
   void mouseReleased() {
     if (mouseButton == RIGHT) {
       inver=!inver;
     }else{
     i++;
     if (i>8) {
       i=0;
     }
     maskimage = loadImage(maskname+i+".jpg");
     }
   }