maskTest

maskTest.pde

PGraphics img1;
PGraphics img2;

void setup() {
  size(400, 400);
  background(255);
  
  img1 = createGraphics(400, 400);
  img2 = createGraphics(400, 400);
  
  // draw the circle that will be masked
  img1.beginDraw();
  img1.clear();
  img1.noStroke();
  img1.fill(0);
  img1.ellipse(200, 200, 100, 100);
  img1.endDraw();
  
  // draw the mask
  img2.beginDraw();
  img2.background(0);
  for (int x = 0; x < width; x += 10) {
    img2.stroke(255);
    img2.line(x, 0, x, height);
  }
  img2.endDraw();

  alternateMask(img1, img2);

  image(img1, 0, 0);
}

void alternateMask(PImage img1, PImage img2) {
  img1.loadPixels();
  img2.loadPixels();
  for (int i = 0; i < img2.pixels.length; i++) {
    float a1 = alpha(img1.pixels[i]);
    if (a1 != 0) {
      float a2 = brightness(img2.pixels[i]);
      float r = red(img1.pixels[i]);
      float g = green(img1.pixels[i]);
      float b = blue(img1.pixels[i]);
      img1.pixels[i] = color(r, g, b, a2);
    }
  }
  img1.updatePixels();
}