sclass G22BurnIn {
  int w, h;
  int[] mask;
  
  void processFrame(BufferedImage frame) {
    int w = frame.getWidth(), h = frame.getHeight();
    if (mask == null || this.w != w || this.h != h) {
      this.w = w;
      this.h = h;
      this.mask = new int[w*h];
    }
    
    // We assume the frame has no transparency
    var gp = grabbableIntPixels_fastOrSlow(frame);
    int n = w*h, iMask = 0, iFrame = gp.offset;
    
    for y to h: {
      for x to w: {
        int fpix = getPixel frame x y
        mpix <- getPixel theMask x y
        theMask[iMask++] = ...
      }
    }
  }