color blue = color(0, 0, 255);
color yellow = color(255, 255, 0);

PImage gr, maskImage;
PGraphics gradient, mask;

void setup() {
  size(600, 400, P2D); 
  gr = createImage(width, height, RGB);
  mask = createGraphics(width, height);
  gradient = createGraphics(width, height, P2D);

  gradientRect(0, 0, width, height, blue, yellow);
  createMask();
}

void draw() {
  background(0);
  gr.mask(mask);
  image(gr, 0, 0);
}

void gradientRect(int x, int y, int w, int h, color c1, color c2) {
  gradient.beginDraw();
  gradient.beginShape();
  gradient.fill(c1);
  gradient.vertex(x, y);
  gradient.vertex(x, y+h);
  gradient.fill(c2);
  gradient.vertex(x+w, y+h);
  gradient.vertex(x+w, y);
  gradient.endShape();
  gradient.endDraw();
  gr = gradient.copy();
}

void createMask() {
  mask.beginDraw();
  mask.background(0);
  mask.stroke(255);
  mask.strokeWeight(20);
  for (int i=0; i<5; i++) {
  mask.line(50+i*100, 50, width-50, height-50);}
  
  mask.endDraw();
}