masking an image with a gradient

gradient_mask.pde

PImage theImage;
PImage theMask;

int imageWidth = 140;
int imageHeight = 140;

void setup(){
  size(500,500);
    noStroke();

  theImage = loadImage("cat.png");
  theMask = loadImage("mask.png");
}

void draw(){
  background(255);
  image(theImage, 0, 0);
  
  float percentHidden = map(mouseY,0, width -1 , 0, 1); // 0.0 to 1.0
  image(theMask, 0, imageHeight * percentHidden);
  
  fill(255);
  rect(0,0, imageWidth, imageHeight * percentHidden);
}