PImage kitten;
int index;

void setup(){
  size(368,300);
  smooth();
  fill(0);
  kitten = loadImage("kitten.png");
  kitten.loadPixels();
}

void draw(){
  background(255);
  int mx = constrain(mouseX, 0,300);
  for (int i = 0; i < kitten.width; i++){
  color k = kitten.pixels[mx*height+i];
    rect(i, height, 2,k);
  }
}