PImage danimage;

void setup() {
  size (400,400);
  danimage = loadImage("danimage.jpg");
}

void draw() {
  background(danimage);
  loadPixels();
  for (int i = 0; i < width*height; i++) {
    color p = pixels[i];    //read color from screen
    float r = red(p);       //change value of red
    float g = green(p);     //change value of green
    float b = blue(p);      //change value of blue
    
    float bw = (r+g+b) / 3.0;
    bw = constrain(bw + mouseX, 0,255);
    pixels[i] = color(bw);  //assign modified value
  }
  updatePixels();
  line(mouseX,0,mouseX,height);
}