PImage arch;

void setup() {
  size(100, 100);
  arch = loadImage("arch.jpg");
}

void draw() {
  background(arch);
  loadPixels();
  for (int i = 0; i < width*height; i++) {
    color p = pixels[i]; // Read color from screen
    float r = red(p); // Modify red value
    float g = green(p); // Modify green value
    float b = blue(p); // Modify blue value
    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);
}