PImage img;
size(600, 400);
img = loadImage("robotmarko-g.jpg");
loadPixels(); 
img.loadPixels();

float[][] filter = {{ 0,  1, 0 }, 
                    { 1, -4, 1 }, 
                    { 0,  1, 0 }};

for (int y = 1; y < img.height-1; y++) 
  for (int x = 1; x < img.width-1; x++) {
    float response = 0;
    for (int ky = -1; ky <= 1; ky++) 
      for (int kx = -1; kx <= 1; kx++) {
        int index = (y + ky) * img.width + (x + kx);
        float r = brightness(img.pixels[index]);
        response += filter[ky+1][kx+1] * r;
      }
    pixels[y*img.width + x] = color(response);
  }
updatePixels();