float[][] kernel = { { -1, -1, -1 },
                     { -1,  9, -1 },
                     { -1, -1, -1 } };
  

PImage img = loadImage("house.jpg"); 
int w = img.width;
int h = img.height;
size(2*w,h);
image(img, 0, 0); 
img.loadPixels();

PImage edgeImg = createImage(img.width, img.height, RGB);
for (int y = 1; y < img.height-1; y++) { 
  for (int x = 1; x < img.width-1; x++) { 
    float sum = 0; 
    for (int ky = -1; ky <= 1; ky++) {
      for (int kx = -1; kx <= 1; kx++) {
        int pos = (y + ky)*img.width + (x + kx);
        float val = red(img.pixels[pos]);
        sum += kernel[ky+1][kx+1] * val;
      }
    }

    edgeImg.pixels[y*img.width + x] = color(sum);
  }
}
edgeImg.updatePixels();
image(edgeImg, w, 0);