float[][] kernel = {
  { -2,0,.5 },
  { 0,0,-.2 },
  {-1,0,4 }};

size(533,400);

PImage landscape = loadImage("landscape.png");
landscape.loadPixels();
PImage edge = createImage(landscape.width, landscape.height, RGB);

for (int x = 1; x < landscape.width-1; x++) {
  for (int y = 1; y < landscape.height-1; y++) {
    float sum = 0;
    for (int kx = -1; kx <= 1; kx++) {
      for (int ky = -1; ky <= 1; ky++) {
        int pos = (y + ky)*width + (x + kx);
        float val = red(landscape.pixels[pos]);
        sum += kernel[ky+1][kx+1] * val;
      }
    }
    edge.pixels[y*landscape.width + x] = color(sum);
  }
}
edge.updatePixels();
image(edge, 0, 0);
//background(255);