float[][] kernel = { { 1, 1, 1 },
                     { 1,  0, 1 },
                     { 1, 1, 1 } };
  
PImage img = loadImage("lena3.bmp");
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; 
    int pos = x + y*img.height;
    int pos_sasiad = (x+1) + y*img.height;
    float val = red(img.pixels[pos]);
    float val_sasiad = red(img.pixels[pos_sasiad]);
    
    if (val == 0) {
      for (int ky = -1; ky <= 1; ky++) {
        for (int kx = -1; kx <= 1; kx++) {
        int pos2 = (y + ky)*img.width + (x + kx);
        float val2 = red(img.pixels[pos2]);
        sum += kernel[ky+1][kx+1] * val2;
        val = sum/8;
      }
    }
    }
    else if (val == 255) {
      for (int ky = -1; ky <= 1; ky++) {
        for (int kx = -1; kx <= 1; kx++) {
        int pos2 = (y + ky)*img.width + (x + kx);
        float val2 = red(img.pixels[pos2]);
        sum += kernel[ky+1][kx+1] * val2;
        val = sum/8;
      }
    }
    }
    edgeImg.pixels[y*img.width + x] = color(val);
  }
}
edgeImg.updatePixels();
image(edgeImg, w, 0);