PImage img = loadImage("lena2.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]);
    
    float r = red(img.pixels[pos]);
    float g = green(img.pixels[pos]);
    float b = blue(img.pixels[pos]);
    
    float r_sasiad = red(img.pixels[pos_sasiad]);
    float g_sasiad = green(img.pixels[pos_sasiad]);
    float b_sasiad = blue(img.pixels[pos_sasiad]);
    
    if (val == 0) {
      r = r_sasiad;
      g = g_sasiad;
      b = b_sasiad;
    }
    else if (val == 255) {
      r = r_sasiad;
      g = g_sasiad;
      b = b_sasiad;
    }
    edgeImg.pixels[y*img.width + x] = color(r,g,b);
  }
}
edgeImg.updatePixels();
image(edgeImg, w, 0);