float[][] kernel = { {-1, 0, 1},
                     {-2, 0, 2},
                     {-1, 0, 1} };
size(100, 100);
PImage img = loadImage("arch.jpg"); // Load the original image
img.loadPixels();
// Create an opaque image of the same size as the original
PImage edgeImg = createImage(img.width, img.height, RGB);
// Loop through every pixel in the image.
for (int y = 1; y < img.height - 1; y++) { // Skip top and bottom edges
  for (int x = 1; x < img.width - 1; x++) { // Skip left and right edges
    float sum = 0; // Kernel sum for this pixel
    for (int ky = -1; ky <= 1; ky++) {
      for (int kx = -1; kx <= 1; kx++) {
        // Calculate the adjacent pixel for this kernel point
        int pos = (y + ky) * width + (x + kx);
        // Image is grayscale, red/green/blue are identical
        float val = red(img.pixels[pos]);
        // Multiply adjacent pixels based on the kernel values
        sum += kernel[ky+1][kx+1] * val;
      }
    }
    // For this pixel in the new image, set the gray value
    // based on the sum from the kernel
    edgeImg.pixels[y*img.width + x] = color(sum);
  }
}
// State that there are changes to edgeImg.pixels[]
edgeImg.updatePixels();
image(edgeImg, 0, 0); // Draw the new image