Mmm, starting from a 5 minutes hack to a partial rewriting, adding comments, spaces, etc.

1. PImage photo;
2. final int MAX_LEVEL = 5;
3. int level = 4;
4. 
   
5. void setup() {
6.   photo = loadImage("G:/Images/dr_house_17.jpg");
7.   size(photo.width, photo.height, P2D);
8.   photo.loadPixels(); // Probably do nothing, but good practice to call it anyway
9. }
10. 
    
11. void draw() {
12.   float f = 255.0/ (pow(2, 2 * level) + 1);
13.   loadPixels();
14.   for (int x = 0; x < width; x++) {
15.     for (int y = 0; y < height; y++) {
16.       int pos = x + y * photo.width;
17.       color c = photo.pixels[pos];
18.       if (level == MAX_LEVEL) {
19.         // Just display the original image
20.       } else {
21.         // Compute the threshold to apply, depending on user input
22.         float threshold = level > 0 ? dizza(x, y, level) * f : 128;
23.         // Decompose the color in its components
24.         int red = (c >> 16) & 0xFF;
25.         int green = (c >> 8) & 0xFF;
26.         int blue = c & 0xFF;
27.         if (key == 'c') {
28.           // Above threshold, black, otherwise, white
29.           int r = threshold >= red ? 0 : 255;
30.           int g = threshold >= green ? 0 : 255;
31.           int b = threshold >= blue ? 0 : 255;
32.           // Recombine the color channels
33.           c = (r << 16) | (g << 8) | b;
34.         } else if (key == 'n') {
35.           // Black & White, compare threshold to the average of the three channels
36.           if (threshold >= (red + green + blue) / 3.0)
37.             c = color(0); // Black
38.           else
39.             c = color(255); // White
40.         }
41.       }
42.       pixels[pos] = c;
43.     }
44.   }
45.   updatePixels();
46. }
47. 
    
48. void mousePressed() {
49.   if (--level < 0) level = MAX_LEVEL;
50.   println(level);
51. }
52. 
    
53. int dizza(int i, int j, int n) {
54.   if (n == 1) {
55.     return (i % 2 != j % 2 ? 2 : 0) + j % 2;
56.   } else {
57.     // Recursive call
58.     return 4 * dizza(i % 2, j % 2, n-1) + dizza(int(i/2), int(j/2), n-1);
59.   }
60. }

Does do nothing.

If you read the code more attentively, you will see that in draw() there is a test on key.
(I would have just handled this with a keyPressed event, less hidden and cleaner. And it would allow a default...)

Hi guys, I'm a "turbo noob" but even if I don't understand much of dithering algorithms