// f: (x, y) -> 0..255
// TODO: optimize!!
static BWIntegralImage bwIntegralImageFromFunction(int w, int h default w, IF2_Int f) {
  byte[] pixels = new[w*h];
  int i = 0;
  for y to h:
    for x to w:
      pixels[i++] = (byte) f.get(x, y);
  ret BWIntegralImage(BWImage(w, h, pixels));
}