static Hi15Image hi15_imageMinusImage(Hi15Image img1, Hi15Image img2, double scale default 1) {
  assertSameSize(img1, img2);
  short[] pixels1 = img1.pixels, pixels2 = img2.pixels;
  int n = pixels1.length;
  short[] pixels3 = new[n];
  double shift = 16; // gray = zero
  
  for i to n: {
    short p1 = pixels1[i], p2 = pixels2[i];
    int r = iround((hi15_r(p1)-hi15_r(p2))*scale+shift);
    int g = iround((hi15_g(p1)-hi15_g(p2))*scale+shift);
    int b = iround((hi15_b(p1)-hi15_b(p2))*scale+shift);
    pixels3[i] = hi15_clamp(r, g, b);
  }
  
  ret new Hi15Image(img1.w(), img1.h(), pixels3);
}