docs: public static float Variance(ImageSource fastBitmap, float mean, int startX, int startY, int width, int height) {
  
          float sum = 0;
          if (fastBitmap.isGrayscale()) {
              for (int i = startX; i < height; i++) {
                  for (int j = startY; j < width; j++) {
                      sum += Math.pow(fastBitmap.getRGB(j, i) - mean, 2);
                  }
              }
              return sum / (float) ((width * height) - 1);
          } else {
              throw new IllegalArgumentException("ImageStatistics: Only compute variance in grayscale images.");
          }
      }