// have the same bounds.
func averageDelta(m0, m1 image.Image) int64 {
	b := m0.Bounds()
	return averageDeltaBound(m0, m1, b, b)
}

// averageDeltaBound returns the average delta in RGB space. The average delta is
// calculated in the specified bounds.
func averageDeltaBound(m0, m1 image.Image, b0, b1 image.Rectangle) int64 {
	var sum, n int64
	for y := b0.Min.Y; y < b0.Max.Y; y++ {