pub fn root_mean_squared_error<I, J, P>(left: &I, right: &J) -> f64
where
    I: GenericImageView<Pixel = P>,
    J: GenericImageView<Pixel = P>,
    P: Pixel,
    P::Subpixel: Into<f64>,
{
    mean_squared_error(left, right).sqrt()
}
Returns the square root of the mean of the squares of differences
between all subpixels in left and right. All channels are considered
equally. If you do not want this (e.g. if using RGBA) then change
image formats first.
src/stats.rs