Function imageproc::stats::root_mean_squared_error

pub fn root_mean_squared_error<I, J, P>(left: &I, right: &J) -> f64 where
    I: GenericImage<Pixel = P>,
    J: GenericImage<Pixel = P>,
    P: Pixel,
    P::Subpixel: ValueInto<f64>, 

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.