Function retroimg::reduce

pub fn reduce<I>(
    img: &I,
    nwidth: u32,
    nheight: u32
) -> ImageBuffer<I::Pixel, Vec<<I::Pixel as Pixel>::Subpixel>>where
    I: GenericImage + 'static,

This is just another name for a cubic resize.

Prefer using small values of nwidth and nheight.