Function unsharpen 

pub fn unsharpen<I, P, S>(
    image: &I,
    sigma: f32,
    threshold: i32,
) -> ImageBuffer<P, Vec<S>>
where
    I: GenericImageView<Pixel = P>,
    P: Pixel<Subpixel = S> + 'static,
    S: Primitive + 'static,

Performs an unsharpen mask on the supplied image.

Arguments:

• sigma - is the amount to blur the image by.
• threshold - is the threshold for minimal brightness change that will be sharpened.

This method typically assumes that the input is scene-linear light. If it is not, color distortion may occur.

See Digital unsharp masking for more information.