pnmhsy miscellaneous functions

Highlights expansion

Highlights expansion is meant to restore blown out highlight by emphasising detail in them. It does this by increasing contrast in the highlights only.

Left: original pic, highlights in statue appear blown out. Middle: ppmhsy -H 0.1. Highlights look better. Right: ppmhsy -H 1. Illustrates which parts of the picture are considered highlights.

Hue-specific saturation

Hue-specific saturation modifies saturation in a specific part of the colour wheel. The effect smoothly falls off to zero past the edges to avoid discontinuous edges. The different hue-specific saturation effects have significant overlap. The functions are designed in such a way that it is possible to completely desaturate a range of colours. Increasing saturation in neighbouring colours does have the effect of doubly increasing saturation in the area in-between the colours. Therefore, use with care, and preferably use the overall saturation setting when possible.

Hue specific saturation can be used to emphasise or de-emphasise certain parts of a picture.

Left: original. Right: greens and cyans deaturated, ppmhsy -2 0.5 -3 0.5, to de-emphasise the background.