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.
