selectivecolour - highly experimental clone of Photoshop's Selective Colour as filter

[wonkey_monkey]

WARNING: This filter is highly experimental, and also probably very badly written. It was written by me for very specific purposes, but I thought it might just be useful to somebody, somewhere.

Update: the filter's now written a bit better, doesn't suffer from pitch problems, and is multithreaded.

This filter partially copies the actions of Photoshop's "Selective colour" image adjustment action. Apart from an input clip (which must be RGB32) it takes 19 arguments.

The first 18 are floats in the range -1 to 1 (other values are accepted, but that behaviour is not well defined). These are in groups of three, representing the Cyan, Magenta, and Yellow %ages (normalised) present in Photoshop's Selective Color dialogue for each of the six colour sections (reds, yellows, greens, cyans, blues, magentas).

The last is a boolean to choose either relative (false) or absolute (true) mode, as per the Photoshop dialogue box.

Usage:

avisource("...")
converttorgb32
selectivecolour(last,\
0.5,0,0,\
0.25,0.1,0,\
1,-0.5,-0.25,\
-0.5,0.5,-0.5,\
0.1,0.2,0.3,\
-0.1,-0.2,-0.3,\
true)

The six triples act on the following sets of pixels (a pixel may be a member of up to 2 adjacent sets):

Reds: those pixels where R>G and B
Yellows: R and G>B
Greens: G>R and B
Cyans: G and B>R
Blues: B>R and G
Magentas: R and B>G

The first number in a triple alters the Red channel, the second the Green channel, the third the Blue channel. Because Photoshop calls these values Cyan, Magenta and Yellow, the numbers work backwards - if you want to increase the intensity of a channel, use a negative number, and vice versa.

The filter doesn't currently implement the "Black" value in Photoshop.

The filter may fail to work correctly when a clip has been cropped horizontally (because source and destination may not have the same pitch). This bit no longer true!

Any use to anyone?

David