[Image-SIG] Removing specific range of colors from scanned image

[Guy K. Kloss]

On Mon, 27 Apr 2009 22:56:00 jcupitt at gmail.com wrote:
> RGB isn't the best colour space for this, but it would sort-of work.

Another issue is that RGB is (usually when encountered "in the wild") the sRGB 
colour space. sRGB as well as most other RGB colour spaces is *not* a linear 
colour encoding per channel. They have been encoded with a geometrical 
function with an exponent of (in the case of sRGB) about 2.2. This is a 
historical effect to compensate for the non-linearity of CRT (cathode ray 
tube) based screens.

If you want to be at least "sort of" linear, you need to undo the gamma 
compensation first:

c' = c ** (1 / gamma)

gamma: the gamma coefficient, usually in the case of sRGB = 2.2
c': linearised RGB colour channel
c: un-linearised (s)RGB colour channel

Now you can perform quantitative sensible computations on the R'G'B' colour 
tuples, like comparing colour distance as the Euclidean distance mentioned in 
John's post.

Guy

-- 
Guy K. Kloss
Institute of Information and Mathematical Sciences
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