[PD] colour and brightness (Was: Re: [GEM-dev] Why is [hsv2rgb] implemented as an abstraction?)

[Claude Heiland-Allen]

Hi,

Redirecting from GEM-dev as it's not about GEM development...

Mathieu Bouchard wrote:
> if you do a polar transform on YUV, you have something easier, faster 
> and more correct all at once. I usually just skip the polar transform: 
> if you apply rotations directly on YUV values, you can make very 
> believable hue shifts.

Interesting, I'm in the process of experimenting a bit with different 
colour spaces, got in a real headache with XYZ and CIE L*a*b and so on, 
but YUV's simplicity may win.

> HSV is dubious in part because the apparent brightness at maximum 
> so-called value is very variable and seems to peak high or low at 
> secondaries or primaries: compare yellow (brightness 89%) and blue 
> (brightness 11%). this really makes HSV suck sometimes. YUV does not 
> have this problem.

I tried a hybrid approach:

"$1 1 1"
  |
[hsv2rgb]
  |
[rgb2yuv]
  |
"0.5 $2 $3"
  |
[yuv2rgb]

and that seems to eliminate the bad brightness mismatches, at the cost 
of some colours seeming a bit washed out (blue) or muddy (yellow).

Attached image demonstrates the difference.


Wondering if there's some set of "perceptual brightness curves" similar 
to the "isophonic curves" [1] there are for perceived loudness of 
different frequencies and levels led me to [2], which seems very 
complicated again.

[1] http://lists.puredata.info/pipermail/pd-list/2007-01/046213.html
[2] http://www.fourmilab.ch/documents/specrend/


Claude
-- 
http://claudiusmaximus.goto10.org
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