Computer Graphics with Photoshop - Understanding 'Selective Color' `?

Author

Understanding 'Selective Color' `?

Hannes F.

2006-07-13, 6:17 pm

Hi,

i'm tryng to understand the 'Selective Color' command in photoshop.
I managed to figure out how the command changes the 'control' colors
(yellow, green, blue, red, cyan, magenta, black, grey, white). E.g.
mode 'relative' transforms the color to CMY, multiplies the (C,M,Y)
factor with the (C,M,Y) value of the respective color and re-transforms
the color to RGB.
But i couldn't figure out how the command changes the 'intermediate'
colors. Must be some sort of interpolating, but there are many possible
choices for interpolation, and in which color model is interpolated ?

Is there some 'selective color' expert who can give me a hint ?

Mike Russell

2006-07-13, 6:17 pm

"Hannes F." <hannes@fassold.com> wrote in message
news:1152815121.940395.32390@b28g2000cwb.googlegroups.com...
> Hi,
>
> i'm tryng to understand the 'Selective Color' command in photoshop.
> I managed to figure out how the command changes the 'control' colors
> (yellow, green, blue, red, cyan, magenta, black, grey, white). E.g.
> mode 'relative' transforms the color to CMY, multiplies the (C,M,Y)
> factor with the (C,M,Y) value of the respective color and re-transforms
> the color to RGB.
> But i couldn't figure out how the command changes the 'intermediate'
> colors. Must be some sort of interpolating, but there are many possible
> choices for interpolation, and in which color model is interpolated ?
>
> Is there some 'selective color' expert who can give me a hint ?

Probably not, since Adobe does not document their color algorithms, nor
discuss them publicly. My guess is that Selective color is implemented in
Lab mode, and that ranges of a and b are used to define a particular range
of hues. Most of the other color calculations use Lab, with the exception
of layer blending calculations, which seems to be mostly based on HSB.

The free LabMeter download at curvemeister is a good experimental image -
for example you can use Select>Color Range and see the resulting square
selection regions on the Lab image.
http://www.curvemeister.com/tutoria...Meter/index.htm
--
Mike Russell
www.curvemeister.com/forum/