[Tig] color vision - contrast affects saturation

[Rob Lingelbach]

>
>> I've often wondered why tonal manipulations can affect colour  
>> saturation.

I'd like to know what you mean Jack by 'tonal manipulation' - does it  
mean
manipulations of luminance, or of some component of color?

On Mar 20, 2007, at 2:36 PM, TSassoon at aol.com wrote:

> Saturation results from differences between color channels. If all  
> channels
> are the same, you have a shade of gray. At the other extreme, if  
> one channel is
> full on, and the others off, then you have a primary color at full  
> brightness
> and saturation. If one is full on, and the others half on, then you  
> have a
> primary color at full brightness and half saturation.
>
> When you, for instance compress levels, you're also increasing the  
> contrast
> between channels, and thus increasing saturation.

on reflection, this makes a lot of sense and accounts I believe for  
the contrast
phenomenon we've been discussing *in the electronic realm* (video).    
I don't
know however if you can apply this explanation to non-electronic  
contrast
differences when you extract color from an image observed for example  
by the
achromat, where the effect takes place entirely in the brain/eye system.

I'm wondering if there's a way one can remove "only color" from a  
painting
(subtractive color as opposed to additive color in video) and see  
that the
resulting grayscale image suffers a lack of contrast?  Because if the
achromat sees a lack of contrast when looking at color images due to his
lack of color sense, then we are not referring to 'channels' any longer,
correct?

Rob
--
Rob Lingelbach
http://www.colorist.org/robhome.html
rob at colorist.org  rob at lingelbach.us