[Tig] color vision - contrast affects saturation

[Rob Lingelbach]

On Tue, March 20, 2007 9:39 am, Jack Kelly wrote:

> Apologies for hijacking this thread and asking what is probably quite a
> beginners... But this question has been nagging me for ages and this seems
> like a good time to ask!

"no question is ever too basic or simple" -- a TIG creed.  The group exists to
help answer any questions, particularly those that may seem "simple", as those
can be hard to find answers to.

> I've often wondered why tonal manipulations can affect colour saturation.
> For example, take a picture of a sunset... subtle tweaks to the RGB curve
> can have quite a profound effect on the saturation of the sky.  For example,
> I often find myself using curves to try to make the clouds a little more
> contrasty and this can result in the sky changing from a light, watery cyan
> to a really saturated cyan.  Why is this?  I guess that saturation and
> contrast are linked in as much as desaturation and reduction in contrast
> both result in making the image more gray but I can't quite wrap my head
> around the relationship between tonality and saturation - any hints would be
> most welcome!

yes, you're relating the other  side of the effect that was under discussion,
that
if a color scene suffers low contrast when the chroma is subtracted, then if
contrast (in your case RGB curve) is increased while there is chroma in the
scene,
the chroma will have to be reduced.  Experienced colorists know this
instinctively
(hence the original thread title - "instinctive contrast") and have a hand on the
chroma knob (an argument for using a dedicated panel- not a mouse or qwerty
keyboard) for an instant adjustment to the color when the contrast (including
gamma, black level, white level) is increased or reduced.   It avoids the client
saying, when you adjust the contrast, "but now the color's changed."  ....which
can open up a can of worms; it's much nicer to "couple" the color and contrast
changes by using two hands.

Now that doesn't really answer your question, it only verifies the condition, and
I think the answer can be found in the recent posts about saturation and
contrast,
if a more scientific explanation is what you need.

regards
Rob
-- 
Rob Lingelbach
http://www.colorist.org/robhome.html   rob at calarts.edu
TIG admin