* Steven Woody <[EMAIL PROTECTED]>:
> Pasi Savolainen <[EMAIL PROTECTED]> writes:
>>> when i worked on individul channels, i always got color cast. i just can not
>>> image that how people working on channels can still keep color balance well
>>> sicne you can not do the r,g,b simultanously. any tips there?
>> This depends heavily on the image that's worked on.
>> Generally I'd say procedure is to go through r,g,b and adjust only black
>> and white point, leaving gamma (middle number) as it is. Most images
>> have about linear channels, only shifted+compressed in some way, so this
>> will get you mostly castless image (again, this is very taste-specific
>> operation and some people, of which I may be one, just don't see much
>> difference in some shades). When image has cast that I notice, I start
>> working with gamma sliders and try to achieve something that look a bit
>> like the anticolor of the cast.
>> For example if cast is blue, go to blue color and move gamma to the
>> right, making it smaller (the colorstrip will turn yellow). In case of
>> yellow cast one would move it to the left. etc.
>> Often after one cast is removed, another creeps up, so this is a game of
>> balance.
>> Turning preview on/off helps a lot, as does having a <Image>->Duplicate
>> around at all times.
> i think you were talking about level tool, did you mean you will not prefer
> curve tool?

This all is doable trough curves tool, it's just that I'm used to only
correcting contrast in curves. It's also not that long that curves tool
didn't have channel histogram visible, so adjusting black/white wasn't
as 'precise'.
If I understand correctly, 'Gamma' on Levels is same as if you add
control point in the centre of Curves tool and move it up/down.

>> Using color pickers and choosing black/grey/white colors from _really_
>> black grey and white spots will likely get you into a good start. They
>> just tend to clip whites/blacks too much, losing detail that way.
> okay, i got your point about using the black/white point in the picture to set
> the relative ends of level tool. but how you use grey?

color pickers should be used in 'Value' channel, grey estimates relative
balance of channels, which is too strong, which is too weak. For example
if you click on color RGB(120, 130, 90), it will (likely) boost blue and
dampen green channel so that they will become rgb(120,120,120) where you
clicked. (this depends on how exactly GIMP decides the value that it
tries to reach, the main point is that it will become similar across
channels by adjusting gamma of individual channels).

   Psi -- <http://www.iki.fi/pasi.savolainen>

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