Pasi Savolainen <[EMAIL PROTECTED]> writes:

> * 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.

i think so.

>>> 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. 

thank you, i've got the point.

more interesting, in the levels window, if i pick a point p1 as grey using grey
picker, then pick another point p2 as another grey point. in this case, what
gimp will do? i think there are two candidates,

1, do adjust to make p1 grey, and from that point on do another adjust to make
p2 grey. the result will be: p1 and p2 are both natural grey.

2, as if user did not click p1, gimp adjust p2 from scratch and simply forgot
what he did before.

which one above is the answer?

> (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).
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