Correcting color cast problem *manually*

I use GIMP to retouch pictures. Unlike PhotoShop Elements, it doesn't
have a color cast tool that can automatically correct white balance
problems based on a supposedly white/grey part of the picture.

Can somebody tell me how to do color cast correction using the *same
approach*, but *manually* (i.e. by sampling a supposedly grey color &
adjusting the RGB channels)?

Thanks.

 

From: John Smith

I don't know GIMP, but in Photoshop if you KNOW you have a good neutral
(ideally a grey card) what I'd do is put a measurement point on it with the
Color Sampler Tool (if GIMP doesn't have this you can just jot down the RGB
numbers read from a given point).

Note how far these RGB values are from neutral (ie, all should be the same
value but they're not if you have a cast so you need to get them closer to each
other), then use curves on each channel separately to pull the mid-tone values
up or down to where they match.

I'd do this on a curves Adjustment Layer so you can modify it later as needed
(dunno if GIMP has adjustment layers). If you put a measurement point on with
the CST you can watch the value change in the Info palette dynamically as you
move the curves ... if GIMP doesn't offer this option then you'll need to make
a curves move and then measure the nominal gray point and make another move if
required.

This should remove the cast, assuming you started with a valid grey point and
you got the RGB values to be pretty close to each other when done with the
curves.

Bill

 

GIMP does have a color sampler tool.

That's the tricky part. Given a particular grey sample, I don't know
mathematically what kind of transfer functions to apply to the RGB
channels to bring the colors back to normal.

GIMP supports layers.

 

You don't need to. Select your sample that needs to be neutral grey. Let's
say it comes out as red - 160, green - 155, blue - 150.

Now open the curves editor. Select "Red" from the drop down selector, and
click somewhere in the curves box to create a new point. Drag it about until
it says "X 160, Y 155". Now select Blue (we'll leave green alone, since it's
the one in the middle) and do the same, but drag the point so it reads "X
150, Y 155".

Click "OK". Your image is now white-balanced.

You can do this for multiple points as well, so you can white-balance the
highlights and midtones seperately, for example.

This is a bit easier in Photoshop, as you can just type the source and
target values in directly, but this doesn't seem to be available in this
version of The GIMP (1.2.4).

Adjustment layers are not the same thing as ordinary layers. It looks like
1.2.4 doesn't have them, so ignore this bit.

 

Just use the Curves tool on the separate channels. Sounds like Chris Brown
knows GIMP well (other reply to this thread) so follow his good advice (the
implementation is a bit different in Photoshop but the end result should be the
same).

If I were you I'd make up a test color, say RGB => 130/125/165 or similar, with
a blue cast and then practice (with curves) getting the balance right. Once
you've done it a few times it's not too hard. A blue cast is common when
shooting slide film in open shade, especially at altitude and that's my most
common color correction, though if you're shooting daylight film indoors you
might have a yellowish cast instead ... make a couple of test files to practice
on with the cast you're most likely to see in your images.

As for the "tricky part", for me the # 1 tricky part is locating a good neutral
in my image, which can be harder than you think since other than white clouds
there aren't THAT many truly neutral objects in a good landscape photo (for
example). The # 2 tricky part is removing the cast without slightly throwing
off the colors in another part of the image, which often requires a second pass
with Curves in a different tonal range. The actual tonal/color transfer with
Curves is relatively straightforward once you know the values you want to
target, I feel.

Bill

 

Actually, I'm not *that* familliar with GIMP - mostly use Photoshop, but I
have used GIOMP a bit, do the manual white balancing thing in Photoshop a
fair bit, and just had a play with GIMP until I was able to do the same
thing.

I find that using a bright object seldom gives good results either. For a
simple white-balance, you want something that has good, mid range values
(100-200ish) in all 3 channels.

 

The procedure to do manual white balance with GIMP is relatively
straightforward, but somewhat time consuming. If you really want to do
this, it's well documented in the "removing colour casts" section of the
online gimp book at:-

http://gimp-savvy.com/BOOK/

That said, I think you'd be better off using one of the white balance
plug-ins available for GIMP, which let you pick a white point in the
image, and balance based on that. Look at:-

http://phpweb.hig.no/oey_kola/yuvadj/
http://www.geocities.com/lasm.rm/wb2.html

It's also well worth reading Eric Jeschke's tutorials on GIMP use for
digital photography for any photographers out there who use GIMP as their
primary image editing tool. They're at:-

http://cs.uhh.hawaii.edu/~jeschke/photography/articles/gimp/tutorials.shtml

HTH,

Mike.

 

Thanks (and to the other posters who have responded as well).