Correcting color cast problem *manually*

Discussion in 'Digital Photography' started by John Smith, Dec 17, 2003.

  1. John Smith

    John Smith Guest

    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)?

    John Smith, Dec 17, 2003
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  2. John Smith

    Bill Hilton Guest

    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

    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

    Bill Hilton, Dec 17, 2003
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  3. John Smith

    John Smith Guest

    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.
    John Smith, Dec 17, 2003
  4. John Smith

    Chris Brown Guest

    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.
    Chris Brown, Dec 17, 2003
  5. John Smith

    Bill Hilton Guest

    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

    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 Hilton, Dec 17, 2003
  6. John Smith

    Chris Brown Guest

    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
    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.
    Chris Brown, Dec 17, 2003
  7. 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:-

    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:-

    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:-


    Mike Brodbelt, Dec 18, 2003
  8. John Smith

    John Smith Guest

    Thanks (and to the other posters who have responded as well).
    John Smith, Dec 18, 2003
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