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  1. #1
    Jamesc359's Avatar
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    Default GIMP White Balance Tutorial

    If your like me you've most likely have take a photo or two that just doesn't have the right color temperature. For a good long while now I've searched for a simple way to correct that. Recently thanks to a discussion on the subject I figured out my own way, and thanks to another member found out another.

    The first and probably the easiest is using Levels.

    We're going to start off with this picture.

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    Levels

    Levels can be found under the Layer > Colors menu.


    Levels Dialog


    On the Levels dialog there is only one thing that really interests us. That's the middle eyedropper -which I've highlighted in yellow- under the All Channels section. This eyedropper sets the middle gray point. So lets select it.

    Setting the Gray Point


    This is really easy in this particular image as I already know where a good gray point is. Right on the center of the tripod's foot. You'll note that this color doesn't have to be a middle gray, it can be a light - dark gray, so long as the object in the photo is gray-ish. Otherwise the resulting color temp will be slightly skewed. You'll also note that it may take a couple of tries to get it just right.

    The Results


    That was rather easy wasn't it? But what if your image doesn't have a gray enough object? One answer to that question is the Channel Mixer.

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    Channel Mixer

    The Channel Mixer can be found under the Filters > Colors menu.


    Removing the Yellow


    Note: You'll want to make sure that the Preserve luminosity checkbox is checked.

    From the Output Channel menu select Blue. We're going to increase the amount of blue in the image from the amount of the Red and Green found in their respective channels. I used a value of Red: 50 - Green: 50, I came to this value just by experimentation.

    As can be seen in the preview field, our image has a more or less correct white balance. However the reds are just a little flat.

    Correcting the Red


    On the same dialog as before select Red from the Output Channel menu. Here we want to change the blue field to -20. Again, I arrived at this value though a little trial and error.

    The Results


    At first the Channel Mixer may seem like a more complicated method, but I feel the results are a lot easier to fine tune when compared to using Levels.
    Last edited by Jamesc359; 02-09-2008 at 10:39 PM.

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    Setting the gray point is essential. For my try on the same photo, I picked the large gray band halfway down the left-hand side, which I gave a result I preferred (granted that the choice is quite subjective and that it will render differently on different monitors).

    However, you can take some of the guess work out of deciding on a gray point. Create a duplicate of the image. Set the active colour to mid-gray (HTML Notation: 808080) and then, with the fill mode set to "difference", use that to fill the new layer with gray. The result will be funky, with the colour of each pixel reflecting how far it is from mid-gray. Next, use the threshold tool to change the picture into black and white - set the low end to 0 and move the top slider so that you get a big enough area to target with the eyedropper. The white blobs that result are the grayest areas of your image (with the photo we are working on nothing larger than scattered pixels are true gray but there is a reasonable blob of near gray on the area I had selected by eye).

    You then proceed as described above, although I normally also set black and white points and shrink the output range to about 7-248 (from 0-255).

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    Redshift is offline I'm new here!
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    I have another method that gives a less drastic result, but may give a decent starting position for your methods. Use the levels dialog, put it on logarithmic mode. Set the input levels to the min and max values for all 4 channels (value, red, green, blue). Ignore any small blips, but go for the edge of the actual full body of the histogram. This stretches the range of those values out to the max possible. In this case, the blue channel needed expanding.
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    OK to edit and repost on DPS only

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    Quote Originally Posted by Redshift View Post
    I have another method that gives a less drastic result, but may give a decent starting position for your methods. Use the levels dialog, put it on logarithmic mode. Set the input levels to the min and max values for all 4 channels (value, red, green, blue). Ignore any small blips, but go for the edge of the actual full body of the histogram. This stretches the range of those values out to the max possible. In this case, the blue channel needed expanding.
    This is basically what "Auto" does, isn't it?

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    No, when I hit auto on that picture, it expands the red and green (by shrinking the input values), but leaves the blue alone. I'm not sure how it decides the values it uses.
    OK to edit and repost on DPS only

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