Custom White Balance

From OSP Wiki

This tutorial was last tested for the Gimp 2.4 and 2.2.

Overview

After reading this tutorial, you should be able to create a custom white balance curve. This curve can be used to correct red/cyan, blue/yellow as well as green/magenta casts on all photos taken under the same lighting conditions as the scene you used to create the curve.

Prerequisites

• You have a photograph with neutral content in high tones, mid tones and low tones. That is, you have light gray, mid gray and dark gray in you original scene. Since you want to balance the image, they will not appear as gray, and that's exactly what will be fixed.
• You are proficient with the Curves tool.

Steps

1. Open the reference photograph.
2. Click <Image> / Layer / Color / Curves

   1. Alternatively <Image> / Color / Curves for GIMP 2.4

3. Click on the reference light gray object in the photo. A vertical line will appear with a x value in Curves.
   1. The x value is the highest channel value for that sample. It is also the strongest component in the color cast you want to remove.
4. On the channels combo box, inspect the vertical line and x values for the other channels.
   1. From the inspection you should choose an appropriate value for that sample. You could just pick one of them or calculate the mean.
5. Create a point in the curve of each red, green and blue channel with the x value exactly the same as the value marked by the vertical line. In each channel, make it so that all y values are equal to the sample value you determined in the previous step.
   1. This will make the object you selected turn into neutral gray. Inspect by enabling Preview in the Curves dialog and looking at the object in the picture.
6. Repeat the procedure for the medium and dark gray reference objects in the picture. Tune until satisfied with the results.
7. When done, save the channel to a file.
8. Click OK.

Applying to other photos

1. Open the photo. It will only work if it was shot in the same lighting conditions. If the lighting conditions changed slightly, it will provide a good starting point.
2. Open the Curves tool again.
3. Click Open and select the file you just saved.
   1. Check with the Preview if you are satisfied with the results.
4. Click OK.

Tips

• You can make your life easier by finding neutral gray references in the lighting condition you're shooting and use that photo as reference, instead of searching your normal pictures for neutral candidates.
• People usually don't like much exact white balance. Instead, a little warmth in the colors is often pleasing to the eye. To achieve this, make the red channel points slightly higher than the value you've determined and the blue channel slightly lower. Blue channel variations are usually more sensitive and you might want to leave it alone.
• Green channel is crucial for skin tones - too green and people look pale and sick, too little (too much magenta) and people appear raging and pimples pop.
• If you want to increase contrast, choose the higher of the three channels as reference value for high tones and the lower of them as reference for the low tones. Conversely, if you want to decrease the contrast, do the opposite.