The White Balance command automatically adjusts the colors of the active layer by stretching the Red, Green and Blue channels separately. To do this, it discards pixel colors at each end of the Red, Green and Blue histograms which are used by only 0.05% of the pixels in the image and stretches the remaining range as much as possible. The result is that pixel colors which occur very infrequently at the outer edges of the histograms (perhaps bits of dust, etc.) do not negatively influence the minimum and maximum values used for stretching the histograms, in comparison with Stretch Contrast. Like “Stretch Contrast”, however, there may be hue shifts in the resulting image. This command operates on layers from RGB images. If the image is Indexed or Grayscale, the menu item is insensitive and grayed out.

10.22.2.  “White Balance” example

Figure 15.92.  Original image

The active layer and its Red, Green and Blue histograms before “White Balance”.

Figure 15.93.  Image after the command

The active layer and its Red, Green and Blue histograms after “White Balance”. The pixel columns reach the right end of the histogram (255): pure white is created (255, 255, 255).

Histogram stretching creates gaps between the pixel columns, giving it a striped look.