Identifies modes used for changing channel depths of an image.

Values:

IG_DEPTH_CHANGE_NO_SCALE	Changes channel depth without scaling of channel values. This results in unchanged channel values (if the new depth is large enough to accommodate the values). The image will likely change in appearance using this option.
IG_DEPTH_CHANGE_SCALE	Changes channel depth with scaling of channel values. This causes channel values to be scaled so that the ratio of new channel value to new channel depth is as close as possible to the ratio of old channel value to old channel depth. The image will likely maintain the same appearance using this option, unless channel depths are reduced too much to maintain accuracy.