Feb 28, 2001

Color Processing

Although the RGB model seems to match the physiology of the eye with its cones sensitive to three different colors, application of processing methods to RGB pictures frequently causes unwanted results. The eye is particularly sensitive to changes in hue or color, and finds even small changes in a picture's tonality noticeable and frequently objectionable, while it tolerates changes in intensity or saturation much more readily. Thus if modifications are to be made to a color image, any changes in an RGB format must change all the colors equally so as to preserve the hue. This will be true if the transformation is linear and applied to each of the RGB planes. Non-linear operations such as histogram equalization and median filtering should generally not be attempted. If these latter operations are desired, then the RGB planes should be changed to HSI planes, and the modifications applied to just the I-plane, or possibly to the I-plane and the S-plane. The H-plane should almost always be left alone. One exception is when the "color-balance" is incorrect, and the image seems to be wrongly tinted, e.g., white does not appear white, or flesh colors do not correspond to our expectation of flesh colors. In these cases, a small addition or subtraction can sometimes be made to the H-plane to rotate the color wheel and change the overall image tints.

Examples of modifications to:

   tree.tif

, , (koala red.tif, koala green.tif, koala blue.tif  --  need all three)

  baboon.jpg

Last modified on February 28, 2001