Desaturation is the removal of color information from an image. It converts color images to black and white images. Desaturation can also be done partially to reduce the punch of overpowering colors. This is useful in digital photos of people where the subjects' faces come out too red.

Desaturating an image on the computer is done by combining the red, green, and blue information into a single brightness value for each pixel. A common method is to simply average the three channels. In other words, brightness = (red + green + blue) / 3.

This seems straight-forward, but the results are not very realistic looking. That's because the eye is more sensitive to green than red, and more sensitive to red than blue. Therefore, the channels need to be mixed in unequal proportions. Empirical tests have arrived at a mixture of 30% red, 59% green, and 11% blue. This combination more accurately reflects how our eye judges brightness. With this formula, yellow appears much brighter, almost white, just as it does in real life. The averaging method causes yellow to be a muddy gray.

Compare the sample below. The black and white photo at left was desaturated using a simple average. The one at right was desaturated with the proper color proportions.

Original image
Mathematical desaturate	Perceptual desaturate

Notice the yellow flower petals in particular. The averaging method flattens out a lot of details, while the perceptual desaturate is much more authentic. The averaging method gets muddied from too much from the blue channel, which, as you can see below, is barely present in this photo.

The red, green, and blue channels from the color photo above.

Adobe Photoshop is a bit mixed in its desaturation methods. For example the Desaturate command (under "Image", "Adjustments", "Desaturate") averages, but converting the image to grayscale (under "Image", "Mode", "Grayscale") is perceptual. To avoid this ambiguity you can desaturate manually with the channel mixer. Go to "Image", "Adjustments", "Channel Mixer". Check the "Monochrome" checkbox at the bottom. Then type 30, 59, and 11 in the red, green, and blue boxes, respectively. Click "OK".

Photoshop's channel mixer dialog.

Optionally, you can save these settings to a .cha file from the channel mixer dialog by clicking "Save..." so that you don't have to remember the numbers next time.