Aspects of Contrast and White Balance

 

by Karen Hanson
When retouching pictures you should first address white balance and contrast. White balance is usually what you ought to consider first, then contrast.This order is important, because you can not set color contrast properly if the image has a color cast.White balance is concerned with the color of the light within the image and sets white as an ideal. White balance applications attempt to retouch the tint of the illumination to neutral and in order to do that, the program needs some whites or grays in the picture to calculate the correct correction tint from. There are dedicated white cards, but one can also do with a sheet of white paper or a white wall. Gray cards are manufactured for the purpose of adding a neutral gray to the photo.White balance software usually has both a manual and an automatic mode. Manual correction comes as a temperature slider, which is fine for incandescent light, but not for fluorescent light or mixed light. When opening RAW images, one normally has a temperature slider. Some RAW converters also have three color sliders for red, green and blue. Fluorescent and mixed light can be somewhat corrected with color sliders, but unfortunately color sliders usually tone the blacks and whites in an undesirable way. For automatic corrections, the software normally needs neutrals in the image, like a gray card and/or a white card. Some programs can dispense with that, but usually neutrals are needed.

There are three kinds of contrast: hue, saturation and brightness. Very few applications have more than a single slider for contrast, that addresses all three kinds of contrast at once. It is not ideal with a single slider for all three, since the result usually suffers from over saturation and colorfulness. At best the software will have a control for luminance contrast and for color contrast.

The usual way to manipulate contrast is simply by altering the difference between the individual red, green and blue values and the average value (128); like this: R= (R-128) * contrast + 128; and likewise for green and blue. This method is only suitable for images that cover the entire brightness range. What if the image is very pale or very dark? In that case you need to change the algorithm to use the average values of the image’s R, G and B channels, like this: R=(R-RAverage)*contrast + RAverage. And so on for G and B. The algorithms are essentially the same since a full brightness range image will have 128 as an average value.

If the darkest and brightest areas are not black and white a different situation arises. If that is the case, one should also be able to expand the brightness range to reach black and white. This is essentially what levels adjustment does. If one’s software does not offer the option to expand brightness range, one can do it with Photoshop’s levels adjustment like this: First convert the image to Lab mode, select the L channel only and run auto levels on that. Then convert back to RGB mode.

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