R. Craig Collins > Intro to Computer Graphics >Brightness and Contrast
Brightness and Contrast © R. Craig Collins, 2008
Many digital images, whether from a camera, or a scanner, often seem to be too dark or two light. This can especially be troublesome if you plan on combining images.
Most high end computer graphics software has tools to adjust the brightness and contrast. However, for new users this is more art than science. Brightness has to do with how light or dark the image appears... often you can 'add light' to a photo in a dark room, or tone down an overly radiant daylight photo so the sky is bluer, rather than a washed-out with white.
Compared to brightness, contrast is fairly difficult to explain. While at first blush it appears to brighten or darken, it actually is sharpening or dulling the differences within the image. Adding contrast tends to make items stand out, removing contrast allows items to sort of fade into the background.
In the GIMP, the brightness and contrast tool is accessed from the Tools/ColorTools menu, and presents two sliders: one to adjust brightness, and of course one to adjust contrast. This tool works best if the Preview box is selected.
You first perhaps drag the brightness control a little... then look at the effect to decide if you need to move it more, or back up some. It is often preferable to first adjust the brightness, then try the contrast.
Click here for a video demonstrating the GIMP brightness-Contrast tool.