GIMP/Levels

The levels tool is an easy and powerful tool for adjusting the tones of an image. It allows more precise control of contrast than the Brightness-Contrast tool, but doesn't give the user the complete control of Curves.

Input Levels
Input levels control the contrast of the picture. The end points control the black points, and the white points. The eyedropper buttons next to the black and white input level markers allow black and white points to be selected in the image. The middle pointer can be used to selected the gamma correction value for the image.

This tool can be used to simulate setting a white point. For images such as Image:PSF B-70002.png, setting a white point is critical in extracting useful images such as Image:Backstroke (PSF).png.