Details

Histogram equalization is a classic image processing technique that adjusts the contrast of an image so that the image utilizes the full range of possible values from 0 (black) to 255 (white). In the equalized image, each value occurs with (roughly) the same frequency. The method is described in many places, including the Wikipedia article and the book cited below. This implementation can operate on grayscale images (where it is probably most effective) and on color images, where two choices are possible: equalization of the three RGB channels separately or equalization in the HSB colorspace, where the equalization is applied only to the brightness channel. This implementation also allows the addition of dither (using the slider), which can help to fill in empty spaces in the histogram. An example is the tank image; without dither, the equalized histogram is spikey and does not use all the possible grayscale values. Adding dither smooths this out, though too much dither is tantamount to adding noise to the image. A good rule of thumb is to add dither until you can start to actually see the graininess in the image, then reduce the dither somewhat.

References

[1] Wikipedia. "Histogram Equalization." (June 28, 2012) en.wikipedia.org/wiki/Histogram_equalization.

[2] R. C. Gonzalez and R. E. Woods, Digital Image Processing, 3rd ed., Upper Saddle River, NJ: Prentice Hall, 2007.