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Histogram equalisation
Posted: 2016-02-19T00:41:51-07:00
by life24
Hello,
Why do histogram equalisation programs usually not produce images with flat histograms?
Thanks
Re: Histogram equalisation
Posted: 2016-02-19T02:52:38-07:00
by snibgo
It is flat, in the sense that the range 0-50 has the same number of pixels as the range 50-100, and 100-150, etc. In addition, the mean is at about 50%.
Although the peak counts are higher at the low end, you'll also see that they are very sparse: there only 6 values in the range 0-50. In contrast to this, there are nearly 50 values in the range 200-250.
This is a typical situation when the input is JPEG, at 8 bits/channel/pixel, and most pixels are at one end of the value range. When they are spread evenly ("equalised") we get a comb-shaped histogram at that end.