Histogram equalisation

Discuss digital image processing techniques and algorithms. We encourage its application to ImageMagick but you can discuss any software solutions here.
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life24
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Histogram equalisation

Post by life24 »

Hello,
Why do histogram equalisation programs usually not produce images with flat histograms?
Thanks
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snibgo
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Re: Histogram equalisation

Post 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.
snibgo's IM pages: im.snibgo.com

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