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Tell me how do I find out gray level variance of the image? Is it any different from the normal variance of the image.

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I would think that it is indeed the "normal" variance of the image. You look at the distribution of the pixel values (i. e. gray levels), and compute its variance.

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I mostly agree... but what about for color images? :-) – Peter K.♦ Feb 14 at 16:12
Well, since this is a gray level variance, you would have to convert the image to grayscale first. :) – Dima Feb 14 at 19:53
Yes, I figured that... just wondering whether the type of grayscale conversion matters. – Peter K.♦ Feb 15 at 3:32
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@PeterK., depends on what you need this for. And, of course, you can also look at the variance of each color channel, or at the covariance of the joint distribution of the color values. – Dima Feb 15 at 5:03

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