PHP - image sharpness comparision

I need an algorithm to compare/rate the sharpness of given zones between two images.

For example: Which image is sharper in the zone given? Image1 - zone (x:10,y:10) to (x:100,y:100) Image2 - zone (x:10,y:10) to (x:100,y:100)

the two zones coord开发者_开发知识库inates may not be the same and the zones may vary in area/size.

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Look at the average amplitude of the differential of the image. Not exactly rocket science.