detection - Detect which image is sharper

Question

I'm looking for a way to detect which of two (similar) images is sharper.

I'm thinking this could be using some measure of overall sharpness and generating a score (hypothetical example: image1 has sharpness score of 9, image2 has sharpness score of 7; so image1 is sharper)

I've done some searches for sharpness detection/scoring algorithms, but have only come across ones that will enhance image sharpness.

Has anyone done something like this, or have any useful resources/leads?

I would be using this functionality in the context of a webapp, so PHP or C/C++ is preferred.

Answer 1

The simple method is to measure contrast -- the image with the largest differences between pixel values is the sharpest. You can, for example, compute the variance (or standard deviation) of the pixel values, and whichever produces the larger number wins. That looks for maximum overall contrast, which may not be what you want though -- in particular, it will tend to favor pictures with maximum depth of field.

Depending on what you want, you may prefer to use something like an FFT, to see which displays the highest frequency content. This allows you to favor a picture that's extremely sharp in some parts (but less so in others) over one that has more depth of field, so more of the image is reasonably sharp, but the maximum sharpness is lower (which is common, due to diffraction with smaller apertures).