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

As e.g. shown in this Matlab Central page, the sharpness can be estimated by the average gradient magnitude.

I used this in Python as

from PIL import Image import numpy as np  im = Image.open(filename).convert('L') # to grayscale array = np.asarray(im, dtype=np.int32)  gy, gx = np.gradient(array) gnorm = np.sqrt(gx**2 + gy**2) sharpness = np.average(gnorm) 

A similar number can be computed with the simpler numpy.diff instead of numpy.gradient. The resulting array sizes need to be adapted there:

dx = np.diff(array)[1:,:] # remove the first row dy = np.diff(array, axis=0)[:,1:] # remove the first column dnorm = np.sqrt(dx**2 + dy**2) sharpness = np.average(dnorm)