How to detect subjective image quality

Question

For an image-upload tool I want to detect the (subjective) quality of an image automatically, resulting in a rating of the quality.

I have the following idea to real

Answer 1

You could try looking in the EXIF tags of the image (using something like exiftool), what you get will vary a lot. On my SLR, for example, you even get which of the focus points were active when the image was taken. There may also be something about compression quality.

The other thing to check is the image histogram - watch out for images biased to the left, which suggests under-exposure or lots of saturated pixels.

For image blur you could look at the high frequency components of the Fourier transform, this is probably accessing parameters relating to the JPG compression anyway.

This is a bit of a tricky area because most "rules" you might be able to implement could arguably be broken for artistic effect.