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

I'm also really interested working out how blurry a photograph is.

What about this:

1. measure the byte size of the image when compressed as JPEG
2. downscale the image to 1/4th
3. upscale it 4x, using some kind of basic interpolation
4. compress that version using JPEG
5. compare the sizes of the two compressed images.

If the size did not go down a lot (past some percentage threshold), then downscaling and upscaling did not lose much information, therefore the original image is the same as something that has been zoomed.

Answer 2

Assesing the image (the same goes for sound or video) quality is not an easy task, and there are numerous publications tackling the problem.

Much depends on the nature of the image - different set of criteria is appropriate for artificially created images (i.e. diagrams) or natural images (i.e. photographs). There are subtle effects that have to be taken into consideration - like color masking, luminance masking, contrast perception. For some images a given compression ratio is perfectly adequate, while for other it will result in significant loss of quality.

Here is a free-access publication giving a brief introduction to the subject of image quality evaluation.

The method you mentioned - compressing the image and comparing the result with the original is far from perfect. What will be the metric that you plan to use? MSE? MSE per block? For sure it is not too difficult to implement, but the results will be difficult to interpret (consider images with high-frequency components and without them).

And if you want to delve more into the are of image quality assessment there is also a lot of research done by the machine learning community.

Answer 3

But what if the photos are "commercial?" Does the value of the existing technology work if the photos are of every-day objects and purposefully non-artistic?

If I hire hundreds of people to take pictures of park benches I want to quickly know which pictures are of better quality (in-focus, well-lit) and which aren't. I don't want pictures of kittens, people, sunsets, etc.

Or what if the pictures are supposed to be of items for a catalog? No models, just garments. Would image-quality processing help there?

Answer 4

I'd like to shoot down the "obviously incorporate resolution" idea. Resolution tells you nothing. I can scale an image by a factor of 2 , quadrupling the number of pixels. This adds no information whatsoever, nor does it improve quality.

I am not sure about the "compress to JPG" idea. JPG is a photo-oriented algorithm. Not all images are photos. Besides, a blue sky compresses quite well. Uniformly grey even better. Do you think exact cloud types determine the image quality?

Sharpness is a bad idea, for similar reasons. Depth of Field is not trivially related to image quality. Items photographed against a black background will have a lot of pixels with quite low intensity, intentionally. Again, this does not signal underexposure, so the histogram isn't a good quality indicator by itself either.

Answer 5

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.