Image downsampling

I'm sort of curious -- there are a lot of filters and masks out there, but I have heard very little about automated or algorithmic image detection, insofar as being able to, for example, determine that a portion of an image has been downsampled -- or to detect if an image has been obviously corrupted, without needing to visually preview the image. This sort of capacity would be quite useful, but I can't seem to find any references to that or any closely related topics when I research on Google. Anyone with some image analysis experience have a word or two on this subject?

>>7
Just because you're a stupid fuck doesn't make what you say necessarily true.

>>1
You should create a feature vector of an image. Base it on things like measures of edge detection, grainyness, variance etc. Generate a feature vector and then throw it at various machine learning algorithms. I doubt you'll get 100% but you might be able to make some determinations. Detecting a resizing is probably simpler than a resample. Go the machine learning route. You won't know if it works til you try.