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What I mean by "true" resolution is the resolution at which the maximum available amount of real detail is present. For an ideal picture this resolution is the same as the resolution encoded in the image file. However, in other cases the actual true resolution may be less than the resolution of the image file:

1. If the image file was digitally scaled - for instance, if the camera has a "digital zoom" feature where it upsamples the individual pixels.




2. If the image is not completely sharp and in focus.

In either of these cases, it is useful to me to have some mechanical method of identifying what the actual true resolution of the image is independent of what the image file says. How can I do this? Is there some software or algorithm which can take an image and analyze it and tell me the highest resolution it can be displayed at while showing detail at the resolution?

My initial naive expectation that it should be "obvious" and easy to determine by some simple mechanism such as compressed sizes is thwarted in cases where the upscaled image has noticable artifacting or noise not present in the original.

You could compute the Fourier transform of the picture and analyze it in frequency space. If the image was upsampled, higher frequencies will be missing in the transformed image.

Note that the absence of the high frequencies could also mean that the original image did not contain enough detail, or it was not sharp in the first place.

Related: How to analyze images with Fast Fourier Transform method?

Assuming accurate meta data (EXIF) is present, it would be possible to determine possible camera model resolutions.

Without metadata it's a statistical guess from common available possibilities.

What you refer to as "True Resolution" is a difficult concept. To start with, digital cameras don't have pixels in the first place, they have sensors. The sensors in turn are processed to produce pixels, but many sensors produce one pixel and one sensor is part of many pixels.

There are image analysis/forensic tools for determining things like edited, cloned, recompressed, resized, and other alterations. Sometimes the JPG cosine compression tables or noise patterns can strongly indicate a particular camera.

In general, unless you can establish the camera model, I don't believe you can realistically get what you want.

What I mean by "true" resolution is the resolution at which the maximum available amount of real detail is present. For an ideal picture this resolution is the same as the resolution encoded in the image file.

To put that another way, you're trying to differentiate images where one pixel represents one pixel on the camera sensor (i.e., a 100% crop) from those that don't. So a shot taken with a full-frame lens on a full-frame SLR would be considered "true," as would one taken with a cropped-frame lens and cropped to just show what was in the image circle.

In either of these cases, it is useful to me to have some mechanical method of identifying what the actual true resolution of the image is independent of what the image file says. How can I do this?

Most cameras with digital zoom provide the DigitalZoomRatio EXIF tag which will tell you when scaling happened in-camera. You will not, of course, be able to tell if the image was scaled elsewhere or the metadata was tampered with unless you have some way to verify that the image and metadata were actually produced by the camera.

Assuming you trust the metadata, the ration could be sued to figure out that a 6000x4000 image that had a zoom ratio of 2.0 started life as 3000x2000. That doesn't mean the image can be scaled back down to that size and all will be well; any changes made during upscaling would have an effect on the downscaled version.

You can't do anything with the not-completely-sharp case for the simple reason that given just the image, there's no source of ground truth for what light fell on the sensor and no insight into the photographer's intent. Shooting with intentionally-small depth of field, putting a diffusing filter in front of the lens, producing motion blur or intentionally shooting out of focus would be captured at the camera's "true" resolution. Any algorithm trying to root out scaling would produce a wrong result.

The best you can do is read the metadata and decide whether or not you trust it.