Pixel shift and spatially-correlated noise.

Started 4 months ago | Discussions thread
hjulenissen Senior Member • Posts: 2,106
Re: Pixel shift position accuracy

John Sheehy wrote:

hjulenissen wrote:

I have tried this software:


It worked ok on my 7D, but I guess that I was hampered by the OLPF. I believe that this sort of sensor alignment super-resolution depends on spatial aliasing to work.

Well, an aliasing sensor helps the final product, because the result, if it has sufficient source positions and is properly aligned, is not aliased, but still has all of the high-frequency SNR at the Nyquist that an aliasing system allows. I'm not so sure that it helps with alignment, at least if the alignment intelligence code is well-written. One of the effects of aliasing is that it puts points and edges in the wrong places. An anti-aliased point of light has an analog-correct center of intensity; an aliased one does not, especially when there is some blind or insensitive space between photosites, after all the various optical PSFs do their thing.

Reading my sentence a second time, I see now that I had an unfortunate choice of words.

What I wanted to express was:

"I believe that this sort of super-resolution _depends_ on spatial aliasing to work."

I agree that aliasing should not help alignment itself, and quite possibly makes it harder. What we are trying to accomplish is to "find new information" by shifting an aliased sampling grid over a scene. The more different each frame is, the more information it can potentially add to the processed output, but the harder it may be to match to other frames.  That sounds like a paradox.

For this tech to work, one must probably assume something about the scene content (a mix of small and large features?) and the camera capture (global or large regions having the same (sub-)pixel shift) such that a global/regional non-ambiguous offset can be found that will let the subtle aliasing from each frame add to the (true) information in the assembled frame.

The only difference between a properly-registered shifted stack from an aliasing sensor and a non-aliasing sensor is the contrast of the higher frequencies (while noise at the Nyquist is unaffected, giving higher SNR at higher image frequencies from the aliasing sensor. AA filters drop SNR at the top of the spectrum; their only major downside, IMO, with sharp lenses that need them.

A nice feature of multi-shot SR using aliased sensors is that each frame is quite usable on its own. I.e. if there is object movement or some other problem during the exposure, it should be simple to mask out an entire frame of parts of it, and still have quite good results. If you are stitching a scene, loosing one exposure is catastrophic.

On the other hand, multi-shot SR only compensates for sensor issues, not limitations in lense, unlike stitching where you can get a wide and sharp panorama using a simple non-wide lense.


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