[foxfluff]

> high frequency noise happens when you subtract two images, one of which is shifted by half-pixel

> What you are left with is the edges (high frequency)

So you subtract a slightly phase shifted high frequency signal, you're left with a high frequency signal that may be amplified at the edges depending on your phase shift. Nothing surprising here?

The question is can you create a high frequency residual by subtracting a lowpass filtered (gaussian blur?) image? I don't think so. You're just left with whatever high frequencies you had but you aren't creating any new ones.

[ghusbands]

Yes, that would be correct, under the assumption of a lowpass filter. As I've just mentioned elsewhere in the thread, though, I don't think kalal accepted that there was a lowpass filter involved, and even stated such, indirectly, and without that it's easy to have a delayed subtraction that amplifies higher frequencies, as in the image example kalal gave.