Unfortunately, Gibb's phenomenon, which produces ringing like effects, is usually mistaken for ringing produced by convolution (or deconvolution) operations. They are not the same in general, and the typical ringing associated with image restoration is mistakenly identified with Gibbs phenomenon on this thread.
Well, Joofa, you obviously appear
to know what you are talking about. I confess I have almost zero knowledge about the Gibb's phenomenon, but I can appreciate that it may be useful to be able to indentify and name any artifacts one may see in an image, especially if one is examining an X-ray of someone's medical condition, or indeed searching for evidence of alien life on a distant planet.
I wouldn't attempt to argue points of physics or mathematics with the eminent Emil Martinec. However, when Emil implies that my ACR 6.1 'detail enhancement' has significantly more ringing artifacts than Bart's Richardson Lucy rendition, I'm plain confused. I just don't see it; at least not at 400% enlargement.
Here's the comparison again.
As it appears to me, the edges of the white sliver at the bottom left of the tree has slightly more noticeable ringing artifacts in Bart's image. Furthermore, if one examines the plain blue area at the top of the crop, immediately above the uppermost white bar, one can see 4 or 5 faint horizontal lines in Bart's image, but only one line in the ACR image (exluding the very dark edge adjoining the white bar, which is apparent in both crops).
I presume these faint lines in the blue paint-work are ringing artifacts, but I'm not certain. Perhaps those faint blue lines actually exist in the paint-work. If I were a doctor examining an X-ray, I'd be concerned about such matters.
As a matter of interest, I tried another shazpening experiment using Focus Magic. Those who are familiar with this program will know that there are several options for different types of image source. 'Digital Camera' is the default and the one I used earlier, but at the bottom of the list is 'Forensic'. It sounds as though that option would produce a better result at restoring detail, and so it does.
Showing 300% crops (above) of the same part of the image, Focus Magic is now doing a much better job in delineating the individual slats of the blind. Bart's image has the edge regarding the clarity of those slats, but I think one could say the FM (forensic) image displays slightly lower noise in the blue area at the top. The crop on the far right is my first result using the default 'Digital Camera' source. The blue paint-work is clearly much smoother, but the detail of the slats much worse, even non-existent. Trade-offs again.
So much for pixel-peeping!