Why does Guava Comb only work on static scenes?

[Chainmax]

Will it ever be updated so as to work on motion areas?

[JuanC]

From the read me:

...This comb filter only works temporally, not spatially. As a result, it only helps with dot crawl, rainbows, and shimmering in stationary portions of a video.

... The effect is most noticeable on computer generated images like subtitles, weather maps, and station logos using a composite or poorly separated S-Video signal ...

[Chainmax]

That doesn't answer my question. I am asking why does it only work on static scenes.

[JuanC]

It is designed that way, in order to help reduce the artifacts related to this phenomena: "chrominance/luminance crosstalk caused by imperfect separation from a composite signal". So it will work temporally (several contiguous frames taken into account to produce each output frame), not spatially (just one frame analyzed each time), and that’s why the results are outstanding in slow scenes (several contiguous frames very similar to each other).

This is my humble opinion, to help you keep your thread up so that, hopefully, Mr. Lindsey Dubb (High Speed Dubb) the author of the filter could see it and answer your question more accurately. ;)

[Wilbert]

Those comb effects are periodic, with a period of 2 (NTSC) or 4 (PAL) frames. If there is no motion, you can get rid of them by averaging over those frames. Compare it with the average of a sinus over its period, which is zero.

If there is motion, it means that the comb effects are still periodic, but move also in the spatial direction. If you average that (like above), it doesn't cancel.

At least, that's my understanding, correct me if I'm wrong :)

[Chainmax]

I understand. However, there's something that's not quite clear. I read the filter's readme and some other stuff several times before posting this thread, and from what I could gather the averaging gets rid of the shimmering, not the dot crawl or the rainbows. Am I wrong here? Bear with me, I'm just an aficionado and am just starting to get into this stuff :).

[JuanC]

Originally posted by Wilbert
If there is motion, it means that the comb effects are still periodic, but move also in the spatial direction. If you average that (like above), it doesn't cancel. I think you're right, that’s why the filter will only help reduce those artifacts, especially around steady objects, but won't cancel them.
Originally posted by Chainmax
...the averaging gets rid of the shimmering, not the dot crawl or the rainbows. Am I wrong here? From the read me:

About Activation: “If you don’t care about shimmering and just want to take care of dot crawl, you can safely set this near 100%.

About MaxVariation: ” There isn’t an intuitive way to choose MaxVariation, so here’s a guide to help you set it: First, find the worst stationary dot crawl or rainbows you have. … Reduce your MaxVariation down until the dot crawl or shimmering returns. Then increase it just enough to make the artifacts disappear, again. You’ve now got yourself the optimal value for it…”

In brief: if you, besides getting rid of dot crawl, want to get rid of rainbows / shimmering, you need to set “Activation” to lower values. And you need to experiment with MaxVariation.

[High Speed Dubb]

Wilbert has it right — Temporal averaging will only remove color crosstalk if there’s no motion. With motion it causes really nasty looking artifacts.

A properly designed spatial filter would be able to remove crosstalk even in moving areas. (But it would have a similar limitation — A spatial filter can’t remove crosstalk across vertical color boundries.)

No one has written a good software spatial comb filter, yet. For that matter, Guava Comb is far from an ideal temporal comb filter. So there’s still lots of improvement possible.


Chainmax,

Guava Comb’s averaging does get rid of rainbows and dot crawl as well as shimmering. As it happens, I didn’t even intend for the filter to reduce shimmering — That was just a fortuitous accident.

[Chainmax]

I get it now. Thanks a bunch for the replies, guys :).

[MasterYoshidino]

i had a thought. when I unsharp masked guavacomb treated combing anime the combing showed again...

wonder if there was some way to detect combing with unsharp filters and either interpolate jagginess or extract... just a stupid comment but hey...