weird interlacing/deinterlacing artefacts

[phelissimo_]

Hello,
does anyone know, what causes those weird artifacts when deinterlacing?
(VirtualDub2 - deinterlacing (yadif double TTF))


[Attachment 56559 - Click to enlarge]
I download the video so I can't recapture it.

This is the source

[Attachment 56560 - Click to enlarge]

greetings

[jagabo]

The video was resized vertically while still interlaced. The two fields are now comingled.

[johnmeyer]

Jagabo, as always, is correct. This is a well-known problem. While I always recommend NOT deinterlacing, there are certain times when you MUST deinterlace, and this is definitely one of them.

When you resize a video, the resizing algorithm has to add or remove (depending on whether you are up- or down-scaling) pixels between rows and columns of pixels. The problem happens because with interlaced video, odd rows of pixels were recorded at a different moment in time than the even rows. If the resizing algorithm doesn't know this, and if there is lots of motion, especially in the horizontal direction, you end up with the artifact you are showing. It cannot be undone, although I did come up with an approach to somewhat mitigate the problem (long thread over in the doom9.org forum).

So, to re-size interlaced video you should deinterlace first, and then re-size. I think you can also split the video into even and odd fields, resize each independently and then weave everything back together into an interlaced video. I know this works for temporal filters, but have never tried it for re-sizing, so I am not 100% certain that this will work, but I suspect that it will.

[phelissimo_]

I think you can also split the video into even and odd fields, resize each independently and then weave everything back together into an interlaced video. I know this works for temporal filters, but have never tried it for re-sizing, so I am not 100% certain that this will work, but I suspect that it will.

Ok. I tried it in VirtualDub2 but it doesn't work well. Am I doing something wrong?


[Attachment 56562 - Click to enlarge]

[jagabo]

The video can no longer be deinterlaced into two separate pictures. Your best bet is to fully blend the two fields together: resize to half height, then resize back to full height.

[phelissimo_]

The video can no longer be deinterlaced into two separate pictures. Your best bet is to fully blend the two fields together: resize to half height, then resize back to full height.

Thanks!

[johnmeyer]

If you blend the two fields from the original, unaltered video together, you'll lose a lot of detail. I don't think that is the best way to proceed. Follow the suggestions I already gave and it will work. Better yet, don't resize.

[jagabo]

If the "source" image in the first post is an accurate representation of your source -- the damage has already been done. You will not be able to deinterlace the video.

Upload a sample (not reencoded) of your source video for analysis.

[johnmeyer]

Wasn't the second image in #1 (Attachment 56560) the source?

However, if the damage has already been done, and you don't have access to the undamaged original, you can salvage a little bit by following the steps I developed years ago in this thread:

repair bad deinterlacing

It will be far worse than having done the resizing properly, but it won't be totally borked like what you showed above.

[manono]

Wasn't the second image in #1 (Attachment 56560) the source?

That's the impression I got, too.

A sample video and not pictures should have been included from the beginning.

[jagabo]

However, if the damage has already been done, and you don't have access to the undamaged original, you can salvage a little bit by following the steps I developed years ago in this thread:

repair bad deinterlacing

I hope he's well versed in AviSynth. He'll have to modify the script to split each field into about 40 bands.

[johnmeyer]

[QUOTE=jagabo;2606017]

I hope he's well versed in AviSynth. He'll have to modify the script to split each field into about 40 bands.

Yeah, I always meant to go back to that work I did and make a real function out of it. The first thing would be to have an input variable that would specify the number of rows in each band. My work was "hard-wired" for the resizing ratio in the particular video we were working on.

Perhaps it would be a good project for the new year.