artifacts on very thin black or white lines - why?

[phædrus]

I've been backing up with DivX for about a year now, and I've been using the 5.0.2Pro codec. In the beginning I was mostly doing Babylon 5 episodes, and did not notice this artifact -- probably because they are a bit fuzzy and the edges on everything are not usually that sharp. But lately, doing more movies, especially those with edge enhancement, I have noticed an artifact that occurs especially on thin light diagonal lines against a dark background or thin dark diagonal lines against a white background. For example, in Black Hawk Down, a dark powerline against a light sky. Or in anime, lots of black lines for mouths or noses on faces.

If the lines are moving in the frame, or if the camera is panning up or down so that the lines are changing position within the frame, then there is a sort of "twirling licorice stick" effect, like the old spinning barber poles but less pronounced, of course. I don't know if I can describe it any better than this. The lines are not perfectly solid, but it isn't stair-stepping, either. The artifacts move whenever the thin lines change position in the frame.

Perhaps I am too picky, but in certain types of program material this is quite noticable. Especially on anime films with edge enhancement, there is a lot of this effect on chin lines, mouth lines, and many other thin dark lines against a light background.It also seems to occur on movies without edge enhancement, but maybe that's not true -- maybe the edge enhancement was there but it was less pronounced and I didn't notice it. But I THINK it occurs regardless of edge enhancement in the source.

Is this an artifact of the DivX codec? It seems to happen no matter what kind of resizing I use. Have they fixed it at all in the 5.1 codec? Would using Xvid encoding be any better?

[DigitAl56K]

Are these any different from general ringing artifacts?

[jggimi]

A picture is worth a thousand words. Please post an image.

If you don't have a place on the web to host an image, PM me and I'll send you an e-mail address to use.

[Soulhunter]

Originally posted by jggimi
If you don't have a place on the web to host an image, PM me and I'll send you an e-mail address to use. Or upload it directly here... (http://www.imageshack.us/) ;)


Bye

[phædrus]

I'd be glad to find some examples, for example from Black Hawk Down and Castle in the Sky, but I hope this doesn't sound too dumb -- how do I get a save a still frame from a DivX avi file and save it as a jpg or gif for posting? I've never had reason to do it before.

[Soulhunter]

There are many ways to do this !!!


- Open the file with VDubMod

- Go to the frame you wanna save

- Go: Menu -> Video -> Snapshot input frame


Bye

[phædrus]

OK, thanks. I don't have VDubMod yet, I've just been using plain VirtualDub, but I should be able to extract a short clip with that and put it up so people can take a look. That way you can see the "twirling" effect on motion. I'm very sick today, so it may be a couple of days. I'll be back. :-) I am very curious about what this artifact is and how I might avoid it in future. Most of the time it occurs on something in the background and therefore it is tolerable. But on some files it drives me crazy.

[jggimi]

phædrus sent me a couple of clips from Castle in the Sky (Laputa). I happen to have the R1 discs, so I've gone back to the source for comparison.

phædrus wrote:...What should be a solid light line indicating the top of the pane of glass is in fact striped, and the stripes are jiggling back and forth a little bit. Yes, they do go back and forth in the clip he sent me. It's too large to upload into the thread, but I've left a link for it below.

We don't know the process that was used to produce the video for encoding ... but I didn't see the artifact in the source frame with Force FILM on or off. I did seem to find the source for the artifact in a previous Telecined frame -- and that frame remains if Force FILM is used. Sometimes, having a FILM percentage greater than 95% isn't enough. If phædrus used Force FILM, this may be one of those sources where IVTC is recommended.

First, here is one of the frames from one of his clips. I've circled the area of concern:http://mywebpages.comcast.net/jggimi/phaedrus1.jpg

Second, the same frame from the original source (720x480):http://mywebpages.comcast.net/jggimi/phaedrus2.jpg

Where that artifact may have originated. This frame is obviously Telecined, but remains after Force FILM:http://mywebpages.comcast.net/jggimi/phaedrus3.jpg

I'll leave the pictures and this clip (http://mywebpages.comcast.net/jggimi/laputaclip4.avi) available for a week or two.

[phædrus]

OK, here is the procedure I used for this backup:

ripped with Smartripper 2.41
made project with DVD2AVI
converted that file with VFAPIConvEn
VirtualDub1.5.0 (an older version dated Feb 18, 2003 -- what can I say? If it ain't broke, don't fix it.)
Divx5.0.2Pro codec 1-pass
Yes, did do the 3:2 pulldown with the internal VirtualDub routine (supposed to be "adaptive") I guess this is the equivalent of "force film."

Yes, having used IVTC (i.e. Donald Graft's decomb) with DVD2SVCD, I think that would be a better option here. I'll have to upgrade my VirtualDub version to do that.

And as an aside, really that's quite amazing -- VirtualDub has gone through 10 iterations in about a year's time (from 1.5.1 to 1.5.10)?

Thanks for the analysis.

phaedrus

[manono]

Hi-

Is phaedrus's DVD also R1? Did he Force Film?

In any event, those look to me like deinterlacing artifacts, perhaps good frames being deinterlaced by not tweaking the deinterlacer settings. Do we have an .avs for that encode?

Edit: phaedrus responded while I was looking at the .avi and writing. Nevermind what I wrote. His methods are so completely obsolete that anything could have happened.

[jggimi]

You're welcome.

Of course, it could also be your resizing method within Vdub, or some sort of color space conversion from YV12 to RGB used with VFAPI.

My "analysis" was no more than a guess. However, if you can examine the vdub input and output prior to saving the .avi, you could determine if the artifact was: comping through the VFAPI frameserving process (color space conversion issue)

Vdub filter related (resizing, or your reverse pulldown)

Codec related (doesn't appear in output screen, but appears in .avi)

[phædrus]

I don't believe I saw those types of artifacts on the output screen of VirtualDub, therefore your (c) option might apply. Perhaps it is codec related. I tried various resizing filters, bicubic, bilinear, and Lanzos (sp?), but it was all pretty much the same.

Well, I'll try something else and see what happens. :-)

I just recently tried installing the DivX 5.1.1 Pro codec, but I found that it made playback of all my 5.0.2 encoded files slightly jerky. That's after the "bigfix" DivX software, and unchecking "smooth playback" as suggested on the DivX.com forums.

At least the bigfix program allowed me to get back to 5.0.2.

If I can fix the 5.1.1 bug, I'll try GKnot with 5.1.1. Or at least upgrade my VirtualDub version.