What's different about GMC in DivX vs. XviD?

[absinthe]

It's rather common knowledge that most standalone DivX/XviD players do not support playback of files encoded with QPEL or GMC. I think the player I own is among the most common out there: the Philips DVP-642, which is a great bargain for only 70 bucks.

The manual plainly states that GMC and QPEL are NOT supported, and attempting to play back my own XviD encodes with GMC certainly bears that out.

But then, someone in a newsgroup made the claim that the DVP-642 most certainly will play files encoded with GMC (though not QPEL, of course). She even referred to the unit's inability to play such files as a persistent "myth." She sent me a short clip of a file encoded with GMC, and I'll be doggoned if it didn't play back just fine.

We scratched our heads over this for some time. We finally concluded that the only difference in files that would play vs. files that would not was that the GMC-encoded files that would play back were DivX files, but files encoded with the XviD codec would NOT play back.

Thus far, every file I can get my hands on that's a DivX file with GMC will play beautifully. But no XviD files encoded the same way will play. As far as I can tell, I am using the same specs for both of the codecs when encoding (1 consecutive B-VOP, packed bitstream, etc.)

Can anyone explain?

-abs

[celtic_druid]

XviD, Nero Digital, etc. use 3 warp point GMC. DivX only uses 1 warp point GMC. Thing about 1 point GMC though is that it is basically useless. This is probably all covered (in much greater detail) in the faq.

[absinthe]

Thanks. That pretty well clears that up. :)

-abs

[yaz]

@absinthe
if the manual states 'no gmc' u'd better keep that. one point is what celtic_druid mentioned (why to use useless options ? :-)
another point is that playing back films with s-frames (gmc'd frames) is another issue than playing back s-frames correctly. I know 'cheat-wares' which handle s-frames as dropped frames. it means that the film 'plays fine' but sometimes looks quite funny.

the bests
y

[absinthe]

Yaz,

No, I won't be using GMC. In fact, I won't be using DivX for my encodes at all :-). I was just trying to figure out why DivX/GMC files would play back while XviD/GMC files wouldn't.

It has been helpful, though. I had a number of DivX files just sitting around on my hard drive that I hadn't watched because they had GMC and I had just assumed they wouldn't play. I hadn't found them time to convert them to something else. But now I can watch them, and they all play flawlessly on the DVP-642.

Do you by chance know any brand/model of standalone players that DO handle GMC and QPEL. I know there's the Yamaha DVX-6600, but I believe that's a European model. I think Sony has one, but I can't seem to locate the exact model.

-abs

[yaz]

@absinthe
sorry, i'm not a hw expert. u should try the hw a/o divx forum. anyway, i've seen Waitec addvertizing a 'full divx pro' compatible player (vision slx, or so) which means (by their interpretation) qpel/gmc/b-frame support. in an other forum i've seen a MediaTek thingy with Elta 8883 decoder chip which was also stated as 'f.d.p' ... just some hints, anyway, u'd better check it by yourself.
the bests
y

[celtic_druid]

As far as I know none of the current chipsets support 3 warp point GMC. Seem to recall reading in the hardware player forum that Sony's player doesn't even handle the fourCC XVID.

I have a MTK based player (Zensonic Z330) and it handles 1 warp point GMC and qpel fine.

[chilledoutuk]

Xbox media center handles almost anything you can through at it. Including any mpeg-4 asp features. it even uses the xvid decoder for anything with an xvid fourcc for all other mpeg4 videos it uses the ffmpeg decoder.

[absinthe]

Wow. Xbox Media Center certainly sounds intriguing. I did a quick search, and this appears to be an open source project that's some sort of add-on to MS's Xbox, eh? Too bad my kids have a PS2 :). But that certainly does open the doors to some interesting possibilities for the future.

-abs

[Rebos]

I have recently ran into a file that is GMC and automattically skips to the next file on my disc. It shows like 1sec of video and then skips to the next file. But what I don't get is why will it let me fast forward through the file and see the Xvid GMC? This is also with the Phillips DVP642/37.

[akupenguin]

GMC is used in some frames and not in others. There may be large sections of video for which GMC didn't help, and thus wasn't used.
Your player probably gives up only when it sees a frame it can't decode, not just because the header says the video will contain GMC.