GMC (DivX 5.03) makes files bigger!!?

[Big_Berny]

Hiho,
I wanted to know if I should use gmc. Some people said GMC is good some said that's bad. So I made a test. I encoded about 1:39 minutes of Terminator II. I know it's a very short sequence (I'll make a bigger test this day), but why the hell is the movie with gmc bigger?
The settings were:
Div 5.03
Qualitybased: Quantizer 2
PsychovisualEnhancement: Normal
Max keyframeinterval: 250
The rest I let to default

Results:
With GMC: 29514kb
Without GMC: 28916kb

Has someone a reason? I thought that with gmc the movie needs less space so you can set a higher quantizer.

Big_Berny

[Big_Berny]

I tested the same sequence but without psychovisualenhancement, because I thought that this could be the reason, but...

Same settings as above, but without Psycho...
With GMC: 31950kb
Without GMC: 31154kb

A very small difference, but nevertheless GMC needs MORE space!

[Big_Berny]

Nox I encoded a sequence of 12:57 minutes of Terminator II.

Settings:
DivX 5.02
1-pass qualitybase : Quantizer 2 (100%)
psychovisualEnh.: no
max keyframe interval: 250
b-frames: yes

The Results:
With GMC: 236'520kb (EncodingTime 19:02)
Without GMC: 230'914kb (EncodingTime 17:56)

I think that are enough tests! What do you think about it? So why the hell should I activate GMC? It's slower (about 6%), it's bigger with the same quantizer (about 2.5%) and it's less compatible!

[OvERaCiD23]

Thus why I'm one of the people who speaks down upon GMC. It doesn't aid compressibility, and can introduce artifacts into the video. Stay away from it, as well as QPel and PsychoVisual effects. B-frames are wonderful though, don't ever stop using them.

[N_F]

Originally posted by OvERaCiD23
Thus why I'm one of the people who speaks down upon GMC. It doesn't aid compressibility, and can introduce artifacts into the video. Stay away from it, as well as QPel and PsychoVisual effects. B-frames are wonderful though, don't ever stop using them.

I second that. In my tests the compressability have suffered and in general the picture have looked worse.

[Teegedeck]

Well, I feel compelled to quote XviD-developer Christoph Lampert from someplace else:
On Thu, 6 Mar 2003, elcabesa wrote:

> is supposed that gmc make filesize gorwing? or this is the fact that it halp
> few compressing and the bit overhead defeat saved bit?

No, of course GMC should _reduce_ filesize. But with GMC you need one
extra bit per MB (called mcsel) so if GMC is not very efficient, the
overhead dominates the gain.

...and also I have no idea whether DivX actually switches off GMC in frames where it doesn't help compression. Hm, obviously not. And we don't know, from how many warp-points GMC is calculated, either. No wonder it causes larger files.

[Evil Andy]

I'm pretty sure that GMC is still bust in 5.0.3

Overall I'm disappointed with the direction DivX is taking now. Each new release seems aimed more at getting support in the hardware market rather than actually improving it or fixing bugs. GMC and Qpel are still bust and rather than fix the lousy bit distribution they give us n-pass.

[theReal]

Each new release seems aimed more at getting support in the hardware market rather than actually improving it or fixing bugs.I never thought Divx would make it to standalones that quickly - that is a pretty good thing. I can live without GMC but I'm looking forward to my first Divx capable standalone DVD Player :)