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View Full Version : Anyone know for sure about quant of b-frames?

dragoman
26th March 2002, 02:05
Hi,

Just a question that has been bugging me. Does anyone know what exactly are the guidelines that DivX5Pro uses when compressing b-frames?

It has been said (unconfirmed) that the quantizer is merely doubled for the b-frame. If we are talking about VBR, then which quantizer is doubled, since in VBR the quantizers are varied in size.

Anyone know for sure? I'm gonna post this at the divx.com forum too, will post any answers I get here.

dragoman
dragoman
29th March 2002, 01:01
Damn!! Nobody? Shit.....

dragoman
-h
29th March 2002, 01:42
I have no idea how DivX5 decides on its B-frame quantizer, since I can't see the source ;)

In 1-pass quality mode, however, all it does is double the desired quantizer. In that case, I dare say that it would decide on a quantizer for the frame (be it I, P or B) in the 2-pass code, and should the frame happen to be a B-frame, it gets doubled.

That's its default behaviour of course - if you turn off B-frame aggressiveness via the registry, you'll see that the output is actually larger than P-frame only encoding :)

-h
dragoman
29th March 2002, 14:37
@-h

This is confirmed? In 1-pass the codec simply doubles the quantizer for b-frames?

dragoman
-h
29th March 2002, 14:40
This is confirmed? In 1-pass the codec simply doubles the quantizer for b-frames?

Yes.

If you change the "Bidir quant multiplier" registry value to 4096 (decimal), B-frames will get the same quantizer as other kinds of frames. Setting it to 6144 will be 1.5 times the desired quantizer. Etc.

-h
dragoman
30th March 2002, 05:18
Okay, cool. Now that's done.

Next thing is 2pass VBR mode. How does the codec determine the quant for b-frames? I posted this at divx.com forums, still no answer to it....

dragoman
-h
30th March 2002, 05:59
Next thing is 2pass VBR mode. How does the codec determine the quant for b-frames? I posted this at divx.com forums, still no answer to it....

I have no idea - I can't see the source code, so it'd take a while to figure it out manually. I don't really have much interest in it, since DivX5's 2-pass system is "not good".

-h
Acaila
30th March 2002, 10:21
I posted this at divx.com forums, still no answer to it....

That's because the developers hardly read the forums, if ever. At least they didn't when I stopped reading there last year.
dragoman
30th March 2002, 14:30
Hi,

Well, bez and thekid have been fairly active ever since DivX5 debued.....

dragoman