Compressibility Test

[SingerWang]

I'm courious to as how the compressibility test works. lets say I pick 5% to test.

What 5% of the movie does it take? random 5%? or what?

[buzzqw]

no.., not random, only the better looking parts ! :D :D :D

If you open the avs for compressibility (something link xxx_comptest.avs) test you will see something like SelectRangeEvery(280,14).

On Avisynth manual www.avisynth.org is better explained but line means select to compress 14 frames every 280. (that's like 5%)

BHH

[SingerWang]

mmm

but wouldn't that over-predict the bitrate?

would units of 14 frames each be able to fully take advantage of I/B frames and stuff?

assuming that each 14 frame is from a different section of a film and no cross encoding between the sections..

[buzzqw]

try this link as first

http://forum.doom9.org/showthread.php?s=&threadid=50714&highlight=Enc

and a little search around.

BHH

[manono]

Hi-

It takes 14 frames from every 280 frames throughout the entire movie. If I'm not mistaken, the first frame of the 14 is then tossd out, because a new I-Frame would be set, while that wouldn't happen every 14 frames during the movie. So, the compressibility is based on the remaining 13 frames of each 280. It has been tested for several years now, and while not perfect, it's usually within 5% or so in its prediction of the compressibility of the entire movie. It neither over nor under predicts the bitrate. Actually, it doesn't have much, if anything, to do with the bitrate, since if the filesize is fixed (1 CD, for example), and the movie length is the same, the bitrate is the same, no matter what the compress test results are. It predicts the compressibility for a given .avs script file (which includes such things as the chosen resolution and the filters used).

[len0x]

Originally posted by manono
If I'm not mistaken, the first frame of the 14 is then tossed out

The actual procedure discards the first and the last frame of the snip for DivX and XviD without b-frames. For XviD with bframes the first three and the last three are discarded.