x264: "Bitrate variability" question

[LoRd_MuldeR]

I'm currently updating Avidemux' Wiki article about x264.
But now I got a question about the "Bitrate variability" setting :confused:

There are to settings: "Bitrate variability (%)" and "Bitrate Variance"
Where is the difference between those two settings ???

Furthermore there is a "Quantizer Compression" setting.
Until now I thought "Bitrate variability (%)" is also known as "Quantizer Compression"
So why are there both of them ???

[akupenguin]

yes, "bitrate variability" and "quantizer curve compression" are the same. "quantizer curve compression" is the official name from x264cli. "bitrate variability" was invented by some frontend, maybe GK or VfW.

[LoRd_MuldeR]

Thank you very much for reply :)

So this means "Bitrate variability (%)", "Bitrate Variance" and "Quantizer Compression" should be one option in fact, right?


Also I'm confused to see both of those options in x264 CLI:
--ratetol <float> Allowed variance of average bitrate [1.0]
--qcomp <float> QP curve compression: 0.0 => CBR, 1.0 => CQP [0.60]
:confused:

[akupenguin]

http://mewiki.project357.com/wiki/Video_configuration_dialog/X264_Configuration#Page_3_-_Rate_control.2C_motion_estimation_and_GOP

[LoRd_MuldeR]

# Bitrate Variance: Allowed deviation from the target bitrate or filesize. 1.0 = +/- 1%.
# Quantizer Compression:' Allowed variability of quantizer. 1.0 = pure CQ, 0.0 = maximum change

yes, "bitrate variability" and "quantizer curve compression" are the same.

Now this doesn't fit together:
On the one hand "bitrate variability" and "quantizer compression" are considered to be the same.
But on the other hand it's two different options. Maybe I'm dimwitted, but I don't get it :(

[akupenguin]

"bitrate variability" != "bitrate variance"

[LoRd_MuldeR]

"bitrate variability" != "bitrate variance"
:stupid:

okay okay, just to make sure I got it 100% now:

Quantizer Compression (aka Bitrate Variability): This controls how much the bitrate can jump up/down through the video. So 0 would enforce an absolute constant bitrate, while 100 would produce a CQ video.

Bitrate Variance: This controls how precise x264 will hit the target average bitrate (or target filesize). The default of 1.0 would allow a discrepancy of 1%, while 0.0 would hit the desired bitrate exactly.

BTW: Is there any reason to allow the bitrate variance/discrepancy instead of hitting the disired size exactly?
I guess there is one, because otherwise this feature wouldn't exist :p

[akupenguin]

Bitrate Variance (--ratetol) only affects 1pass ABR. ABR's goal is: get as close as possible to CRF's quality, while still being somewhere near the target filesize. The more you restrict the filesize, the more CBR compromises it has to make in order to be sure it's within the tolerance. 1% is already pretty restricted, though still much better that CBR. (note: the effective range isn't unlimited. --ratetol inf is still usually within +/- 30%, and is still not as good as CRF)

[LoRd_MuldeR]

Bitrate Variance (--ratetol) only affects 1pass ABR. ABR's goal is: get as close as possible to CRF's quality, while still being somewhere near the target filesize. The more you restrict the filesize, the more CBR compromises it has to make in order to be sure it's within the tolerance. 1% is already pretty restricted, though still much better that CBR. (note: the effective range isn't unlimited. --ratetol inf is still usually within +/- 30%, and is still not as good as CRF)

Very interesting :cool:

[DarkZell666]

You got confused by avidemux's new x264 dialog right ? ;) I've reported the "problem" over at the avidemux forum.

Quantizer Compression ([0.0 -> 1.0]) and Bitrate Variability ([0% -> 100%]) are the same value indeed, so I'm gonna pull SadaraX's ears :p