[x264-devel] Making sense out of x264 rate control methods

[Jason Garrett-Glaser]

>  ABR Mode
>   - Tries to keep the encoded media around the desired bitrate. No
>     guarantees.
>   - To enable in x264 simply set bitrate parameter.
>   - qp_min, qp_max and qp_step can be used to control the
>     quality of ABR mode?
>   - qp, ipratio and pbratio are ignored in ABR mode?
>   - Good for generating the first pass log file that can be used for
>     subsequent VBR or CBR second passes.
>
>     x264 --bitrate 1500 -o <output> <input>

ipratio applies to all modes.  pbratio applies to all modes whenever
MB-tree is disabled.

>  VBR Mode
>   - By running ABR mode two or more passes we automatically get VBR
>     mode.
>   - Does a better job at keeping the media around the desired bitrate
>     than ABR but takes longer encoding time
>   - Good for generating second and subsequent log pass files. Run as
>     many times as desired but after 3 passes quality improvements are
>     negligible.
>
>     x264 --pass 1 --bitrate 1500 -o <output> <input>  # ABR Mode
>     x264 --pass 2 --bitrate 1500 -o <output> <input>  # VBR Mode

This is called 2-pass mode, not VBR mode.  ABR is a form of VBR.  CRF
is also a form of VBR.  In fact, anything not CBR is a form of VBR.

After 2 passes the quality change is negligable, not 3.

It doesn't necessarily take a longer time, since you can use faster
settings to compensate (and may still get better results than 1-pass
ABR).  In general, never use ABR mode, ever.

>  CBR Mode
>   - By running ABR mode and setting vbv_maxrate equal to bitrate we get
>     CBR mode.
>   - The hard limit in maxrate can cause high degradation in quality.
>     Only use if you really need to limit the bitrate below a certain
>     value.
>   - Not useful to generate first pass log files.
>
>     x264 --vbv-bufsize 2000 --bitrate 1000 -o <output> <input>

CBR is perfectly useful to generate first pass log files for a 2-pass
CBR encode.  2-pass generally doesn't help much with CBR though and
can sometimes hurt.

> For single pass encodings I should use use CQP, ABR, CBR, CRF or Lossless.

No, you shouldn't use ABR ever.

>  - ABR if you care about bitrate (e.g. streaming)

ABR is useless for streaming.  For streaming, the best option is
capped CRF (CRF + VBV) or CBR.

>  - CRF if you care about quality and bitrate/size is irrelevant.

No, CRF if you care about quality and you don't need a _specific
average bitrate_.  You can still cap the local bitrate using VBV.

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