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x264 – Rate Control Choices

Firstly, you need to decide on a rate control method. This controls how bits are allocated within the stream. At a rudimentary level, you want the quality to remain constant but this implies that more complex frames use more bits and less complex frames use less bits.

Rate control methods available:

  • constant quantisation parameter – not recommended anymore unless you know you want it
  • constant rate factor – good for one pass when the priority is quality and file size/bit rate is not really a concern
  • one pass average bit rate – good for streaming purposes or targeting a bit rate when two-pass is unfeasible
  • two pass variable bit rate – good for targeting a bit rate when you have the time to spend on two passes (though the first pass can be quite fast) and are writing to a file

Or in prose, CQP mode is mostly deprecated by CRF as CRF maintains more constant quality which was usually the aim of someone using CQP mode. If you care more about quality than bit rate then I would recommend using one pass CRF to save yourself a bit of time. Evaluate various rate factors between about 25 and 15 (a lower value is higher quality and higher bit rate) until you find a quality level/approximate bit rate with which you’re happy and then you can use that value forever more.

One pass average bit rate is good if you need to aim for an approximate bit rate but don’t have time to run two passes. If you have the time then two passes are recommended as the bits will be better distributed to maintain constant quality.

To use one pass CQP:

-cqp

To use one pass CRF:

-crf

To use one pass ABR or two pass VBR:

-b BIT_RATE

http://rob.opendot.cl/index.php/useful-stuff/ffmpeg-x264-encoding-guide/