Overview: Rate control

Rate control allocates encoded data under quality, bitrate or buffering objectives.

Some encodes aim at a data budget; others let the rate vary to maintain a selected quality objective. Buffer constraints add a timing boundary that an average bitrate alone does not describe. This branch explains the objectives and controls behind those choices, including multi-pass analysis and quantization.

• Video bitrate
  Video bitrate is the data budget for the encoded picture stream and can be a target, a limit, or a measured result.

• Constant bitrate encoding
  Constant bitrate encoding controls video output toward a specified rate over the encoder's buffering model.

• Variable bitrate encoding
  Variable bitrate encoding allows the coded video rate to change as the content and encoding constraints require.

• Constant quality encoding
  Constant quality encoding targets an encoder-defined quality level while allowing bitrate and output size to vary.

• Constant rate factor (CRF)
  Constant rate factor is an encoder-specific quality control that produces variable bitrate without targeting an exact output size.

• Quantization parameter (QP)
  A quantization parameter selects quantization strength in a video coding process and is not a universal picture-quality score.

• Two-pass encoding
  Two-pass video encoding analyzes a source before a final encode so the encoder can allocate an overall bitrate budget across it.

• VBV buffering
  VBV constrains encoded video using a modeled buffer capacity and input rate rather than a whole-file average alone.