Constant-Quality Modes: CRF and RF

Target-bitrate encoding answers a simple question: how do we fit this material into a fixed pipe? Constant-quality encoding inverts the question: how do we spend exactly as many bits as the picture needs? The second framing is usually the more efficient one, and it underpins the modern configuration vocabulary.

The idea behind a quality parameter

A constant rate factor is a dimensionless dial. A lower value instructs the encoder to preserve more detail across every frame; a higher value permits more quantisation and therefore a smaller file. Crucially, the dial does not set a bitrate. The resulting rate floats with scene complexity, which is precisely why allocation becomes smarter: an explosion of grain gets the bits it needs, and a static shot does not waste them.

Hardware paths expose the same concept under the name rate factor. The naming differs across implementations, and the numeric scale is not comparable between them, but the mental model is identical: one dial trades fidelity against size.

Capture characteristics such as grain and motion drive how a quality dial behaves

Why two encoders with the same dial differ

The mapping from a dial value to actual picture quality is implementation-specific. Two encoders can both offer a value of twenty and produce visibly different results on identical content. Comparisons of dial values across implementations are therefore invalid unless a measurement layer sits between them, which is the role of the metrics described on the measurement page.

Setting the dial intelligently

Anchor on a perceptual target rather than a file size.
Sample a representative clip before committing a value across a whole library.
Account for delivery: a value acceptable for streaming at HD may be too coarse for archival at UHD.
Remember that the dial interacts with the preset; a slower preset improves efficiency at the same value.

A common error is treating a quality dial as portable across resolutions. The same value at 1080p and 4K does not produce the same subjective result, because compression artefacts scale with frame area.

Where the mode is unsuitable

Constant quality is inappropriate when a hard ceiling is contractual, for example a channel with a strict multiplex budget. In those settings a capped variable-bitrate mode is appropriate, with the quality dial used only to steer average behaviour. That hybrid is covered on the bitrate ladder page.

See also the container comparison and the acceleration page for how hardware exposes the same control.