The anchor, or baseline, is the encoder that every other configuration is measured against when reporting BD-rate. Because BD-rate is always a difference between two encoders, a comparison needs a fixed reference, and the anchor is it. A common choice is x264 at its medium preset, the universal baseline that runs everywhere, so a BD-rate of minus 40% means a given configuration reaches the same quality as x264 medium at 40% less bitrate, while plus 5% means it needs 5% more. The sign convention is the constant source of confusion: negative is a saving against the anchor. The catch is that the whole comparison only holds if it is apples-to-apples - the same content, the metric and model, the resolution handling, and the speed effort held comparable across every configuration and the anchor. Change the anchor and every reported number changes with it, which is why the anchor and its exact version must be named in any result.
The handbook · 7 volumes
Fora Soft Video Engineering Handbook
The anchor encoder decides what every BD-rate number in a comparison means, and a poorly chosen anchor makes a codec look better or worse than it is. We wrote seven volumes covering the whole video stack, and for anchor choice start with Volume 1: encoders and presets, H.264, HEVC and AV1 trade-offs, rate-quality curves and quality metrics. Written by Nikolay Sapunov, CEO at Fora Soft.
Comparing encoders and not sure which anchor makes your BD-rate numbers credible?
Tell us the codecs and content you are testing: we set up the anchor, test points and BD-rate methodology so the results hold up in a decision meeting. 250+ video products shipped since 2005.
