BD-rate, short for Bjontegaard Delta rate, is the single number behind almost every codec comparison: the average percentage of bitrate one encoder saves to reach the same quality as another, measured across a range of quality rather than at one point. It is computed by fitting each encoder's rate-quality curve, reading the horizontal gap between the curves at equal quality in the log-bitrate domain, and averaging that gap over the quality range both encoders actually reach. The sign convention trips up newcomers: a BD-rate of minus 44% means the test encoder needs 44% less bitrate than the anchor for the same quality, so negative is good. The crucial catch is that BD-rate is a bitrate saving at equal quality, not a quality score like VMAF or PSNR, even though it is computed from one; the two are constantly confused. Its twin, BD-PSNR or BD-VMAF, reads the same curves vertically.
