BD Rate (Bjontegaard delta rate) allows the measurement of the bitrate reduction offered by a codec or codec feature while maintaining the same quality as measured by objective metrics. The BD Rate method was described in 2001 by Bjontegaard. It is a function of two RD curves: a reference curve and a test curve (order is important). BD-Rate is measured in percent. BD-rate equals 0% when calculated metrics are the same. When the test shows better results than reference BD-Rate value less than zero. The rate change is computed as the average percent difference in rate over a range of qualities. Metric score ranges are not static - they are calculated either from a range of bitrates of the reference codec or from quantizers of a third, anchor codec.
 Bjøntegaard, G. (2001). Calculation of Average PSNR Differences between RD-curves.