On the Computation of BD-Rate over a Set of Videos for Fair Assessment of Performance of Learned Video Codecs

The Bj{\o}ntegaard Delta (BD) measure is widely employed to evaluate and quantify the variations in the rate-distortion(RD) performance across different codecs. Many researchers report the average BD value over multiple videos within a dataset for different codecs. We claim that the current practice in the learned video compression community of computing the average BD value over a dataset based on the average RD curve of multiple videos can lead to misleading conclusions. We show both by analysis of a simplistic case of linear RD curves and experimental results with two recent learned video codecs that averaging RD curves can lead to a single video to disproportionately influence the average BD value especially when the operating bitrate range of different codecs do not exactly match. Instead, we advocate for calculating the BD measure per-video basis, as commonly done by the traditional video compression community, followed by averaging the individual BD values over videos, to provide a fair comparison of learned video codecs. Our experimental results demonstrate that the comparison of two recent learned video codecs is affected by how we evaluate the average BD measure.