Impact of reference distance for motion compensation prediction on video quality

Transmitting high-quality, real-time interactive video over lossy networks is challenging because data loss due to the network can severely degrade video quality. A promising feedback technique for low-latency video repair is Reference Picture Selection (RPS), whereby the encoder selects one of several previous frames as a reference frame for predictive encoding of subsequent frames. RPS can operate in two different modes: an optimistic policy that uses negative acknowledgements (NACKs) and a more conservative policy that relies upon positive acknowledgements (ACKs). The choice between RPS ACK mode and NACK mode to some extent depends upon the effects of reference distance on the encoded video quality. This paper provides a systematic study of the effects of reference distance on video quality for a range of video coding conditions. High-quality videos with a wide variety of scene complexity and motion characteristics are selected and encoded using H.264 with a bandwidth constraint and a range of reference distances. Two objective measures of video quality, PSNR and VQM, are analyzed to show that scene complexity and motion characteristics of the video determine the amount of degradation in quality as reference distance increases. in particular, videos with low motion degrade in quality more with an increase in reference distance since they cannot take advantage of the strong similarity between adjacent frames. Videos with high motion do not suffer as much with higher reference distance since the similarity between adjacent frames is already low. The motion characteristics also determine the initial quality under the bandwidth constraint. The data presented should be useful for selecting ACK or NACK mode or for modeling video repair techniques.