Comparison of Constant Rate Factor and Constant Bitrate Mode Encoding for rPPG Detection

Remote photoplethysmography (rPPG) has gained attention as a non-intrusive camera-based contactless method for the remote monitoring of vital signs. rPPG is especially appealing for its suitability in telehealth settings as a way to provide healthcare professionals physiological measurements of their patients during video consultations, without the need of specialist equipment. Limited data transmission is a key concern in video conferencing and necessitates the need for video compression to mitigate the communication bottleneck. To ensure that their service is manageable at large scales, video communication providers often use a form of Constant Bitrate (CBR) mode encoding to keep bitrates constrained and consistent regardless of video content. In this paper, we study how video compression using Constant Bitrate mode encoding affects the accuracy of rPPG methods and compare this to video encoded using a Constant Rate Factor mode (CRF), more commonly used for asynchronous video. To this end, we analyse the behaviour of three typically used video codecs (AV1, H.264, and H.265) in both CRF and CBR modes on 129 videos from 3 different databases (UBFC-RPPG, LGI-PPGI-FVD and PURE). Our results show a noticeable difference between CRF and CBR mode encoded videos on rPPG signal quality that is inconsistent between encoders.