Design And Application Of Rate Control Technology Of Real-Time H.264Encoding For Hybrid Screen Image Sequence

Hybrid screen image sequence, image sequence that containsinterleaved scenes of nature video and computer-synthesized video, iswidely observed in tele-education, web conferencing and remote desktopaccess. To encode a hybrid screen image sequence, the quality becomesunsatisfactory when using traditional video compression technology suchas MPEG-4, because a sequence of computer-synthesized image usuallyshows less temporal continuity and spatial correlation than a sequence in anature scene. And the requirement of rate control target and frame ratecould not be satisfied at the same time when using screen codectechnology based on image compression to encode scenes of nature video.To solve the problems mentioned above, we propose a new H.264ratecontrol method which classifies screen content as “slow-motion” phase or“fast-motion” phase on the fly, based on the temporal and spatialframe-area update pattern of recent frames. Then, for frames in“slow-motion” phase, which usually result from a presenter’s GUIoperations, a new rate control algorithm named Frame Rate Adaptive-CQPis applied, which puts priority on the quality of individual frame ratherthan the frame rate. For frames in “fast-motion” phase, which usuallyresult from playing a movie during the presentation, the classicalCRF+VBV rate control algorithm is applied.We implement this rate control algorithm into PPClass, a broadcastingsystem for M-learning for evaluation purpose. The experimental resultsshow that this new content-adaptive algorithm can achieve better quality, 3.5db~8.3db PSNR improvement in computer-synthesized image sequenceand20%~30%higher subjective quality assessment score withsingle-stimulus method, than existing algorithm while meeting the ratecontrol object.