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Optimized H.264 Encoder for Live Courses Broadcasting

Author: JinLei
Tutor: XieWeiKai
School: Shanghai Jiaotong University
Course: Educational Technology
Keywords: distance education screen encoding video encoding H.264 standard compound image compression rate control
CLC: TN919.81
Type: Master's thesis
Year: 2011
Downloads: 43
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Abstract

Screen codec is a special codec to compress computer-synthesized video. It real-time captures the computer screen content and compress them into a video and live broadcasting it to the network or store it locally for playback. Traditional screen codec is based on compound image compression and then integrates the changed regions detection and simply motion search steps into the algorithm. However, the compression ratio of these hybrid compression methods are far from being enough if the screen video contains complicated motions.Currently, the general video encoder with highest compression effiency is H.264 encoder, which to develop screen encoder based on has become a new research direction. X264 is one of the best implementation of the encoders following this standard. We firstly investigated its framwork and encoding algorithms deeply and then implement a screen encoding system by modifying it.However, to apply x264 directly into real-time screen video compression will bring two majors problems: (1) its high computational complexity and high CPU occupancy rate will affect other operations on the sender’s computer; (2) H.264 encoder’s real-time bitrate control algorithm is sometimes ineffective in limiting the peak bitrate. Furthermore, the fluctuation of the picture quality of the compressed video will deteriorate the user experience.To solve the problems above, in one hand, we integrate a Mirror Driver-based changed region computation step into the classic H.264 encoder, x264. After the step, we can find the unchanged macroblocks and then quick-find appropriate prediction modes for them. Intel VTune profiling results show that CPU time has been reduced by about 30%. On the other hand, we propose an adaptive frame rate based constant Quantization Parameter rate control algorithm (FRACQP), which limits peak bitrate by reducing the local frame rate. FRQCQP can strictly control the peak bitrate while smoothing the video quality and finally achieves 3 ~ 8db PSNR improvement.

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