Rate control and constant quality rate control for MPEG video compression and transcoding

The focus of this thesis is the design of rate-control (RC) algorithms for constant quality (CQ) video encoding and transcoding, where CQ is measured by the variance of quality in PSNR (peak signal-to-noise ratio). By modeling DCT coefficients as having Laplacian distributions, Laplacian rate/models are developed for MPEG-4 encoding and transcoding. These models accurately estimate the rate and distortion (in PSNR) of MPEG-4 compressed bitstreams. The rate model is applied to a CBR (constant bit rate) encoding algorithm. This algorithm offers a better or similar PSNR as compared to the Q2 [7] algorithm with a lower variation in bitrate. Thus, it outperforms Q2. These models are then applied to CQ video coding and transcoding. Most CBR control algorithms aim to produce a bitstream that meets a certain bitrate with the highest quality. Due to the non-stationary nature of video sequences, the quality of the compressed sequence changes over time, which is not desirable to end-users. To provide a solution to this problem, six CQ encoding algorithms are proposed: the first two are VBR (variable bit rate) algorithms with a fixed target quality (FTQ), the next two are CBR algorithms with FTQ, and the last two are CBR algorithms with a dynamic target quality (DTQ). Within each group of two, the quality is controlled either at the frame level (using the Laplacian rate/distortion model) or at the macroblock level (using the actual distortions). With the success of these algorithms, the CQ DTQ encoding algorithms are extended to MPEG-4 video transcoding (bitrate reduction with requantization). These CQ transcoding algorithms can handle the problems that are uniquely present in transcoders, such as the lack of the original sequence and requantization. Similar to their encoding counterparts, these CQ transcoding algorithms have an extra degree of freedom to balance the quality variation with the accuracy to the target bitrate and the average quality. Simulation results indicate that these algorithms offer lower PSNR variance while having similar/lower average PSNR and bitrate when compared with Q2T and TM5T (transcoding version of Q2 and TM5). Besides proposing MPEG-4 CQ RC algorithms, an MPEG-2 rate-control algorithm is also developed based on TM5. It aims at improving the subjective quality measured by using Watson's DVQ (digital video quality) metric. When compared with TM5, it provides a better DVQ. However, since Watson's DVQ metric is not a standard way to estimate the subjective quality, PSNR is still used in the rest of the researc

An efficient rate control algorithm for a wavelet video codec

Rate control plays an essential role in video coding and transmission to provide the best video quality at the receiver's end given the constraint of certain network conditions. In this paper, a rate control algorithm using the Quality Factor (QF) optimization method is proposed for the wavelet-based video codec and implemented on an open source Dirac video encoder. A mathematical model which we call Rate-QF (R - QF) model is derived to generate the optimum QF for the current coding frame according to the target bitrate. The proposed algorithm is a complete one pass process and does not require complex mathematical calculation. The process of calculating the QF is quite simple and further calculation is not required for each coded frame. The experimental results show that the proposed algorithm can control the bitrate precisely (within 1% of target bitrate in average). Moreover, the variation of bitrate over each Group of Pictures (GOPs) is lower than that of H.264. This is an advantage in preventing the buffer overflow and underflow for real-time multimedia data streaming

Q-AIMD: A Congestion Aware Video Quality Control Mechanism

Following the constant increase of the multimedia traffic, it seems necessary to allow transport protocols to be aware of the video quality of the transmitted flows rather than the throughput. This paper proposes a novel transport mechanism adapted to video flows. Our proposal, called Q-AIMD for video quality AIMD (Additive Increase Multiplicative Decrease), enables fairness in video quality while transmitting multiple video flows. Targeting video quality fairness allows improving the overall video quality for all transmitted flows, especially when the transmitted videos provide various types of content with different spatial resolutions. In addition, Q-AIMD mitigates the occurrence of network congestion events, and dissolves the congestion whenever it occurs by decreasing the video quality and hence the bitrate. Using different video quality metrics, Q-AIMD is evaluated with different video contents and spatial resolutions. Simulation results show that Q-AIMD allows an improved overall video quality among the multiple transmitted video flows compared to a throughput-based congestion control by decreasing significantly the quality discrepancy between them

Comparing temporal behavior of fast objective video quality measures on a large-scale database

In many application scenarios, video quality assessment is required to be fast and reasonably accurate. The characterisation of objective algorithms by subjective assessment is well established but limited due to the small number of test samples. Verification using large-scale objectively annotated databases provides a complementary solution. In this contribution, three simple but fast measures are compared regarding their agreement on a large-scale database. In contrast to subjective experiments, not only sequence-wise but also framewise agreement can be analyzed. Insight is gained into the behavior of the measures with respect to 5952 different coding configurations of High Efficiency Video Coding (HEVC). Consistency within a video sequence is analyzed as well as across video sequences. The results show that the occurrence of discrepancies depends mostly on the configured coding structure and the source content. The detailed observations stimulate questions on the combined usage of several video quality measures for encoder optimization

Transcoding of MPEG compressed video

Many video services use pre-encoded video for the distribution of video programs to end users. The transmission of compressed video over channels with different capacities may require a reduction in bit rate if the transmission media has a lower capacity than the capacity required by the video bitstream, or when the communication network is congested. The process of converting a compressed video format into another compressed format is known as transcoding. This thesis addresses the specific transcoding problem of bitrate reduction of a previously compressed MPEG video. Fully decoding a compressed video then re-encoding it at a lower bit rate, as a second generation video, has two disadvantages. First, it is not an efficient solution in terms of implementation complexity, delay and cost. Second, errors are introduced in the repeated compression/decompression of MPEG video, known as multigeneration. In this research, five mechanisms contributing to the continued degradation in multigeneration of MPEG video are identified: Pixel Domain Quantization (PDQ), Pixel Domain Clipping (PDC), Compression Control Parameters Variation (CCPV), Motion Vector Re-estimation (MVR) and Error Propagation due to Motion Compensation (EPMC). The degradation caused by each mechanism is illustrated and quantified by experiments. Next, the research addresses transcoding of MPEG compressed video. Two methods to reduce the requantization errors in transcoding are proposed. The first method assumes Laplacian distributions for the original DCT coefficients. A Laplacian parameter for each coefficient is estimated at the transcoder from the quantized input DCT coefficients. These parameters are used in transcoding to improve the quality of the transcoded video. The second method, selective requantization, is based on avoiding critical ratios of the two cascaded quantizations (encoding versus transcoding) that either lead to larger transcoding errors or require a higher bit budget. The experimental results show that both methods improve the quality of the transcoded video. Moreover, the thesis addresses the problem of multi-program video transmission over heterogeneous networks and provides a joint transcoder for transcoding multiple MPEG video bitstreams simultaneously. It is shown that joint transcoding provides better picture quality than independent transcoding of each sequence at a constant bitrate. Furthermore, joint transcoding minimizes the variation in picture quality between the sequences, as well as within each sequence. Consequently, joint transcoding results in a better utilization of the channel capacity