Two-pass rate-distortion optimized rate control technique for H.264/AVC video

Rate control for video encoders can be partitioned into procedures of rate allocation and bit achievement at several levels. For non-real-time applications, we propose to adopt a two-pass video encoding mechanism, where the framelevel rate allocation in the second pass is characterized of using content-aware models constructed by using information collected in the first pass. In our proposed scheme, a video sequence is divided into units of non-overlapping window, where a two-pass procedure is sequentially applied. The goal of the first pass is to find the encoding modes and motion vectors (MVs) for each MB and generate the proposed R-λ (rate vs. Lagrangian multiplier) and D-λ (distortion vs. Lagrangian multiplier) models to represent content characteristics of each frame. In the second pass, the encoding modes and MVs for MBs found in the first pass are retained, but the QPs are re-assigned MB-by-MB according to bit-rate and distortion predictions based on the above-constructed models. The performance of the proposed two-pass video encoder was compared with the H.264 reference model JM8.0. From the results, the proposed method outperforms JM 8.0 in PSNR by up to 1.38 dB for the test images used. Besides, the proposed algorithm is capable of keeping a controlled bit rate of much more precision (< 0.017% of error, two orders better than JM8.0), as well as a slightly smoother video quality, than that obtained by JM 8.0.