H.264為現在壓縮效果及畫面品質都很好的視訊標準，相較於以往MPEG-2和H.263，H.264在相同位元率下之影像品質獲得大幅改善，因此H.264在多媒體裝置及網路應用上扮演著越來越重要的角色，然而，在不同網路環境下，其傳輸頻寬不同，如何使視訊串流滿足各種頻寬要求是一大課題，而轉換編碼是其中一種可以克服此問題的技術，而本篇論文利用畫面跳張(frame-skipping)的方式將高位元率之H.264視訊串流轉換編碼成為較低位元率的H.264視訊串流，使其能適應不同的頻寬限制，而轉換編碼最直接的作法就是不利用原始資訊直接重新編碼，然而這樣運算複雜度過高，但若完全再利用(reuse)原始資訊將導致編碼效能嚴重下降，因此，為了降低複雜度並維持一定的效能，我們利用原始資訊分別針對區塊模式與移動向量提出了轉換編碼演算法來幫我們簡化移動估測，實驗結果也顯示，能在維持一定的畫面品質效能下，大幅降低轉換編碼的時間。The emerging H.264/AVC achieves better coding performance compared with prior video coding standards, and applications of H.264/AVC become popular and important in networked multimedia services, such as teleconferencing, video on demand, and distance learning. However, because different networks might have different limitations of channel bandwidth, it is often needed to reduce the bit-rate of the coded video bit streams. Frame-skipping transcoding is one key technology to provide such interoperability between different systems and devices. The most straightforward method for frame-skipping transcoding is to decode the video stream and re-encode the reconstructed video sequences after frame-skipping. However, the computational complexity of the fully decoding and re-encoding process, referred to as complex cascaded pixel domain transcoding (CCPDT), is extremely high. Reusing information existing in the original incoming video stream, referred to as simple cascaded pixel domain transcoding (SCPDT) can reduces computational complexity but lead to severe degradation in coding performance due to mode and motion vector mis-matched problems. Consequently, to reduce the computation cost and maintain high coding performance during transcoding, we propose an efficient inter/intra mode decision algorithm and an efficient motion vector composition method. The experimental results show that our proposed transcoding algorithms can maintain high coding performance and speed up transcoding process simultaneously for a reduced frame rate video transcoder.
