由於目前使用者網路頻寬不同的限制，要將廣播視訊傳送到不同解析度的用戶端設備是一項重要議題，而降解析度轉換編碼是一種將已編碼視訊串流的位元率或畫面大小經過轉換編碼後更能適應傳輸網路的限制與不同設備之解析度的技術，並可以有效提供這方面的應用。由於H.264中加入很多了高複雜度的預測和壓縮技術，使其可以提供比以往的視訊壓縮標準更佳的壓縮效能，但同時也因此讓H.264編碼端複雜度過高，所以將H.264的視訊串流解碼後再進完整編碼（被稱為複雜型串接式像素域轉換編碼）是相當耗時的，若要將之運用於即時之轉換編碼系統是有難度的。由於直接利用原始解碼端之預測資訊進行再編碼，雖然能使轉換在編碼時間大幅下降，但其編碼效能並不好。在本篇論文，即是針對H.264轉換編碼架構中，探討如何在降低解析度之應用下以最快的速度進行轉換編碼，所以我們提出了降解析度轉換編碼中快速區塊模式決策與移動再估測演算法，能在維持一定的畫面品質效能下，有效的加速轉換編碼時間。Through the network limitation of client devices, it is a challenging matter to transmit broadcasting videos to different resolutions terminal devices. Down-scaling video transcoding is a technique for adapting the bit rate or frame size of the encoded video to suit better the constraints of transmission network and target devices, then it can offer effient solutions for this type of application. H.264 supports many advanced compression techniques that can achieve better coding performance than the previous standards. However, the compression techniques require high computational complexity, so if we fully decoding and re-encoding the video bitstream in H.264 process, referred to as complex cascaded pixel domain transcoding (CCPDT), is quite involved and time consuming, which would be difficult to achieve real time implementation. Because of direct reusing information existing in the original incoming video stream can reduces computational complexity, but the coding efficiency is not good. In this thesis, we will explore how to perform H.264 downscaling transcoding as fast as possible, so we propose fast mode decision and motion re-estimation algorithms for downscaling transcoding. The experimental results show that our propose transcoding algorithms can maintain the coding performance and speed up transcoding process simultaneously.
