High Efficiency Video Coding (HEVC)為新一代的視訊壓縮技術，由JCT-VC (ISO/IEC MPEG和 ITU-TVCEG)所開發，其目標是達到比目前主流H.264視訊壓縮標準更高的編碼效率，其適用範圍從Ultra HD (4000x2000或更高)到行動裝置上HD (720p或1080p) 的影片。在眾多新技術中，轉換單位(Transform unit, TU)採用以樹狀編碼結構(residual quadtree)，對於各種不同的畫面特性提供多種轉換大小，雖然可以提供更好的編碼效能，但也因為其複雜的樹狀架構，比起以往視訊編碼標準大幅增加了計算複雜度，因此發展TU之模式決策快速演算法有其必要性。本論文提供一個轉換單位之快速模式決策演算法，其演算法利用轉換編碼前的資訊來作模式決策，可分為兩部分：首先參考[11]所提出之零區塊繼承性，將此與零區塊偵測法作結合來提前中止轉換單位之編碼。第二部分為利用殘餘值之梯度來猜測子區塊AC係數之相似性，省略或是提前終止該轉換單位之編碼，省下不必要模式來降低編碼端的複雜度。根據實驗結果，利用本演算法在些許RD效能失真下，可以減少平均63%，最高到76%的轉換單位編碼時間。 High Efficiency Video Coding (HEVC) is a new video coding standard that improves the coding efficiency significantly. A residual quadtree (RQT), which provides the nested quadtree-based transform coding for various characteristics of prediction blocks is adopted in transform unit (TU). The heuristic examination of TU sizes cost additional computational complexity comparing with previous video coding standard. Thus, to develop a method that can efficiently eliminate inefficient TU mode candidates is critical. Two early termination schemes are developed in this thesis. All required information for the proposed mode decision criteria can be derived before transform coding. First, Zeng et. al. [11] has concluded the existence of the zero block inheritance (ZBI) property that could be used to skip many child TUs. We propose an early termination algorithm by combining the ZBI and all-zero block detection technique. Second, we use the gradient of residual to represent the similarity of coefficients. The similarity is employed to skip current TU mode or stop the TU splitting process. Experimental results show that our proposed method saves about 63% on average and up to 76% TU encoding time with negligible rate-distortion loss compared to HM6.1.
