High Efficiency Video Coding (HEVC)是一種新一代的視訊壓縮技術，其目標是達到比H.264/AVC視訊壓縮標準更高的編碼效率，其適用範圍更從HD增加到Ultra HD的影片。此外HEVC提供的編碼單位(coding unit, CU)，其大小從8x8到64x64不等，取代H.264/AVC中固定大小的編碼單位(marcoblock, MB)，使得位元率(bitrate)得以下降，卻也大幅增加了計算複雜度。
HEVC框內預測所使用的預測模式已被大幅增加至35個，其中包含33個角度預測模式、DC模式和planar模式。在HEVC中，一張圖會先被切割成許多同一尺寸的最大編碼單元，而每一個最大編碼單元會再藉由四分樹架構被切割成不同尺寸的編碼單元。因為HEVC必須藉由計算Rate-Distortion Optimization(RDO)來決定預測單元(prediction unit, PU)的模式選擇，所以RDO的計算量佔了絕大部分的編碼時間。為了減少RDO的計算量，此篇論文提出藉由梯度運算子，預先偵測出該PU可能的趨向於的方向，以減少不必要的RDO計算。本篇論文所提出的演算法可使編碼時間平均減少31%，而只有少量的PSNR損失與位元率提高。

High Efficiency Video Coding (HEVC) is a new video coding standard, its goal is to achieve higher compression coding efficiency than H.264/AVC, and its supported resolutions is from HD (720p or 1080p) to Ultra HD (7680x4320). Furthermore, coding unit, CU, is provided in HEVC. Its size is ranging from 8x8 to 64x64, replacing the fixed-size marcoblock in H.264/AVC. The new technique results in the decrease in bitrate but the increase in computational complexity.
The modes of HEVC intra prediction was increased to 35 modes with 33 angular modes, DC mode and planar mode. In HEVC, a frame is divided to Largest Coding Unit (LCU) with uniform size, each LCU is then divided into several Coding Unit (CU) with various size. The coding unit partition corresponds to a quad-tree structure. Since HEVC needs to calculate RDO to determine the prediction unit mode selection, the calculation of RDO usually consumes a lot of coding time. In order to reduce the calculation of RDO, the proposed algorithm will use the gradient operator, pre-select the possible direction in each prediction unit, the calculation of unnecessary RDO will be reduced.
Experimental result shows that the proposed algorithm is capable to speed up the coding time of intra prediction averagely 31% with negligible PSNR loss and slight bitrate increase.

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