360度視訊可提供使用者身臨其境的三維視覺體驗，但現有的視訊編碼器多以二維長方形影像為輸入，因此須先將360度視訊的三維球體域資料投影至二維平面再進行視訊編碼。等距長方投影(equirectangular projection, ERP)及立方體投影(cubemap projection, CMP)為目前360度視訊最常使用之二維投影格式，而立方體投影因幾何失真度較低因此編碼效率較佳，但目前卻未有針對編碼立方體投影格式的位元率控制(rate control)之位元分配(bit allocation)設計。因此，本論文提出基於立方體投影的360度視訊編碼之位元分配方案，其分成兩部分，第一部分為基於支援向量機(support vector machine, SVM)的立方體投影之高編碼代價區域偵測，參考每一最大編碼單元(largest coding unit, LCU)之紋理複雜、動量、動量密集度、與沿時間軸動量方向變異性，以支援向量機(SVM)偵測高代價最大編碼單元(LCUs)。第二部分則以經由曲面擬合(surface fitting)所得之函式分配每一面(face)的高代價最大編碼單元與非高代價最大編碼單元之位元。實驗結果顯示本論文所提出之演算法優於原始HM16.16之參考軟體所採用之R-λ model 的位元率控制，平均BDBR下降3.24%，平均BDWS-PSNR上升0.13dB。;360-degree videos provide users immersive visual experiences. Since most video encoders take two-dimensional rectangular images as inputs, the three-dimensional sphere domain data of 360-degree videos must be projected onto the two-dimensional image plane before video coding. The eqirectangular projection (ERP) and cubemap projection (CMP) are the most commonly used 2D projection formats of 360-degree videos, where the cubemap projection enables better coding performance because of it has smaller geometric distortions. Since currently there are not any bit allocation schemes proposed for video coding of the cubemap projection, this thesis proposes a bit allocation scheme that consists of two parts for video coding of the CMP. First, high coding cost largest coding units (LCUs) of six faces of the CMP are detected using support vector machine (SVM), referring to the texture complexity, motion vector magnitude, motion density, and temporal variance of motion. Second, bit allocation between high coding cost LCUs and non-high coding cost LCUs is applied by functions attained by surface fitting of coding statistics based on HEVC/H.265. Experimental results show that our proposed performance is better than HM16.16 with R-λ model. with 2.256% BDBR decrease and 0.13 dB BDPSNR increase.
