H.264/AVC之碼率控制技術研究

Abstract

在本文中，我們針對影像編碼當中編碼端一個相當重要的部分作研究。碼率控制主要目的是藉由調整編碼而達到預期的資料大小，我們的討論將建構在H.264/AVC標準上，首先分析量化參數、移動補償資料與壓縮後資料量之間的關係，進而針對壓縮後的檔頭資料作分析，然後重新建立一個針對H.264/AVC編碼特性的碼率失真模型。對於每張畫面的位元配置，我們也利用前後張影像的資料關係去調整，以改善影像品質跟穩定度。最後，為了改良原本使用在JM上的MAD值預測，我們利用移動向量來預測每個大區塊的MAD值。整合上述的方法，能改善原本在低碼率時，在緩衝器上的不佳效果，特別是藉由較精準的碼率失真模型能預測出準確的量化參數。經過實驗，可以發現在緩衝器的穩定性上能有不錯的效果，在影像品質上更能獲得明顯的效果。

In this thesis, we study the rate control issue of a video compression system. The purpose of rate control is to adjust the encoder so that the number of encoded bits can match the number of desired bits. Here, we focus on the rate control of an H.264/AVC encoder. First, we analyze the relation between the quantization parameter, MAD, and the coded bit number. We also analyze the encoding of header bits. Based on these analyses, we build up a new rate-distortion model. The bit allocation of each picture is adaptively determined depending on the relations among the picture and its front and rear pictures. This adaptive change can improve the quality and stability of the coded videos. Finally, we use motion vectors to predict the MAD value, which is originally predicted by the JM model. By combining the above methods, we can improve the performance of the buffer in low bit-rate encoding. Especially, with a more accurate R-D model, we can better predict the quantization parameter. In experiments, we show that our approach not only improves the stability of encoder buffer but also makes an obvious improvement of visual quality.