整合前處理機制與具感知能力去方塊效應濾波器之H.264/AVC壓縮效能提升方法

Abstract

近年來，數位多媒體內容 (digital multimedia content)的相關技術及創作內容已快速蓬勃地發展，各種不同形式的影音特效是造就多媒體內容如此受大眾歡迎的主因，正由於多媒體資料是如此的豐富且複雜，相關的多媒體資料壓縮技術也已經成為重要的研究方向，其中，區塊壓縮 (block-based) 搭配移動補償 (motion compensation) 的方法已被許多國際壓縮標準所採納且行之多年，例如: JPEG、MPEG1/2/4、H.264/AVC。雖然區塊壓縮可達到不錯的壓縮效率，但它同時也造成了視訊影像的失真，其中最明顯的就是方塊效應 (blocking effect)，為了減低此效應的影響，各個壓縮標準也分別制定其去方塊效應濾波器 (deblocking filer)，以求失真程度降到最低。 目前最新的視訊壓縮標準 (H.264/AVC) 提供編碼器兩個動態調整去方塊效應濾波器的參數，在H.264/AVC 的標準制定中並沒有規範如何動態調整此組參數，因此，本篇論文利用後處理 (post-processing) 的方法來改進 H.264/AVC去方塊效應濾波器的能力，根據人類視覺系統 (HVS) 模型，分析視訊影像的內容，進而動態調整去方塊效應濾波器的強度，對於解碼後視訊畫質的提升有很大的幫助。 除了後處理機制之外，本篇論文還利用前處理 (pre-processing) 機制搭配後處理的方法來改善H.264/AVC 的整體壓縮效能，也就是能夠用較少的資料來表示原本的多媒體資料且仍能呈現出不錯的畫面品質，前處理的想法是先將原始多媒體資料經由適當的低通濾波器 (low pass filter) 處理之後，然後將此低頻訊號傳給 H.264/AVC 編碼器來進行壓縮，如此，整體的視訊壓縮效能將會比直接壓縮原始訊號來得好。

Block-based video coding cooperating with block transform and block motion compensation is the most widely adopted way to reduce the data redundancy in various video coding standards. Although the goal of de-correlations is achieved effectively by this way, the most annoying artifact known as the blocking effect also comes into existence. To both remove this artifact and improve the coding performance simultaneously, the latest video coding standard, H.264/AVC, enforces the deblocking filters inside its coding loop. In the design of deblocking filters of H.264/AVC, one pair of parameters, OffsetA and OffsetB, are provided, which allow the adaptive control of the deblocking strength in slice level. Thus, finding out better parameters for conducting the deblocking process of H.264/AVC is capable of improving visual quality of reconstructed video. Identifying which edges belong to blocking effect relies on perceptual judgment of human beings. In fact, this subjective assessment may not exactly match existing objective measurements and high PSNR does not always stand for less blocking artifacts. In this thesis, we introduce two new criteria for measuring the blocking distortion by analyzing the perceptual difference between the source and the reconstruction. The experimental results validate the proposed approaches, especially in subjective issues. On the other hand, another implicit advantage of deblocking is ignored by most encoders. It is observed that different coded images may have the same output after applying the mandatory deblocking process. Based on this observation, we integrate this concept into H.264/AVC. For eight different deblocking modes, we first derive the equations to change the input image but do not affect the final output reconstruction. By choosing those of less bitrate consumption, the proposed pre-processing approach successfully improves video coding performance. Combing advantages of both pre-process and post-process, an enhanced H.264/AVC coding system is implemented which maximizes the effect of deblocking filters. The experimental results demonstrate its improvements for H.264/AVC codec both in objective and subjective evaluations