現今的網路視訊和數位電視大都使用 MPEG 標準來壓縮視訊資料，如果在傳播頻寬的限制下，要播放大量的視訊串流勢必要提高壓縮率。然而在高壓縮率下，以方塊為基礎 (block-based) 的視訊壓縮技術；例如，JPEG 和 MPEG，將會在方塊與方塊之間產生不自然的邊界，也就是我們所稱的方塊效應 (blocking effect)。 在本論文中，我們運用四種濾波器來改善方塊效應(deblocking)的問題。四種濾波器將各別運用在平滑區塊有明顯邊界、平滑區塊有模糊邊界、平滑區塊無明顯邊界和凌亂區塊。根據人類視覺系統 (HVS) 在方塊效應下的感覺，色彩平滑的區域比複雜的區域更敏感，所以我們才會特別注重平滑區域的濾波。 我們選用的影像壓縮標準是 MPEG-4，其原始碼來自 XviD 的官方網站。實驗結果顯示，在大部份的情況下，我們所改良的去除方塊效應之濾波器比 XviD的濾波器可移除更多方塊效應，並且在主觀的視覺及客觀的 PSNR 值上皆有提升。也因為我們是在解碼器解壓縮之後，再用去除方塊效應之濾波器來減輕方塊效應，換言之去除方塊效應的程序可以獨立工作，不用修改已經存在的編解碼標準，而得到更好的影像品質。 Currently, most network video and digital television use MPEG compression standard to compress the video data. If the bandwidth is limited, we must increase the compression rate for huge amount of video stream. In such a case, the block-based compression methods will generate unnatural edges between block's boundaries; for example, JPEG and MPEG. This is called blocking effect or blocking artifact. In this study, we use four filters to improve the blocking effect. The filters are used to eliminate the blocking effect in smooth block with edge, smooth block with blur edge, smooth block without edge, and textured block. According to the principle of human visual system (HVS), the blocking effect in a smooth area is more sensitive than that in a complex area; thus, we have to take care of smooth area specially. We select MPEG-4 as the video compression standard for evaluation. The MPEG-4 source code comes from official website of XviD. In the experiments, we show that the proposed method can remove more blocking effect than the XviD filter can remove in both subjective visual comparison and objective PSNR comparison. We apply the deblocking filter after the decompression; in other words, the procedure for deblocking can work independently. So we can get better video quality without modify the decoding standard.
