影像壓縮技術之新方向：減少緩衝暫存空間使用量、加權可適性算術編碼、以及膠原蛋白影像壓縮

New Directions for Image Compression Techniques:Less Buffer Size, Weighted Adaptive Arithmetic Coding, and Collagen Image Compression

黃穎文(Ying-Wun Huang)

指導教授 ： 丁建均

國立臺灣大學/電機資訊學院/電信工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.01480

摘要

現今為了節省記憶體使用率以及減少網路傳輸時間的緣故，影像壓縮不只變得愈越來越受歡迎而且愈來愈受重視。影像壓縮的終極目標，就是盡可能使用少量的儲存空間來記錄大筆的資料，例如JPEG與JPEG2000。其中JPEG的全名為Joint Photographic Experts Group，是一種失真影像壓縮技術。「失真」一詞表示我們無法完美重建或解壓該JPEG壓縮後的圖片，也就是圖片會喪失一些細節、資訊。所以，JPEG在某些應用上無法提供足夠的品質保證，例如：醫學影像、具有複雜紋理的影像等等。縱使JPEG2000提供比JPEG更良好的影像品質以及壓縮率，但它的普及率仍然比不上JPEG；這是因為當我們使用JPEG2000來壓縮影像時，必須將整張影像讀進記憶體的緩衝暫存區裡來做處理，這會造成硬體實現上高昂的記憶體成本與使用量。
誠如上述所提，即使JPEG2000在壓縮領域有著不凡的表現，但是極高的記憶體緩衝暫存區需求導致它仍然不同於JPEG般地普及；對於實作上諸如嵌入式系統和行動裝置等產品而言，記憶體緩衝暫存區的使用量是一個非常重要的考量點。由於大量的緩衝暫存區使用量會導致極高的硬體成本，為此我們提出一套新的壓縮方案來解決該困擾，使用預測編碼並結合JPEG與JPEG2000中離散餘弦轉換以及離散小波轉換的特性來降低緩衝暫存區需求量。
除此之外，我們亦針對可適性算數編碼的機率表提出一套加權後的累計方法，該方法適用於各種既有的資料壓縮技術，像是影像以及文字壓縮等。根據實驗結果，我們提出的加權可適性算數編碼比起靜態算數編碼以及可適性算數編碼，能得到更好的壓縮效果。
近年來，有一類主題在影像壓縮的領域中變得相當熱門，那就是醫學影像壓縮。一般而言，醫學影像中有任何細節上的誤差是不被允許的；因此，失真影像壓縮技術通常是不能使用在醫學影像上的。為此我們針對醫學影像中的其中一種類型，即膠原蛋白影像，提出一種新的無失真壓縮方法；由於膠原蛋白影像的紋理非常複雜，以周圍鄰居像素為基礎的預測方式的無失真壓縮法，並不適用於該類型影像，為此我們提出一套方法，將複雜的紋理簡單化，再進行無失真壓縮。

關鍵字

影像壓縮 ； JPEG ； JPEG2000 ； 離散餘弦轉換 ； 離散小波轉換 ； 緩衝暫存大小 ； 算術編碼 ； 醫學影像 ； 無失真壓縮 ； 膠原蛋白影像

並列摘要

Image compression is more and more important because it is helpful for saving the memories and reducing the transmission time. Two popular image compression standards are JPEG and JPEG2000. JPEG is an acronym for the Joint Photographic Experts Group, which is a lossy image compression method. “Lossy” means that we cannot reconstruct the original image without error (i.e. some information is lost). Therefore, JPEG may not be suitable in some applications, such as medical image encoding and compressing an image with a complicated texture, etc.
JPEG 2000 has a better compression performance than JPEG. However, it is not as popular as JPEG due to the large requirement of buffer size. When we compress an image with JPEG 2000, we have to input the entire image into the buffer. Thus, it costs a lot of memory to implement JPEG 2000.
In practice, the buffer size requirement is an important issue in many embedded systems and mobile devices. Owing to the fact that the cost of hardware is proportional to the buffer size, we propose a new compression method that is a hybrid of the discrete cosine transform, the discrete wavelet transform, and predictive coding techniques. It has a good compression performance and the required buffer size is as small as that of JPEG.
In addition, we also propose a weighted accumulating method for adjusting the probability table in adaptive arithmetic coding, which can be applied in image compression and text compression. We show the simulation results that our proposed weighted adaptive arithmetic coding has better performance than both the static and the conventional adaptive arithmetic coding schemes.
In recent years, medical image compression becomes more and more popular. In general, a lossy compression method cannot be applied to the medical image because the details in a medical image are important for diagnosis. In this thesis, we also proposed a lossless image compression algorithm to encode collagen images, which are important for diabetes and skin cancer diagnosis. The conventional lossless coding which based on the neighbor-based prediction is not suitable for the complicated collagen image. Therefore, we propose a new method to reduce the complexity of texture in the collagen image and lossless compress the simplified texture after all.

並列關鍵字

Image compression ； JPEG ； JPEG2000 ； Discrete Cosine Transform ； Discrete Wavelet Transform ； Buffer Size ； Arithmetic coding ； Medical image ； Lossless compression ； Collagen image

參考文獻

JPEG and JPEG2000

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