[JPEG]4.2 Lossy and lossless compression

在這份規格書中,對編碼與解碼的程序,定義了兩種類別-- lossy and lossless processes.


lossy是利用數位餘弦轉換法(Discrete Cosine Transform，簡稱DCT) .....


以下明天繼續...

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        This Specification specifies two classes of encoding and decoding processes, lossy and lossless processes. Those based on the discrete cosine transform (DCT) are lossy, thereby allowing substantial compression to be achieved while producing a reconstructed image with high visual fidelity to the encoder’s source image.


        The simplest DCT-based coding process is referred to as the baseline sequential process. It provides a capability which is sufficient for many applications. There are additional DCT-based processes which extend the baseline sequential process to a broader range of applications. In any decoder using extended DCT-based decoding processes, the baseline decoding process is required to be present in order to provide a default decoding capability.


        The second class of coding processes is not based upon the DCT and is provided to meet the needs of applications requiring lossless compression. These lossless encoding and decoding processes are used independently of any of the DCT-based processes.


A table summarizing the relationship among these lossy and lossless coding processes is included in 4.11.


        The amount of compression provided by any of the various processes is dependent on the characteristics of the particular image being compressed, as well as on the picture quality desired by the application and the desired speed of compression and decompression.