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JPEG standard generally use two type of scan to transmit the image using entropy coding. One is known as progressive mode, while other is known as sequential mode. In sequential mode, every block of image is encoded in a zigzag fashion from low frequencies to high frequencies and transmitted. So, at the receive, we get information in a sequential manner from left to right blocks and top most block to bottom most. While in progressive mode, the encoding for all blocks are done at the same time for same frequency components. So, the DC component of each block is encoded together, followed by first frequency components of all the blocks and so on. Since, progressive mode uses the information from all blocks at the same time, it is useful to implement JPEG using progressive mode for large image sizes. As in that case, information about DC and low frequency components can give average information about image. User can then download the complete image, if he feels the need to store that image. Progressive scan is also efficient in coding because different Huffman table can be used to encode different frequency components. Hence, performance of progressive scan is expected to be better than sequential mode. |
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