3) Huffman coding

Huffman encoding is achieved by replacing the original data with a fixed-length encoding. Huffman coding was originally established to compress text files, and so far there have been many variations. Its basic idea is that the higher the frequency in the image data is, the shorter the corresponding code length is, and the lower the frequency is, the longer the corresponding code length is.

Huffman coding rarely achieves a compression ratio of 8:1. In addition it has the following two drawbacks: 1 It must accurately count the frequency of occurrence of each value in the original file. Without this exact statistics, the effect of compression Will be greatly reduced, or even not achieve the effect of compression. Huffman coding usually goes through two operations. The first pass counts and the second pass produces codes, so the coding process is slow. 2 It is more sensitive to bit additions and deletions. Since all Huffman encoding bits are combined without considering the byte division, adding one bit or one bit reduces the decoding result.

4) ZIP code

Supported for PDF file formats. Like LZW, ZIP compression is most effective for compressing images that contain a large area of ​​single color.

5) CCITT coding

It is a series of black and white image lossless compression technology supported by TIFF, PDF and Postscript language file formats. CCITT is an abbreviation for the French spelling of the International Telegraph and Telephone Advisory Committee. It is a set of image compression standards for fax-type applications that is dedicated to the compression and transfer of binary images. Today, when we send and receive faxes in the office or at home, we mostly use the CCITT Group 3 compression standard. Some digital network-based fax machines and TIFF files that store binary images use the CCITT Group 4 compression standard.

Lossy compression technology has predictive coding, subband coding, fractal coding, etc. The professional principles involved are quite difficult and will not be described in this article.

3 common image compression file formats

1) RAW

The extension is .raw. It is a lossless compression format. It is an image storage format that is popular for professional or quasi-professional digital cameras. Its data is original files that have not been processed by computer. After uploading to a computer, it needs to be viewed with the supporting image software and can be converted to a lossless TIFF format. , or compressed into JPEG format. Of course, you can also use the PhotoShop software to view or save to other file formats by downloading and installing the plug-in. Its size is slightly smaller than the TIFF format, such as Canon EOS300D digital camera, the original size of the RAW file is about 7M, with the supporting software can be converted to TIFF or JPEG format (Figure 3), extract the size of the TIFF file after about 18M .


(to be continued)

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