Compression Example
The trivial example of a large image of solid color demonstrates the variable-length LZW compression used in GIF files.
------------------------+-------------------------+--------------------------------------------- CODE | PIXELS | NOTES ------------------------+-------------------------+--------------------------------------------- no. | value | length | this code | accumulated | Relations using N¡ apply only to same- N¡ | N¡+256| (bits) | N¡ | N¡(N¡+1)/2 |<-- color pixels until coding table is full. ________________________________________________________________________________________________ 0: | 100h | 9 | | | Clear code table -------+-------+ +-----------+-------------+-------------------------- 1: | FFh | | 1 | 1 | Top left pixel color chosen as the | | | | | highest index of a 256-color palette -------+-------+ +-----------+-------------+-------------------------- 2: | 102h | | 2 | 3 | -------+-------+ +-----------+-------------+-------------------------- 3: | 103h | | 3 | 6 | : : | | : : | 255: | 1FFh | | 255 | 32 640 | Last 9-bit code -------+-------+--------+-----------+-------------+-------------------------- 256: | 200h | 10 | 256 | 32 896 | : : | | : : | 767: | 3FFh | | 767 | 294 528 | Last 10-bit code -------+-------+--------+-----------+-------------+-------------------------- 768: | 400h | 11 | 768 | 295 296 | : : | | : | : | 1791: | 7FFh | | 1791 | 1 604 736 | Last 11-bit code -------+-------+--------+-----------+-------------+-------------------------- 1792: | 800h | 12 | 1792 | 1 606 528 | : : | | : | : | 3839: | FFFh | | 3839 | 7 370 880 | Code table full -------+-------+ +-----------+-------------+-------------------------- : | FFFh | | 3839 | The maximum code may repeat for more same-color pixels. : | | | | Overall data compression asymptotically approaches : | | | | 3839 x 8 / 12 = 2559-1/3 -------+-------+ +-----------+-------------+-------------------------- | 101h | | | | End of image data _____________________________________________________________________________The code values shown are packed into bytes which are then packed into blocks of up to 255 bytes. A block of image data begins with a byte that declares the number of bytes to follow. The last block of data for an image is marked by a zero block-length byte.
Read more about this topic: Graphics Interchange Format
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