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March 2003
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2003.03.26 12:02 "LZW and 4bit tiffs", by John Carter
Hi I'm having great fun here trying to write a LZW decompression function. I've pretty much stuck to the description given in section 13 of the tiff spec (v6) it goes more or less like so 1) read in a compressed strip 2) extract codes until EOI (257 dec) (keeping track of bit boundaries etc) the codes I'm getting out seem ok I have 2 questions 1) should the code you extract be in the table or at least if not in the table the index to where the next new string will be added, 2) if statement 1 is false should the new string be written to location table[code] or should it be written to the first available free space in the table. the reason i ask is that with the following sequence of code extractions and table additions i end up adding two strings of length 0 together ( malloc complains about this ). thanks jc code result 256 init table 153 258 259 260 261 262 87 173 203 247 251 268 269 270 383 ? 253 both strings = 0 length 265 272 275 240 274 276 113 265 274 280 276 250 283 277 399 ? 286 both strings = 0 length 271 277 289 269 111 257 EOI code ____________________________ John Carter BSS O&M Development group, Motorola, Mahon Industrial Estate, Cork. Tel: +353 21 4807 637 (direct) Fax: +353 21 4357635 ( reception ) Mobile: +353 86 3947920 Email mailto:john.a.carter@motorola.com www http://www.cork.cig.mot.com/~jcarte01/ Homer: There's 3 ways to do things, the right way, the wrong way, and the max power way. Bart: isn't that the wrong way. Homer: yea but faster.