2003.03.26 12:02 "LZW and 4bit tiffs", by John Carter
Thread
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
- read in a compressed strip
- extract codes until EOI (257 dec) (keeping track of bit boundaries etc)
the codes I'm getting out seem ok
I have 2 questions
- 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,
- 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.