GIF-encoder
lazarus / 39-Graphics

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stab · 2013-02-13 07:21

Hi all,

I'm trying to make a GIF-encoder in Lazarus - FPC(Object Pascal) and
have been reading a very precise article at
http://giflib.sourceforge.net/whatsinagif/index.html, but I don't
understand the last part on "LZW Image Data - Saving the Code Stream
as Bytes".

The example code stream is:

#4 #1 #6 #6 #2 #9 #9 #7 #8 #10 #2 #12 #1 #14 #15 #6 #0 #21 #0 #10 #7
#22 #23 #18 #26 #7 #10 #29 #13 #24 #12 #18 #16 #36 #12 #5

Jumping back to our sample image, we see that we have a minimum code
size value of 2 which means out first code size will be 3 bits long.
Out first three codes, #1 #6 and #6, would be coded as 001 110 and
110. If you see at Step 6 of the encoding, we added a code of #7 to
our code table. This is our clue to increase our code size because 7
is equal to 2^3-1 (where 3 is our current code size). Thus, the next
code we write out, #2, will use the new code size of 4 and therefore
look like 0010. In the decoding process, we again would increase our
code size when we read the code for #7 and would read the next 4,
rather than the next 3 bits, to get the next code. In the sample table
above this occurs in Step 2.

Why should the codesize be increased at code #2. IMHO 2 fit within 3
bit (or actually in 2 bits)?
I my opinion you should increase codesize when you reach code #9.
I realize that I've misunderstood something fundamental, but cannot
see what.   

Very thankful for a clarification

Regards
stab

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circular · 2013-02-20 12:16

Maybe the code size is increased after a code out of the current range
is used. So for example if all codes fit into 2 bits, you need to
forsee that a new code could be used, outside of that range. I suppose
that if that out-of-range code is used, it means that the last string
of values is associated with that new code.