Need help decompressing GIF raster data - gif

Question

I have a 10x10 gif that consists of 4 colors, white, red, blue, black. I have parsed the gif data below
4749 4638 3961 <-- header
0a00 0a00 9100 00 <-- lsd (pb 91 = 1001 0001) nColors = 4, bytes = 12
ffffff ff0000 0000ff 000000 <-- global color table
#0 FF FF FF
#1 FF 00 00
#2 00 00 FF
#3 00 00 00
21f9 04(00) (0000) (00)00 <-- graphics control extension
(00) = pb (000 reserved 000 disposal method 0 user input flag 0 transparent color flag)
(0000) = delay time
(00) = transparent color index
2c 0000 0000 0a00 0a00 00 <-- image descriptor
02 16 <-- (image data - 02 = lzw min code size, 0x16 size of image (bytes))
8c2d 9987 2a1c dc33 a002 75ec 95fa a8de 608c 0491 4c01 00 <-- image block
3b
Okay so we have our image data above (labeled image block) and I'm trying to decompress this so that I get the original image back. My understanding is that you read bytes left to right and bits right to left starting with lzwmincodesize + 1 (2 + 1 bits = 3 bits at a time).
This is the decompression algorithm I'm following
Initialize code table
let CODE be the first code in the code stream
output {CODE} to index stream
<LOOP POINT>
let CODE be the next code in the code stream
is CODE in the code table?
Yes:
output {CODE} to index stream
let K be the first index in {CODE}
add {CODE-1}+K to the code table
No:
let K be the first index of {CODE-1}
output {CODE-1}+K to index stream
add {CODE-1}+K to code table
return to LOOP POINT
I'm stepping through the decompression algorithm and here's what I've come up with so far... (starting with first 3 byte codes)
Global Color Table
000 FF FF FF
001 FF 00 00
010 00 00 FF
011 00 00 00
100 CLEAR
101 End of Data
3 2 1 6 5 4 8 7
10|001|100 0|010|110|1 100|110|01
last current output cindex exists dictionary value
100 4 CLEAR
100 001 001 1 y RED
001 110 001 001 6 n +001 001 RED RED
001 001 110 001 001 6 y +001 001 001 RED RED
001 001 010 010 2 y +001 001 010 BLUE
010 010 010 2 y +010 010 BLUE
010 110 001 001 6 y +010 001 001 RED RED
001 001 100 4 CLEAR
100 111 111 7 ???? <--- (what do I do here)?
I should be getting 5 values of red, and then 5 values of blue for the first 10 pixels but as you can see it decodes 5 red then 2 blue then 2 red. Can anybody point out what I'm doing wrong here?
Thanks

Answer 1

Your mistake comes from missing the code size increase. Here are the codes, "nextcode" values and current code size:
Code read from bitstream: 100, 001, 110, 110, 0010, 1001
internal 'Next' code value: 110, 110, 110, 111, 1000, 1001
current code size: 3, 3, 3, 3, 4, 4
The missing logic from your decode loop is that you need to maintain a "nextcode" variable which tells you where to insert codes in your table and when to increase the code size. It starts at the value "clearcode + 2" and increases after each code is read from the bitstream (after the first non CC value). The logic basically looks like this:
clear_dictionary:
clearcode = 1<<codestart;
codesize = codestart+1;
nextcode = clearcode + 2;
nextlimit = 1<<(codesize+1);
oldcode = -1;
mainloop:
while (!done)
{
code = getCode();
if (code == clearcode)
goto clear_dictionary;
if (oldcode == -1)
{
write_code_to_output(code);
}
else
{
<LZW logic>
nextcode++;
if (nextcode >= nextlimit)
{
nextlimit <<= 1;
codesize++;
}
}
oldcode = code;
} // while !done