1XV also supports the GIF 'aspect ratio' extension that was discussed on
2comp.graphics.  Here's the description:
3----------------------------------------------------------------------------
4
5There was quite a discussion in comp.graphics a few weeks ago, with a
6lot of people complaining about GIF images coming with many different
7aspect ratios, but no indication what the initial aspect ratio was.
8
9Several suggestions were made, the best of which was Chris Schoeneman
10(i think) suggesting the following extension block be added to specify
11the aspect ratio:
12#   7 6 5 4 3 2 1 0   Byte #
13#  +---------------+
14#  |0 0 1 0 0 0 0 1|    1        '!' - GIF extension block introducer
15#  +---------------+
16#  |0 1 0 1 0 0 1 0|    2        'R' - For 'aspect Ratio'
17#  +---------------+
18#  |0 0 0 0 0 0 1 0|    3         2  - Two bytes in block
19#  +---------------+
20#  |  pixel width  |    4            - First part of ratio (numerator)
21#  +---------------+
22#  | pixel height  |    5            - Second part of ratio (denominator)
23#  +---------------+
24#  |0 0 0 0 0 0 0 0|    6         0  - extension block end code
25#  +---------------+
26#
27#Let byte four equal 'x' and byte five equal 'y'  Then x:y is the _pixel_
28#aspect ratio.  'x' and 'y' should be relatively prime (ie they should
29#have no common divisor except one), but they don't have to be.
30
31Jef Paskanzer [of PBMPLUS fame]  modified his
32gif decoder to recognize this. I also modified my own, not that I count :-)
33Jef and I cross-checked ours and these diffs are compatible with both.
34
35This code, by the way, isn't terrible robust, and will only skip extensions
36found BEFORE the image separator, but it is a lot better than nothing.
37
38Since then, there has been a seemingly official announcement about a GIF89a
39standard from CompuServe, and the possibility of both GIF's becoming outlawed
40because of the Unisys LZW patent,  etc. etc. etc. oh, well... life goes on.
41
42anyway... I hope this is of some help. you are welcome to post these patches
43to wherever is appropriate (i didnt do a patch on patchlevel.h, b.t.w.), or
44incorporate them into your new program, or whatever. I relinquish all claims,
45etc. etc.
46
47-steve
48-------------------------------------------------------+"Come, Watson, come!"
49 Steve Swales                (716) 275-0265,-3857,-5101| he cried. "The game is
50 steve@bat.lle.rochester.edu           (128.151.32.111)| afoot. Not a word!
51 {decvax,harvard,ames,rutgers}!rochester!ur-laser!steve| Into your clothes and
52 University of Rochester            250 East River Road| come!"		S.H.
53 Laboratory for Laser Energetics    Rochester, NY 14623|     'The Abbey Grange'
54
55