Re: GIF encoding

On 26 Sep, 01:53, Joshua Cranmer <Pidgeo...@xxxxxxxxxxx> wrote:
Roedy Green wrote:
On Tue, 25 Sep 2007 16:04:43 GMT, "Andrew Thompson" <u32984@uwe>
wrote, quoted or indirectly quoted someone who said :

I had not realised GIF's could have 'compression'.

This is a sore point Unisys got hold of the patent to LZW compression
used in GIF and they started threatening people with it. It is
expired now, though you never know what a lawyer could do.

I think though your code does do LZW, thumbing its nose at Unisys.

The US LZW patent expired on June 20, 2003. The counterpart patents in
the United Kingdom, France, Germany and Italy expired on June 18, 2004,
the Japanese counterpart patents expired on June 20, 2004 and the
counterpart Canadian patent expired on July 7, 2004. Consequently, while
Unisys has further patents and patent applications relating to
improvements to the LZW technique, the GIF format may now be used freely.

I believe Sun added in to Java 6 full GIF compatibility in ImageIO
because the patents had long since expired by then.

LZW compression as implemented in GIF is now fully free!

--
Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth

Thanks for your replies.

Having looked further into it, I can see the file size of the "remade"
animated gif gets larger when adding the frame information (encoded by
the LZW compression). I think its creating a palette based on the
first frame and thus when adding the additional frames its not able to
compress well as its only taking the first frame into account and
leads to addtional file size, whereas the application ive been
comparing agaisnt (Easy Gif animated) is able to compress well using
all the information from each frame.

To recap:

I have an animated gif (40k). I can split it into 15 frames, each of
which are 256 colours and of good quality. I run these images through
ngif (a c#.net port from Java, can be found on code project) and my
new animated gif is 80k. However, if I run this using an animated gif
application (Easy Gif Animated) it produces an animated gif of 40k,
much like the original.

One thing I have noticed is if I process the original and the Easy gif
animated version both have less colours in each of their frames. For
e.g. The first frame has a yellow border, which should be in each of
the frames but it is not if you look at each of the frames (but still
displays correctly when animated in IE/firefox). The yellow border is
however in each of the frames of the ngif produced version, which
would account for the extra file sizes. I also believe the original
and the easy gif animated version are not storing local colour
palettes, whereas the ngif is.

Therefore I believe its in the encoding that is the issue, basically..
how can I improve this? People must have come across this issue before.

.

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