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Mark said:
I believe that lossless compression for gifs is a reference to dithering, whitch is the process of automatically removing colors that will affect the image least. It's called "compression", but it's not in the same manner of jpg's. Jpg compression is "lossy" rather then "lossless". If it's "lossless" it's not technically compressed. It's also called optimized.

I guess it just depends on what your ideas of compression are to begin with.
err... zip, rar, ace, tar, etc = lossless compression, because the data on the other end is exactly bit for bit the same. jpg, mp3, ogg, etc = lossy because data is removed and is not recoverable... if a file is smaller, it's compressed, whether or not any data has been lost :) far as i know optimized is not a word people generally use for temporarily shrinking a file... you can optimize the compression algorithms, or the creators can optimize their file format to compress better :)
 

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also, i don't think dithering has anything to do with compression... it won't change the size of an image file unless the compression algorithm works better with the resulting data (which i doubt?).

and if it did, then it's definitely not lossless... lossless compression means that the file is smaller, but all data is recoverable. if you dither an image, you've changed the data, and you will not be able to perfectly recover the original... though depending on the dithering algorithm and how you view the result, the human eye might not be able to tell the difference.
 
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