Re: [stella] Homebrew-Homepage-Screenshots

Subject: Re: [stella] Homebrew-Homepage-Screenshots
From: Julian Squires <tek@xxxxxxx>
Date: Mon, 12 Nov 2001 01:57:48 -0330
On Sun, Nov 11, 2001 at 11:12:56PM -0500, Erik Mooney wrote:
> Actually, GIF is in fact lossy!  GIFs can only have a 256 color palette,
> so the image is first quantized lossily to 256 colors, then lossless LZW
> compression is applied.  PNG is the same, except it uses a 16-bit palette.
> GIF _is_ lossless for images generated by computer, like diagrams or
> screenshots, that begin with 256 colors or less.  That's its intended
> function.

Or cartoons, or all sorts of low-color things. But anyway, the compression
_is_ lossless, the format just supports only paletted images. But there's
no data loss from compressing, decompressing, and recompressing a
suitable image.

> IIRC, LZW isn't the patented part of GIF; the whole file format is
> Compuserve's proprietary intellectual property.  LZW is openly available.

No, LZW is Unisys's property, IIRC. They sued CIS.

> There is no good true lossless compression for photographic images.  Any
> format will result in a huge file.

Actually, there are some nice (although patented) extensions to JPEG
for lossless encoding of truecolor images which are supposed to be
pretty good.

 |/|  Julian Squires <tek@xxxxxxx>
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