Re: [stella] Homebrew-Homepage-Screenshots

Subject: Re: [stella] Homebrew-Homepage-Screenshots
From: Erik Mooney <erik@xxxxxxxxxx>
Date: Sun, 11 Nov 2001 23:12:56 -0500
On Sun, 11 Nov 2001 23:08:20 -0330, you wrote:

>On Sun, Nov 11, 2001 at 09:07:48PM -0500, Erik J. Eid wrote:
>> Albert Yarusso, one of the maintainers of AtariAge, told me that he prefers 
>> the "non-lossy" picture formats such as PCX, BMP, and PNG.  I believe that 
>> GIF is a lossy compression format.
>GIF uses LZW compression, which is non-lossy. LZW is patented, however,
>so you end up losing in a different way. PNG is a good alternative.[1]
>Unfortunately PNG is not as widely supported as GIF. 
>(you're thinking of JPEG, which uses quantization and DCTs to aid
>compression, both of which cause loss)

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

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

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

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