W3C home > Mailing lists > Public > www-svg@w3.org > September 2010

Re: Some comments on <image>

From: David Woolley <forums@david-woolley.me.uk>
Date: Sun, 19 Sep 2010 19:22:12 +0100
Message-ID: <4C9654D4.2070903@david-woolley.me.uk>
To: ddailey <ddailey@zoominternet.net>
CC: www-svg@w3.org
ddailey wrote:

> 1. The spec says "Conforming SVG viewers need to support at least PNG, 
> JPEG and SVG format files." Why not GIF? I recall a profound nervousness 

I think the basic reason is that PNG can exactly represent any image 
that GIF can represent, except for animations, and usually does so more 
compactly for equal quality, but also has the option of better quality.

> that spread like squid ink through the open source community [1] 10 or 
> 12 years ago as the holders of the GIF patent threatened to go after 
> those who used it without license. I believe, however, that the patent 
> has since expired. [2] A search of gif in Google images shows about a 
> billion files with close to half that number for PNG. In many cases GIF 

Image formats are often chosen without any real understanding.  There 
are an awful lot JPEG images (or PDF images using DCT) that are totally 
unsuitable for JPEG, either because people believe it produces the best 
compression for everything (and only make one dimensional decisions), or 
because they don't know PNG and paintbrush produces very poor GIFs.

> files are smaller than PNG files, I think, and lots of the older public 

Although it is possible, and may be more common for very small images, 
the compression scheme used in PNG is generally better than that used in 
GIF (the LZW used in GIF, and the actual subject of the patent, is 
designed as a compromise between compression speed and and compression 
ratio - it was really intended for real time compression of streamed 
data.  That in PNG is designed to give good compression, at the expense 
of slow compression speeds.

Apart from the possibility that PNG may have a higher overhead, the 
other reason that you may observe this is that PNG has more possible 
formats, and, for example, paintbrush uses 24 bit unpalletised for PNG 
and uses a non-optimised palette for GIF.

> domain imagery sites on the web used gif because, well, PNG wasn't 
> available then. All the browsers I know of go ahead and support GIF 
> anyhow, but it is one thing we can be certain of that no longer has 
> patent entanglements. PNG?? Who can ever be completely sure until the 20 
> years pass?
It's very likely that any such patent would also affect GIF.

David Woolley
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Received on Sunday, 19 September 2010 18:22:46 UTC

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