Re: IMG tag and allowed image types...

Adam Thodey (athodey@engin.umich.edu)
Fri, 21 Jul 1995 14:24:43 -0400 (EDT)


Date: Fri, 21 Jul 1995 14:24:43 -0400 (EDT)
From: Adam Thodey <athodey@engin.umich.edu>
Subject: Re: IMG tag and allowed image types...
To: lilley <lilley@afs.mcc.ac.uk>
Cc: www-html@www10.w3.org
In-Reply-To: <8330.9507211757@afs.mcc.ac.uk>
Message-Id: <Pine.3.87.9507211443.C755-0100000@chinook.engin.umich.edu>

How would I set up the IMG tag such that would allow many different 
formats to be downloaded depending on the users WEB browser?

Would be be done, by lets say I had image type foo, bar, etc... and just 
used:

<IMG SRC="imagename">

would that automatically look for types or how would that work?

Thanks again,
ADAM


On Fri, 21 Jul 1995, lilley wrote:

> Adam Thodey wrote:
> 
> > I was wondering if there was a standard (i did not notice this in the 
> > 3.00 spec) which would only allow certain types of graphics, i.e., GIF, 
> > XBM etc.? 
> 
> No, ther eis not one in the standard, nor is there likely to be.
> 
> > Would this mean that if the browser supports inline JPEGS or inline TIFF 
> > or inline PICT that this is acceptable HTML inclusion? 
> 
> There are two questions here that need to be disentangled.
> 
> 1) If my browser supports inline foo is it acceptable for my browser 
> to request an inline image of type foo? The answer is yes. If the 
> server has that image in format foo it will send that by preference. 
> 
> It is more likely that your browser will say it can accept inline images 
> in several formats: foo bar etc and the server will send whichever of these 
> it has (or an error saying, nothing suitable was found).
> 
> This is called content negotiation and relies on the client expressing
> which formats it can accept and the server being able to supply the various 
> formats; possibly by performing a conversion on the fly. Not all clients 
> and not all servers can do this. Also, most content authors link to 
> explicit formats. Also, image format conversion while maintaining quality 
> is not as simple as it looks.
> 
> 2) If my browser supports inline foo should I write pages with an 
> explicit link to image.foo
> 
> The answer is, it depends on your audience. If you are fairly sure they 
> can all read foo format that is fine. For example because they all have 
> a particular browser (a browser demo or fan club page for example).
> 
> In general though, no assumptions can be made; in general, servers do not 
> do image format conversion on the fly (if anyone has written a server or 
> server extension that does, feel free to drop me a line as I would like 
> to test it). So it is a case of authoring what the maximum number of people 
> can already accept inline.
> 
> GIF87a     is a safe bet.
> 
> GIF89a     (the one with transparency) is extremely common and GIF87a 
>            readers can read it, they just ignore the extensions.
>          
> XBM        is pretty widespread
> 
> JPEG JFIF  is now common
> 
> other formats supported inline include 
> 
> PICT       (some Mac browsers, only) 
> NCSA HDF   (NCSA Mosaic for X, only)
> PNG        (Amiga Mosaic so far)
> 
> Chimera supports just about any format inline, as it is user extendable.
> 
> > or is there going 
> > to be a strict standard for image types?
> 
> Clearly, this list will evolve over time. Once popular formats may become 
> used less (so newer browsers stop supporting them); newer formats may become 
> more popular. This is why the HTML standard does not dictate a list of 
> acceptable inline image formats.
> 
> -- 
> Chris Lilley, Technical Author
> +-------------------------------------------------------------------+
> |       Manchester and North HPC Training & Education Centre        |
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> 
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|    Adam M. Thodey                         athodey@engin.umich.edu    |
|  Personal Homepage on the WEB    http://www.engin.umich.edu/~athodey |
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