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Re: dynamic images

From: David Woolley <david@djwhome.demon.co.uk>
Date: Tue, 29 May 2001 22:20:53 +0100 (BST)
Message-Id: <200105292120.f4TLKro16426@djwhome.demon.co.uk>
To: w3c-wai-ig@w3.org
> Are there accessibility issues surrounding dynamically generated images,
> such as stock charts etc, which are different to static images? It strikes
> me that it would be difficult to generate meaningful alt text for a dynamic
> image, as the content of the image is always changing. I imagine the issue

That would be, I think, a priority 1 or 2 violation of the accessibility
guidelines.  Images should not be changing until browsers allow the image
to be frozen, in which case they could be made not to change.  The current
presumption is that browsers don't allow freezing, I believe.

I think any practical implementation would also violate the requirement
to provide alternatives to newer technologies.  If you remove this constraint,
you should be thinking of sending the data points and using SVG to 
construct the image.  Longer term, you should be sending the data points
as application specific XML and transforming to SVG client side.  (Even
if SVG takes off commercially, I don't think you should be doing this as
your accessibility solution for five to eight years.)

I suspect many of these actually violate the "use W3C technologies"
constraint, by using Java, but in that case they probably are already
sending the data points.  I suspect that "assistive technology" interfaces
to Java less well than to the core of Internet Explorer.

The only real model that the W3C technologies currenly support is refresh
of static images on user demand.  META/Refresh should probably be treated
as a newer/proprietory technology.  The accessibility guidelines imply
that META/Refresh be overridable, which, currently means that pages using
it must provide an alternative with manual refresh.

> is particularly difficult in the case of a dynamic graph, although I suppose
> you could have an alternate text feed - does this sound like a sensible
> approach?

The only way you will achieve the graphics within W3C implemented technologies
is by refreshing a snapshot.  You could either include the parameters as
alt text for such snapshots, on a page which is manually refreshed, or provide
a manually refreshed text only page containing the parameters in a better
formatted form.

The extreme streaming media solution would be to send an MPEG stream with the
text as subtitles, but that makes unreasonable assumptions about the 
technology available to the user.
Received on Tuesday, 29 May 2001 17:27:38 UTC

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