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Re: Proposal for <canvas src> to allow images with structured fallback by Tab Atkins Jr.

From: Steve Faulkner <faulkner.steve@gmail.com>
Date: Tue, 8 Mar 2011 10:06:24 +0000
Message-Id: <42416B8D-750B-4006-B77B-347B5333F871@gmail.com>
Cc: HTML WG LIST <public-html@w3.org>, HTML Accessibility Task Force <public-html-a11y@w3.org>, "jackalmage@gmail.com" <jackalmage@gmail.com>
To: Laura Carlson <laura.lee.carlson@gmail.com>
Hi Laura 

How did you come to the conclusion it is not a viable solution?


Sent from my iPhone

On 8 Mar 2011, at 09:43, Laura Carlson <laura.lee.carlson@gmail.com> wrote:

> Hi Steve,
> Thank you for bringing Tab's proposal [1] to our attention [1]. I'm
> please that longdesc is being thought about again.
> However <canvas src> is not a viable solution. I added <canvas src>
> [3] to the "Suggested Alternatives Are Not Viable Solutions" section
> [4] of the change proposal  to reinstate longdesc in HTML5 [5].
> Best Regards,
> Laura
> [1] http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2011-March/030766.html
> [2] http://lists.w3.org/Archives/Public/public-html/2011Mar/0156.html
> [3] http://www.w3.org/html/wg/wiki/ChangeProposals/InstateLongdesc#.3Ccanvas_src.3E
> [4] http://www.w3.org/html/wg/wiki/ChangeProposals/InstateLongdesc#Suggested_Alternatives_Are_Not_Viable_Solutions
> [5] http://www.w3.org/html/wg/wiki/ChangeProposals/InstateLongdesc
> On 3/7/11, Steve Faulkner <faulkner.steve@gmail.com> wrote:
>> Resent with full context (apologies for earlier mail sent accidentally due
>> to a combination of small screen and fat fingers)
>> This is a proposal Tab sent to the WHATWG mailing list
>> http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2011-March/030766.html
>> I think it has a lot going for it.
>> What I would add is a native ( non JavaScript) method of displaying the
>> alternative content as a range of people would benefit from being able to
>> switch between the canvas and the structured alternative.
>> Proposal for
>> <canvas src> to allow images
>> with structured fallback by
>> Tab Atkins Jr.
>> "Allow me to quickly summarize
>> the state of affairs and the
>> problem I
>> wish to solve: 1. <img> was designed in a
>> broken manner, as it is a purely
>> visual
>> element with no way for non-
>> sighted users, such as blind
>> people or search engines, to view its
>> contents.  To fix this issue, @
>> alt was
>> added to <img>, to provide a
>> textual alternative to the image. 2. @alt is fine for images with
>> simple textual equivalents, but
>> some
>> images are complex and can't
>> be easily described in a single
>> unstructured paragraph.  For example, a complex graph may
>> be best
>> represented in textual form by
>> presenting the data used to
>> generate it
>> in the form of a <table>. 3. To fix this, @longdesc was
>> added, which is a link to
>> another page
>> with a longer, and possibly
>> structured, alternative
>> representation of the image.  This has several
>> problems, however.  First, data
>> shows
>> that many users of @longdesc
>> don't realize that the value of
>> the attribute should be a link to a
>> page representing the longer
>> description, and instead either
>> point it to the embedding page
>> or the
>> image itself, or just fill it with nonsense.  Second, the fact
>> that
>> the long description is a
>> separate page means that it's
>> possible that
>> the linkage can be broken if the server is reorganized or the url
>> structure of the site is altered,
>> or if the code containing the
>> <img>
>> is copypasted between pages.
>> Since the long description is not presented at all in all major
>> visual user-agents, a breakage
>> can go a
>> long time before being
>> detected. 4. Some other elements, such
>> as <object> and <video>, have
>> conceptually similar issues,
>> where they want to present
>> alternative
>> content in case their main content is unusable or
>> unrecognized.  These
>> elements encode the fallback
>> content as direct descendants,
>> hiding
>> them when they're not necessary. 5. The <canvas> element's
>> situation is *directly*
>> analogous, as
>> <canvas> represents an image,
>> and contains textual/structured
>> alternative content as descendants.  The problems
>> with @longdesc
>> described in #3 are not present
>> in <canvas> - the descendants
>> are
>> clearly alternative content, and travel inline with the element,
>> making them immune to linkrot. It thus seems that <canvas>
>> represents a strictly better
>> alternative
>> to <img> when structured
>> accessibility fallback content is
>> required, except for one problem -
>> <canvas> can only be scripted.
>> To use it as
>> an image, one must also run
>> some javascript with creates an
>> out-of-document <img> element, points it at the
>> appropriate image, and
>> then draws it into the canvas.
>> This is inaccessible for visual
>> users
>> without javascript, or for user- agents like feed readers that
>> don't
>> run script. I suggest that we can retain all
>> the benefits of <canvas> over
>> <img
>> longdesc> while avoiding the
>> above problem by adding an
>> optional @src attribute to <canvas>.  If
>> present, the image linked by
>> the attribute
>> is loaded and drawn into the
>> <canvas> automatically,
>> without any script intervention required.
>> <canvas> would then fire the
>> same
>> events that <img> currently
>> does and generally expose the
>> same API, in addition to the current canvas
>> API.  It would be used like: <canvas src="complex-
>> chart.png">
>> <table>
>>  -data that the chart
>> represents-
>> </table> </canvas> I've discussed this privately
>> already, and one of the
>> objections to
>> this proposal was that @
>> longdesc allows the user-agent
>> to delay loading the alternative until the
>> user actively requests it, as it's
>> not always ideal to drop a large
>> structured alternative into the
>> middle of content (similar to
>> how sighted users can just skip over a
>> complex chart, returning to it
>> later when they have more time
>> or
>> attention).  I don't believe this
>> is a valid objection - there is nothing requiring a screen-
>> reader to automatically present
>> the
>> alternative content to the user.
>> UAs must already have ways to
>> delay showing in-band information
>> until requested, so as to
>> correctly
>> represent the <details>
>> element, for example.  The
>> same mechanism can be used for <canvas>; while I'm
>> not in any place to recommend
>> UI for
>> screen-readers, it may even
>> make sense to present
>> <canvas> and <details> in a unified fashion. Thoughts on the problem or
>> the proposed solution
>> presented here?"
>> Sent from my iPhone
> -- 
> Laura L. Carlson
Received on Tuesday, 8 March 2011 10:11:21 GMT

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