Re: You Got Your SVG in my Canvas! Mmm, Delicious! (was: hit testing and retained graphics)


You were explained to more than once what problems we are trying to solve.

1. We need a vehicle to tell an assistive technology the position and
bounds of an object on the drawing space.
2. In EVERY operating system since 1994 this has been tied to retained mode
graphics information. In these systems hit testing was tied to the same
information in retained mode graphics used to supply platform accessibility
3. Authors have been asking for hit testing

Use cases have been provided and they have been ignored. Frankly, what you
are trying to do with canvas is as as old as X Windows. What we are
proposing is not novel and is proven. Now after reading this you don't know
what precisely to do my assessment is you are simply blocking the effort.

Canvas MUST be made accessible and this hole needs to be filled. You still
have not proposed an alternative working solution to fill these holes. I
have asked.

I believe what Doug is proposing is a good idea and it would allow us to
start working on SVG accessibility sooner rather than later. Another way to
address this is to remove canvas from the W3C HTML 5 specification as it is
inaccessible and simply focus our efforts on SVG.

I think Doug has come up with a novel way to address this problem and in
fact you earlier were proposing having an entirely different model for
addressing retained mode graphics in canvas. This would do that without
having duplicate effort. Kudos to Doug!


Rich Schwerdtfeger
CTO Accessibility Software Group

From:	"Tab Atkins Jr." <>
To:	Doug Schepers <>
Date:	06/30/2011 02:07 AM
Subject:	Re: You Got Your SVG in my Canvas! Mmm, Delicious! (was: hit
            testing  and retained graphics)
Sent by:

On Wed, Jun 29, 2011 at 9:59 PM, Doug Schepers <> wrote:
> I agree with others on this list that a retained-mode Canvas API is a bit
> a contradiction, and that SVG is better suited (designed, in fact) for
> retained-mode graphics.

Note: I don't think "retained mode" and "canvas" are necessarily in
opposition.  I'm relatively opposed to grafting retained-mode
characteristics onto the 2d context as currently defined, because it's
really not meant for it.

> I agree with you and others that there should be a way for applications
> using Canvas to be made more accessible.
> I suggest that rather than adding more retained-mode shape features to
> Canvas API or shadow tree, that we simply allow the SVG and Canvas to be
> used more seamlessly together.  The SVG WG has discussed this in the
> and I think most of them would be open to this idea.
> I wrote up some more detailed notes on my blog [1], and I'm interested in
> hearing feedback from you and others.
> [1]

While I like the general shape of several of your ideas, I must make
the same requests as I did of other people - we need to know precisely
what problems are to be solved before we can decide how to solve them.


Received on Thursday, 30 June 2011 12:49:48 UTC