Re: Canvas positioning issue - possible solution (feedback requested).

I can create a code example but none of it would run right now :-).

Rich Schwerdtfeger
CTO Accessibility Software Group

From:	Dominic Mazzoni <>
To:	Richard Schwerdtfeger/Austin/IBM@IBMUS
Cc:,, Maciej Stachowiak
            <>,, Charles Pritchard
Date:	03/08/2011 03:47 PM
Subject:	Re: Canvas positioning issue - possible solution (feedback

On Tue, Mar 8, 2011 at 11:59 AM, Richard Schwerdtfeger
<> wrote:
> One of the challenges we still have with Canvas accessibility is
computing a
> bounding rectangle for the accessibility API mapped to each object in the
> fallback content. We have looked at:
> - Use CSS positioning. This is inadequate as it only provides us with a
> point and not the full rectangle,

What's wrong with the width and height attributes in CSS?

One advantage I can see to this approach is that authors could simple
comment out the surrounding <canvas> tag to see what the fallback
content looks like and how it lays out.

Also, it looks like for WebKit at least, we might be laying out the
fallback content offscreen, so those coordinates will already be
available. Not sure if other user agents have chosen to implement it
the same way.

> We have had a number of discussions on this and we think the right way to
> this is to bind the element to a clipping region in canvas. This is also
> consistent with how Windows and UNIX graphics subsystems started. You
> get a handle to a device context and apply a number of properties to it
> including the bounding rectangle or clipping region. When assistive
> technology first came out this device context was bound to an accessible
> object and that is where the bounding rectangle would come from. It also
> allowed the graphic subsystem to maintain the rectangle as things were
> on the desktop. This same clipping rectangle was used in hit testing for
> mouse.

I'm totally open to this too, will be easier to understand if I could
see a complete code sample of how it'd be used.

- Dominic

Received on Tuesday, 8 March 2011 22:58:48 UTC