scalable canvas a11y [was Re: canvas notes]

[Bill Haneman <Bill Haneman Sun COM>]

A number of interesting issues get raised when one talks about scalable 
canvases, especially when the complexity of the rendered content changes 
with 'zoom level'.

It may be important to distinguish between scaled-canvas-size, and 
notional "zoom level".  For instance, a user with poor vision may never 
be able to use the "fit to window" view effectively.  At the same time, 
such a user may have an even greater need for a good "30 thousand foot 
view" of the canvas than a user with good eyesight, because high scale 
factors (or other factors such as poor peripheral vision) already make 
their field of view small.  My opinion is that zoom and physical 
scalability must be decoupled for this reason.  It may also be useful to 
"ordinary" users to have some control over the level of detail rendered 
in the various canvas sizes.

Related issues arise for blind users (they, too, need control over the 
"zoom level") and for users with mobility problems (they need control 
over the size of point-and-click targets, and may need for assistive 
technologies like GOK to help them interact with the canvas).  So 
accessibility of a complex canvas must include the ability to represent 
the individual components on the canvas as accessible objects in their 
own right.  We may need to add some attribution to pieces of the 
accessibility hierarchy to indicate the "level of detail" at which an 
object becomes relevant, or may choose for the objects to come and go as 
they go into and out of the visible scope.

Just something to keep in mind... since adding good accessibility 
support now will be much easier than trying to force-fit it later.

Thanks

Bill