Re: draft of hit testing on active regions (actually paths) in canvas

Rich Schwerdtfeger
CTO Accessibility Software Group

Jonas Sicking <jonas@sicking.cc> wrote on 10/27/2011 07:42:40 PM:

> From: Jonas Sicking <jonas@sicking.cc>
> To: Richard Schwerdtfeger/Austin/IBM@IBMUS,
> Cc: franko@microsoft.com, chucko@jumis.com, david.bolter@gmail.com,
> cyns@exchange.microsoft.com, public-html-a11y@w3.org,
> janina@rednote.net, jbrewer@w3.org, public-canvas-api@w3.org
> Date: 10/27/2011 07:43 PM
> Subject: Re: draft of hit testing on active regions (actually paths)in
canvas
>
> Some feedback:
>
> Limiting to only allowing paths seems like a unfortunate limitation.
> For example it misses calls to drawImage which I think is quite
> common. I'd rather prefer a call where you say "all drawing operations
> from this point should be considered drawing for element X", then let
> the page do arbitrary drawing operations, then have a second call
> which says "i'm now drawing for element Y" or "I'm now drawing for no
> element".
>

I see. So what would you use compute the bounds? Do you use the all the
points to compute a single rectangle for hit testing? ... or do you compute
multiple paths for an element to do hit testing on?


> What is the use case for the zIndex argument? The actual pixel drawing
> operations hasn't had a need for that so far and instead rely on the
> painters algorithm. It seems better to me to have a direct mapping
> between the drawing operations and the accessibility API.
>
zIndex: Performance and authors may have overlapping drawing objects.

> Why return false rather than throw an exception if the element doesn't
> exist? Also what do you mean by "doesn't exist". If the element
> doesn't exist then how could the page have a reference to it and pass
> that reference to the function? Do you mean "element isn't inside the
> canvas"?
>
Throwing an exception makes sense. I meant the element did not exist. I am
thinking that the author may have content outside fallback such as a drop
down menu. We could limit the elements to the fallback content. I would
like your feedback on that.

> In general I think I prefer the API I proposed in [1]. Can you
> describe what problems you were trying to solve with your changes
> compared to that proposal?
>
> [1]
http://lists.w3.org/Archives/Public/public-canvas-api/2011JulSep/0195.html
>
> / Jonas
>
> On Thu, Oct 27, 2011 at 4:56 PM, Richard Schwerdtfeger
> <schwer@us.ibm.com> wrote:
> > Hi Frank,
> >
> > Thank you for the call today. I am glad we are on the same page onthe
three
> > basic methods for hit testing. I took the basic methods that Jonas had
put
> > out earlier on the list and the discussion we had in San Diego and put
> > together an active regions and hit testing section. I also agree with
you
> > this is for when we have multiple interactive elements within canvas. I
was
> > glad to see that customers are starting to come to Microsoft to talk
about
> > building more interactive widgets with canvas - not because I want them
to
> > but more because it shows we are making the right moves to address
canvas
> > accessibility.
> >
> > Jonas, Charles, Frank, David, Cynthia
> >
> > This is a first draft and the section I would like you to look at is
section
> > 16. We can change the names to whatever we want. I have not yet added
spec.
> > information as to how existing functions like scrollElementIntoView
should
> > work on these elements as they should be included in the layout engine
> > positioning per Jonas earlier feedback. I have also not stated how we
would
> > convert the paths to rectangles for the underlying OS platform
accessibility
> > APIs. We could either do this in the canvas spec. or in the
accessibility
> > API mapping guide that Cynthia is leading. We just need to decide.
> > Personally, I prefer the one stop shopping.
> >
> > I am sure Ian will want to tweak or rewrite the algorithm section
processing
> > as he has his own consistent way of doing that but I took my best shot
at
> > it.
> >
> > Please review section 16.
> >
> > (See attached file: clickableregion.html)
> >
> > Since, like many of you I am going to TPAC, I will not have time to
make
> > changes prior to the face to face. Janina please put a link to this
document
> > in your canvas agenda for TPAC.
> >
> > See you all in California.
> >
> >
> > Rich Schwerdtfeger
> >
>