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

From: Jonas Sicking <jonas@sicking.cc>
Date: Fri, 28 Oct 2011 16:14:43 -0700
Message-ID: <CA+c2ei-7Bp7E0XUnpMWkdiSRj5kAcwjeNOOz5K-pLri01dJroQ@mail.gmail.com>
To: Charles Pritchard <chuck@jumis.com>
Cc: Richard Schwerdtfeger <schwer@us.ibm.com>, franko@microsoft.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
On Fri, Oct 28, 2011 at 2:19 PM, Charles Pritchard <chuck@jumis.com> wrote:
>> On Fri, Oct 28, 2011 at 1:02 AM, Charles Pritchard<chuck@jumis.com>
>>  wrote:
>>>> On Thu, Oct 27, 2011 at 7:27 PM, Charles Pritchard<chuck@jumis.com>
>>>>  wrote:
>>>>> On 10/27/11 5:42 PM, Jonas Sicking wrote:
>>>>>> 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
>>>>> My primary concern here is path fidelity. Apart from the extra steps I
>>>>> outlined earlier, would a mature setDrawingFor implementation keep all
>>>>> of
>>>>> the paths for accessibility?
>>>>> setPathForElement instructs the implementation to maintain the already
>>>>> constructed path on some kind of stack or other ordered object.
>>>>> setDrawingFor would -possibly- maintain a series of paths opposed to a
>>>>> series of subpaths.
>>>>> Basic implementations are most likely to just keep bounding box
>>>>> information
>>>>> in either case. More advanced implementations will maintain some kind
>>>>> of
>>>>> additional information about the interactive region.
>>>> I don't think keeping just bounding box information should be
>>>> permitted. My idea is that when the user clicks a<canvas>    which has
>>>> used the setDrawingFor API, then we forward the click to the actual
>>>> elements inside the canvas. Similar to how labels work. I.e. any event
>>>> handlers registered on the element would fire. If the element is a
>>> Yes, I like that idea.
>>>> <input type=checkbox>    then it would flip its "checkedness" etc. But
>>>> only pixels drawn while setDrawingFor is in effect acts as a
>>>> forwarding area.
>>> This kind of implementation requires keeping a bitmap for each
>>> interactive
>>> element.
>>> It can be a small bitmap, just 1 bit per pixel.
>> That would be one way to implement it. Another implementation strategy
>> would be for to just store vector information for each issued drawing
>> command. This information is then used to do hit-testing internally in
>> the implementation. So for a path-based drawing command the
>> implementation would remember the path. For a drawImage command the
>> implementation would remember the coordinates that make up the square.
>> This is similar to how we implement image maps in Gecko where each
>> <area>  is parsed into a set of coordinates which are then used to do
>> hit testing.
> I recommend that browser vendors take the vector strategy.
> Is this something that would only apply on "fill" commands?
> fillText may be tricky -- I'd recommend treating it as a rectangle in the
> implementation.
> Should the current clip() be applied? Seems that it would.

Yes. Absolutely.

> I'm trying to flesh out the implementation details.
> Calculating clip against a path can be a bit costly -- having tried and done
> it poorly twice in implementations: I'd recommend that the clip() be added
> onto the path list and used before hit testing subsequent drawing commands.
> That is...  ctx.fillRect(..); ctx.clip();  ctx.fillRect(); would have two
> items on the hit testing path list, the clip rect would have the special
> property of requiring the event fall within it before hit testing subsequent
> paths.

That sounds like a good implementation strategy indeed.

>>>> This way authors have a much greater incentive to use the
>>>> setDrawingFor API. I.e. they would get much simpler event handling for
>>>> all their users. The fact that we'd be able to use the same
>>>> information to drive screen magnifiers etc is "just" a convenient side
>>>> effect from the authors point of view. This is the type of API which
>>>> has worked the best historically. I.e. bolt-on solutions which are
>>>> there only for a11y users has seen much less usage than APIs which add
>>>> enough semantics to help all users. (This is why we prefer if people
>>>> use HTML based UIs, rather than canvas based ones after all, right?)
>>>> (Note, i'm not saying that bolt-on is bad or should be avoided. But
>>>> when we can find better solutions, I think we should use them).
>>> The setPathForElement method is providing the same incentive: simpler
>>> event
>>> handling for users, while also driving screen magnifiers and other
>>> spatial
>>> AT, such as AT running on touch-based devices.
>>> I'm afraid there may be a miscommunication, as I don't believe either of
>>> us
>>> have suggested a "bolt-on solution".
>>> We're looking at the same pointer semantics for both proposals. Setup an
>>> interactive region to delegate pointer events to an element within the
>>> Canvas sub-tree.
>> Hmm.. so is the intent for setPathForElement that it would forward
>> clicks to the appropriate element inside the canvas too? If so, it
>> doesn't seem like a bounding box would be enough to remember for the
>> implementation.
> You're correct: it would forward clicks too. You're also correct, a bounding
> box is rather crude. That said: VoiceOver on Mobile Safari uses bounding
> boxes from the accessibility tree.
> I'd prefer all vendors go beyond simply tracking a bounding box. But, as a
> first step, if they can at least start with the bounding box, that'd
> catch-up Canvas a11y with the rest of HTML5.

As I said before. I don't think we should worry about implementors
complaining before they actually do. My experience is that
implementors are much happier to implement a good API that is tricky
to implement, than a bad API that is easy to implement.

We're all here because we want to improve the web. Not because we want
to do the minimal amount of work necessary.

>> If setPathForElement is indeed intended to work the same way as
>> setDrawingFor, then it simply seems like a question of how much
>> convenience we supply for page authors. I.e. do we request that they
>> construct paths for all drawn "things" that they want hit testing for,
>> or do we let them use any drawing command.
>> My preference would be the latter.
> As an author, my preference is the former; I'd be happy to have anything,
> and I do believe that both semantics are useful.
> Now, my code is based on existing practices, so I'm already doing a lot of
> extra work for hit testing. Plugging setPathForElement would be easier for
> me to support than setDrawingFor, but both can work.

This is the part I don't understand. I'd really like to though. See
questions below.

>>>> I think we'll see significantly more usage if all the author needs to
>>>> do is to sprinkle setDrawingFor calls over the code to mark which
>>>> drawing calls are made for which elements. If setPathForElements
>>>> require authors to create new paths specifically just to mark bounding
>>>> boxes then I think fewer people will use it.
>>> setPathForElement is less work in the code bases I work on.
>> Same in Gecko. But I think it's more work for websites.
> In many cases, setPathForElement will require less lines of code to be
> added.
> Many existing projects already maintain their own hit testing code. In many
> cases the hit testing code is separate from the rendering code. That is,
> it's run in very separate methods. It'd be easier in my own projects to use
> setPathForElement, though I've shown you how I would use setDrawingFor for
> the same effect.

There's a short-term perspective and a long-term perspective here.

Or, put it another way. Would you have maintained your own hit testing
code if setDrawingFor would have been available in all <canvas>
supporting browsers when you originally wrote your code? If so, I'm
very interested to know why. My goal with setDrawingFor was that that
wouldn't be needed. I'm happy to make changes to setDrawingFor in
order to reach that goal.

However there is also a short-term perspective. I'd love to learn why
setPathForElement is easier to deploy on existing code. It seems to me
that the setDrawingFor almost is a complete superset in functionality
and so should almost be able to work as a drop-in replacement for

Is setPathForElement easier because it doesn't require the page to
issue the actual drawing command? I.e. setDrawingFor would require the
page to switch to a transparent "brush" and thus require the context

But if the hit-testing code is separated from the drawing code anyway,
then it would just be a single context save/restore. And inside it a
lot of path-building commands to create hit-test regions for the
various elements. This doesn't sound like a big downside, though I'm
eager to learn why if I'm wrong.

>>> I've shown a few examples of how and when it is more appropriate.
>>> Consider a simple button. In most cases of Canvas based UI, the author
>>> need
>>> only convey the bounding box of the button, or the outer shape of the
>>> button, the rest of the drawing calls are entirely presentational.
>>> That presentational information is certainly useful when/if serialized
>>> into
>>> an SVG document. I do feel that setDrawingFor has a strong tie to the
>>> component model being discussed in other lists.
>> In the button UI example you are talking about, why wouldn't the page
>> simply add a setDrawingFor(element) call before the code it already
>> has to draw the button, and maybe a setDrawingFor(null) after?
>> If it's really concerned about only getting the outermost bounding box
>> covered, it can add move the setDrawingFor calls to be just around the
>> code that draws the button background.
> Yes, that's correct, but that's one additional line of code.
> The following example demonstrates that, with a 100x100 button.
>>> Here is a basic example of supporting both methods in existing clients:
>>> // NOTE: real-world use would include polyfill methods for legacy clients
>>> (using CSS overlays).
>>> // setDrawingFor for legacy clients
>>> var hasDraw = Boolean('setDrawingFor' in ctx);
>>> if(hasDraw) ctx.setDrawingFor(element);
>>> ctx.fillRect(0,0,100,100);
>>> if(hasDraw) ctx.setDrawingFor(null);
>>> // setPathForElement for legacy clients
>>> var hasDraw = Boolean('setPathForElement' in ctx);
>>> ctx.fillRect(0,0,100,100);
>>> if(hasDraw) ctx.setPathForElement(element);
>>> // end.
>> I'm not sure that what this example intends to show?
> element = document.createElement('button');
> It shows how I might support buttons in my current code bases.
> As demonstrated, setPathForElement requires one less line of code.
> Given all the insertion points I'd add the code, that's a lot less code.

Ah, I see. However this only appears to be an issue if you're issuing
a lot of commands which aren't connected to any clickable element.
Since otherwise you wouldn't need to make the setDrawingFor(null)

Is that the case?

Note that if you're drawing a lot of details inside the rect in the
fillRect call above, this wouldn't affect the behavior of the end
result. All drawing commands which happen while setDrawingFor is in
effect are added to the hit-testing area for the element. If some
calls are completely inside a already existing area then the
implementation can even optimize that and not create any additional
vectors for those calls.

> For implementors: It'd be great, if setPathForElement or setDrawingFor are
> implemented, if debugger tools would allow us to see a visualization of the
> active element regions on the canvas. Sure, we can move our mouse around and
> see the cursor change, but seeing everything at once is quite helpful.
> Simply show random colors with a 0.3 opacity, it can be helpful.

This sounds great indeed. Not something we can put in the spec
obviously, but I think it in general shows the power of the approach
of both setPathForElement and setDrawingFor.

/ Jonas
Received on Friday, 28 October 2011 23:15:47 UTC

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