[whatwg] hit regions: clearing of regions

[Dirk Schulze]

On Mar 12, 2014, at 12:13 AM, Rik Cabanier <cabanier at gmail.com> wrote:

> On Tue, Mar 11, 2014 at 10:03 AM, Edward O'Connor <eoconnor at apple.com>wrote:
> 
>> Hi Rik,
>> 
>> You wrote:
>> 
>>> Currently, the specification states that if you create a region and
>>> then create another region that completely covers region, the first
>>> region is removed from the hit region list [1]
>>> 
>>> This is a complex operation that involves either drawing the regions
>>> to a bitmaps and counting pixels, or path intersection. It is also
>>> unintuitive because an author might expect that he could remove the
>>> second region and still have the first region active.
>>> 
>>> What was the reason that this was added to the spec?
>> 
>> The model the spec uses for hit regions is that you have two bitmaps--one
>> for the canvas surface, and the other is used to track which hit region
>> the pixels of the canvas surface are in. See this thread from 2012:
>> 
>> 
>> http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2012-July/thread.html#36556
> 
> 
> Thanks!
> It looks like that thread never came to a conclusion.
> 
> The arguments against using a bitmap presentation still stand:
> - it will be too expensive to keep an actual copy of the canvas pixels in
> memory to do hit testing
> - if you have to mimic the behavior with paths, you need access to
> expensive/difficult path manipulation algorithms
> 
> Hit regions should be redesigned so they work on the path geometry as
> opposed to pixels. We already have the necessary code to do hit testing on
> paths (see isPointInPath)

In SVG we tried to avoid having hit testing based on pixel values obviously for performance reasons.

Greetings,
Dirk