[whatwg] isPointInPath v. set of pixels in canvas hit regions

Rik Cabanier cabanier at gmail.com
Fri Jul 6 22:58:00 PDT 2012

On Fri, Jul 6, 2012 at 5:40 PM, Dean Jackson <dino at apple.com> wrote:

> On 07/07/2012, at 10:11 AM, Charles Pritchard <chuck at jumis.com> wrote:
> >> On Thu, Jul 5, 2012 at 1:05 PM, Edward O'Connor <eoconnor at apple.com>
> wrote:
> >>> As things currently stand in the spec, implementations basically need
> to
> >>> keep N+1 bitmaps per canvas, where N is the number of hit regions. I
> >>> doubt any implementors would be enthusiastic to implement hit regions
> >>> like this. From a WebKit perspective, we'd much prefer keeping a Path
> >>> for each hit region, and then simply using isPointInPath for hit
> >>> testing. This also implies that the current piggybacking of "Clear
> >>> regions that cover the pixels" in clearRect() could go away. Yay! :)
> >>
> >> Bog-standard hit-testing algorithms apply.  Keep a single extra canvas
> >> around, draw each region into it with a different color.  When you're
> >> hit-testing, just see what color that pixel is, and look up which
> >> region is associated with it.  This is extremely fast and simple to
> >> implement, and has all the right properties - the "topmost" region for
> >> a given pixel is the one returned.
> We're aware of this technique, but it has a couple of obvious issues:
> 1. It requires us to create a duplicate canvas, possibly using many MB of
> RAM. It's generally going to be less memory to keep a list of geometric
> regions. And performance won't be terrible if you implement some spatial
> hashing, etc.
> 2. It doesn't allow for sub pixel testing. In your algorithm above, only
> one region can be at a pixel (which also means it isn't our standard
> drawing code with anti-aliasing). Consider a zoomed canvas, where we might
> want more accurate hit testing.
Wouldn't thess problems go away if you use an OpenGL/DirectX backend to
Canvas like so many browsers are doing?
That way, you only need the display list for hit testing and just render
the region for hit testing (ie


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