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

Tab Atkins Jr. jackalmage at gmail.com
Thu Jul 5 23:28:33 PDT 2012

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.


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