[whatwg] hit regions: clearing of regions

Edward O'Connor eoconnor at apple.com
Tue Mar 11 10:03:26 PDT 2014

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:



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