Subject: Re: Intersection of CGPath and CGRect
On 28/06/2009, at 4:43 AM, Martin Hewitson wrote:
Does anyone know a good way to test if a CGPathRef intersects a
CGRect at any point?
This can be a hard problem.
First, naturally discard the trivial case of the bounding rect of the
path not intersecting the rect you're testing. The further approach
you take will probably best be dictated by what you're trying to
If the purpose is to simply test whether a rect has hit a path, for
hit-testing purposes, I've found one of the fastest general methods is
to render the path into a 1 x 1 bitmap context with only an alpha
channel. Using the rect for hit-testing against as a source rect, the
portion of the path bounding rect that intersects that is scaled down
using a suitable transform into the bitmap. If the outcome is some non-
transparent alpha value, you got a hit, otherwise, you didn't.
Somewhat surprisingly, I found this technique faster than any other I
tried and very nice for hit-testing, as it takes i...
nto account what was
actually rendered - strokes, fills, alpha and anti-aliasing. (Thanks
due here to Ken Ferry who first suggested this approach to me).
If on the other hand you actually need to find the points at which the
rect intersects the path, you'll need a very different approach. In
this case testing a line segment against the path can be done, so
you'd need to break the rect down into four line segments then test
each one. It's hard to quickly find the intersection of a line segment
with an arbitrary bezier curve, but somewhat easier if the curve is
first flattened so you're just testing line segments against other
ine segments. It can still be an expensive operation though - it's
well worth breaking a path down into its elements and first finding
the bounding rects of each element, and trivially eliminating all
those that don't intersect, then doing the more brute-force
intersection test on what remains.
DrawKit ( http://apptree.net/drawkitmain.htm
) contains code for doing
both kinds of testing - feel free to use what's there.
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