On Fri, Nov 21, 2008 at 6:33 PM, Greg Titus <[EMAIL PROTECTED]> wrote:
>
> On Nov 21, 2008, at 3:19 PM, DKJ wrote:
>
>> I want to determine whether a line between two CGPoints in a view
>> intersects with a given CGRect. There's nothing in the CGGeometry reference
>> that does this specifically, and I don't yet see how to use the functions
>> that are there to figure this out.
>>
>> Before I dust off my Euclid (a rare first edition), has anyone got a quick
>> solution? It seems the sort of thing that would be handy in many situations.
>>
>> dkj
>
> BOOL lineIntersectsRect(CGPoint a, CGPoint b, CGRect rect)
> {
> float lineSlope = (b.y - a.y) / (b.x - a.x);
> float yIntercept = a.y - lineSlope * a.x;
> float leftY = lineSlope * NSMinX(rect) + yIntercept;
> float rightY = lineSlope * NSMaxX(rect) + yIntercept;
>
> if (leftY >= NSMinY(rect) && leftY <= NSMaxY(rect))
> return YES;
> if (rightY >= NSMinY(rect) && rightY <= NSMaxY(rect))
> return YES;
> return NO;
> }
You'll want an additional check using the reciprocal slope and the X
intercept, as this code will fail on vertical lines and be very
inaccurate on nearly vertical lines. You'll probably want to do both
and make it an OR check, that is if this check succeeds then return
YES, otherwise do the other case and return YES if it succeeds.
Mike
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