UIBezierPath intersect

Question

I\'ve been searching for an answer for hours but have trouble finding anything on the topic.

I have a question related to Objective-c. I\'m making an application in

Answer 1

The problem is in the checkLineIntersection method. With

if (ua > 0.0 && ua < 1.0 && ub > 0.0 && ub < 1.0) { return YES; }

you check only if the interior part of the lines segments intersect. But if the start or endpoint of the first line segment is equal to the start or endpoint of the second line segment, ua and ub will be 0.0 or 1.0.

The solution is to include one end of the interval in the condition:

if (ua > 0.0 && ua <= 1.0 && ub > 0.0 && ub <= 1.0) { return YES; }

This seemed to work as expected in my test program.

Some further remarks:

• I think you should activate the shortcut

  if (denominator == 0.0f) return NO;
  

  again to avoid division by zero.

• In touchesMoved, you could add the new line to the array after checking for intersections. Now the new line is inserted first, which means that it is checked against itself for intersections.

• You have declared Line as subclass of UIView, but this is not really a view class. You could just declare Line as subclass of NSObject.

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ADDED: The following method might work even better, because it avoids the division and therefore possible overflow problems with small denominators:

-(BOOL)checkLineIntersection:(CGPoint)p1 :(CGPoint)p2 :(CGPoint)p3 :(CGPoint)p4
{
    CGFloat denominator = (p4.y - p3.y) * (p2.x - p1.x) - (p4.x - p3.x) * (p2.y - p1.y);
    CGFloat ua = (p4.x - p3.x) * (p1.y - p3.y) - (p4.y - p3.y) * (p1.x - p3.x);
    CGFloat ub = (p2.x - p1.x) * (p1.y - p3.y) - (p2.y - p1.y) * (p1.x - p3.x);
    if (denominator < 0) {
        ua = -ua; ub = -ub; denominator = -denominator;
    }
    return (ua > 0.0 && ua <= denominator && ub > 0.0 && ub <= denominator);
}