Circle and line segment collision in C#

Question

I'm trying to implement a method to check if a circle and a line intersect. I took most of this code (fixed based on the answer), and also modified the code a bit to use Point's instead of Vector2f's`.

This is currently what I have:

private bool CircleLineIntersect(int x, int y, int radius, Point linePoint1, Point linePoint2) { Point p1 = new Point(linePoint1.X,linePoint1.Y); Point p2 = new Point(linePoint2.X,linePoint2.Y); p1.X -= x; p1.Y -= y; p2.X -= x; p2.Y -= y; float dx = p2.X - p1.X; float dy = p2.Y - p1.Y; float dr = (float)Math.Sqrt((double)(dx * dx) + (double)(dy * dy)); float D = (p1.X * p2.Y) - (p2.X * p1.Y); float di = (radius * radius) * (dr * dr) - (D * D); if (di < 0) return false; else return true; }

It looks consistent with this algorithm, so I'm not sure what the problem is.

If anyone could provide guidance it would be much appreciated.

EDIT:

It doesn't seem to be calculating correctly. For example with input x=1272, y=1809, radius=80, linePoint1={X=1272,Y=2332}, linePoint2={X=1272,Y=2544} there shouldn't be an intersection (y+radius is less than both y values of the line segment), but the function is returning true.

What IS your problem? In what way does this code not do what you want? — Aron
– Aron, Commented Sep 2, 2013 at 4:15
@Aron It doesn't seem to be calculating correctly. For example with input x=1272, y=1809, radius=80, linePoint1={X=1272,Y=2332}, linePoint2={X=1272,Y=2544} there shouldn't be an intersection (y+radius is less than both y values of the line segment), but the function is returning true. — Brad
– Brad, Commented Sep 2, 2013 at 4:27

Aron · Accepted Answer · 2013-09-02 04:57:34Z

1

Error exists in your test case. Not only does the it intersect, but your line goes through the center of the circle. The line is a vertical line (X =1272). Your circle is centred about (1272, 1809). ERGO it goes through the centre.

Perhaps you have a misunderstanding between the terms line and line-segment, within mathematics.

answered Sep 2, 2013 at 4:57

Aron

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2 Comments

Brad Over a year ago

I need the function to test for a line segment, not a line. Is there anything I can change to make it test the segment instead of the line? Perhaps testing if the point of intersection is within the line segment?

Brad Over a year ago

I added some code to test if the intersection point lies within the line segment and it seems to work correctly now (although I'm not sure how efficient it is).

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