CODEWITHSUNDEEP
c#geometrycollision-detection
Brad
Brad

Reputation: 10660

Circle and line segment collision in C#

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.

Upvotes: 0

Views: 1523

Answers (1)

Aron
Aron

Reputation: 15772

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.

Upvotes: 1

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