Unity 2D Line Segment Intersection

License.txt

This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <http://unlicense.org/>

LineUtil.cs

using UnityEngine;

public static class LineUtil {
    public static void Swap<T>(ref T lhs, ref T rhs) {
        T temp = lhs;
        lhs = rhs;
        rhs = temp;
    }

    public static bool Approximately(float a, float b, float tolerance = 1e-5f) {
        return Mathf.Abs(a - b) <= tolerance;
    }

    public static float CrossProduct2D(Vector2 a, Vector2 b) {
        return a.x * b.y - b.x * a.y;
    }

    /// <summary>
    /// Determine whether 2 lines intersect, and give the intersection point if so.
    /// </summary>
    /// <param name="p1start">Start point of the first line</param>
    /// <param name="p1end">End point of the first line</param>
    /// <param name="p2start">Start point of the second line</param>
    /// <param name="p2end">End point of the second line</param>
    /// <param name="intersection">If there is an intersection, this will be populated with the point</param>
    /// <returns>True if the lines intersect, false otherwise.</returns>
    public static bool IntersectLineSegments2D(Vector2 p1start, Vector2 p1end, Vector2 p2start, Vector2 p2end,
        out Vector2 intersection) {
        // Consider:
        //   p1start = p
        //   p1end = p + r
        //   p2start = q
        //   p2end = q + s
        // We want to find the intersection point where :
        //  p + t*r == q + u*s
        // So we need to solve for t and u
        var p = p1start;
        var r = p1end - p1start;
        var q = p2start;
        var s = p2end - p2start;
        var qminusp = q - p;

        float cross_rs = CrossProduct2D(r, s);

        if (Approximately(cross_rs, 0f)) {
            // Parallel lines
            if (Approximately(CrossProduct2D(qminusp, r), 0f)) {
                // Co-linear lines, could overlap
                float rdotr = Vector2.Dot(r, r);
                float sdotr = Vector2.Dot(s, r);
                // this means lines are co-linear
                // they may or may not be overlapping
                float t0 = Vector2.Dot(qminusp, r / rdotr);
                float t1 = t0 + sdotr / rdotr;
                if (sdotr < 0) {
                    // lines were facing in different directions so t1 > t0, swap to simplify check
                    Swap(ref t0, ref t1);
                }

                if (t0 <= 1 && t1 >= 0) {
                    // Nice half-way point intersection
                    float t = Mathf.Lerp(Mathf.Max(0, t0), Mathf.Min(1, t1), 0.5f);
                    intersection = p + t * r;
                    return true;
                } else {
                    // Co-linear but disjoint
                    intersection = Vector2.zero;
                    return false;
                }
            } else {
                // Just parallel in different places, cannot intersect
                intersection = Vector2.zero;
                return false;
            }
        } else {
            // Not parallel, calculate t and u
            float t = CrossProduct2D(qminusp, s) / cross_rs;
            float u = CrossProduct2D(qminusp, r) / cross_rs;
            if (t >= 0 && t <= 1 && u >= 0 && u <= 1) {
                intersection = p + t * r;
                return true;
            } else {
                // Lines only cross outside segment range
                intersection = Vector2.zero;
                return false;
            }
        }
    }
}

awesome ! works perfectly thanks for this <3

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Verify Github on Galxe. gid:AnLTmP36d5nYvuvt7eimgd

Very strong project

Very strong project

Verify Github on Galxe. gid:AnLTmP36d5nYvuvt7eimgd

This works great and how cool it is to come across the creator of the mighty Ogre3D after all these years :)

Please define a license for this (e.g. MIT license).
Great solution!

Public domain, use it however you want (I've added License.txt)

In my case I actually just needed to know the distance to the intersection with a square which you can do with the Bounds and a Ray. Code snippet below:

Bounds bounds = square.bounds;
Vector2 direction = point - previousPoint;
Ray ray = new(previousPoint, direction);

if (bounds.IntersectRay(ray, out float distance))
{
                    Debug.Log("Collide Distance: " + distance);
}

There is also bounds.Contains(point) which I found useful too.

Just in case it is useful to anyone else 👍