Checks whether two line-segments interset.

RayCollision.cpp

/**
 * Checks for intersection with another line segment. Touching at
 * endpoints qualifies as intersections.
 *
 * @param math::CRay2 The other line
 *
 * @return TRUE if they intersect, FALSE otherwise.
 * @see http://gamedev.stackexchange.com/questions/26004/how-to-detect-2d-line-on-line-collision
 **/
bool CRay2::CheckCollision(const CRay2& Other, math::CVector2* p_Intersection) const
{
    /**
     * Solve for the intersection point.
     *
     * Derivation:
     *  First line:             y - y1 = m1(x - x1)
     *  Second line:            y - y2 = m2(x - x2)
     *  Equal to each other:    m1x - m1x1 + y1 = m2x - m2x2 + y2
     *  Solve for 'x':          m1x - m2x = -m2x2 + y2 + m1x1 + y1
     *                          x(m1 - m2) = -m2x2 + y2 + m1x1 + y1
     *                          x = (-m2x2 + y2 + m1x1 + y1) / (m1 - m2)
     *  Solve for 'y':          y = m1(x) - m1x1 + y1
     *  Thus (x, y) becomes the intersection point.
     *
     *  Then we must see if (x, y) is in the range of line segments given.
     **/

    math::CVector2 Intersect;
    float m1, m2;

    m1 = this->GetSlope();
    m2 = Other.GetSlope();

    // Test for infinite slopes or parallel lines.
    if(this->Start.x == this->End.x)
        Intersect.x = this->Start.x;
    else if(Other.Start.x == Other.End.x)
        Intersect.x = Other.Start.x;
    else if(m1 == m2)
        return false;
    else
        Intersect.x = (-m2 * Other.Start.x + Other.Start.y + m1 * this->Start.x - this->Start.y) / (m1 - m2);

    Intersect.y = m1 * Intersect.x - m1 * this->Start.x + this->Start.y;

    if(this->OnRay(Intersect) && Other.OnRay(Intersect))
    {
        if(p_Intersection != NULL)
            *p_Intersection = Intersect;
        return true;
    }
    else
        return false;
}