Question

Using a bit of code like this:

    static bool Intersects(Vector2 a1, Vector2 a2, Vector2 b1, Vector2 b2, out Vector2 intersection)
    {
        intersection = Vector2.Zero;

        Vector2 b = a2 - a1;
        Vector2 d = b2 - b1;
        float bDotDPerp = b.X * d.Y - b.Y * d.X;


        if (bDotDPerp == 0)
            return false;

        Vector2 c = b1 - a1;
        float t = (c.X * d.Y - c.Y * d.X) / bDotDPerp;
        if (t < 0 || t > 1)
            return false;

        float u = (c.X * b.Y - c.Y * b.X) / bDotDPerp;
        if (u < 0 || u > 1)
            return false;

        intersection = a1 + t * b;

        return true;
    }

I can determine where two lines collide. However, the coordinates are given relative to the window, not each other. For example, say the two lines are perpendicular to one another. One of them overlaps the second one by one pixel at 0,0. This would mean that to resolve this collision, I would need to move back 1 pixel. However, if I performed this same test at say, 400,400, it would tell me that it intersects at 399, 399. I obviously do not have to resolve the collision by 399, 399.

I’m looking to use this information to find out how much the lines are overlapping so I can move them back appropriately. What must I do to do this?

Answer 1

This answer was pretty obvious, and I figured it out.

Just subtract a1 and a2 from X and Y of outVect:

(outVect.X-a1.X), (outVect.Y-a1.Y)

This negates its location.