矩形の外周を回るオブジェクトを表現したいときに使います。
ベクトルは中心からです。四角い時計を想像してもらえればわかりやすいと思います。

    public static Vector2 GetRectEdgeCrossPointFromCenter(Vector2 direction, Rect rect)
    {
        float dot_vertical = Vector2.Dot(Vector2.up, direction);
        float dot_horizon = Vector3.Dot(Vector2.right, direction);

        Vector2 v_vertical = Mathf.Sign(dot_vertical) * Vector2.up;
        Vector2 v_horizon = Mathf.Sign(dot_horizon) * Vector2.right;

        float rad_vertical = Vector2.Angle(direction, v_vertical) / 180 * Mathf.PI;
        float rad_horizon = Vector2.Angle(direction, v_horizon) / 180 * Mathf.PI;

        float tan_horizon = float.NaN;
        float tan_vertical = float.NaN;

        if (dot_vertical != 0)
        {
            tan_horizon = Mathf.Tan(rad_vertical);
            if (Mathf.Abs(tan_horizon) > 1.0f) { tan_horizon = float.NaN; }
        }
        if (dot_horizon != 0)
        {
            tan_vertical = Mathf.Tan(rad_horizon);
            if (Mathf.Abs(tan_vertical) > 1.0f) { tan_vertical = float.NaN; }
        }

        Vector2 edge_cross_point = rect.center;
        if (!float.IsNaN(tan_horizon))
        {
            edge_cross_point += v_vertical * rect.height * 0.5f;
            edge_cross_point += v_horizon * rect.height * 0.5f * tan_horizon;
        }
        else if (!float.IsNaN(tan_vertical))
        {
            edge_cross_point += v_horizon * rect.width * 0.5f;
            edge_cross_point += v_vertical * rect.height * 0.5f * tan_vertical;
        }

        return edge_cross_point;
    }