float DistanceFromRectangle(Vector2 point, Vector2 center, Vector2 size, float angle) {

    // First, transform the point into the rectangle's local coordinate space.
    Vector2 offset = Rotate(point - center, -1 * angle);

    // Take the absolute value of this offset, on x and y, and subtract the size.
    // This gives zero on the edge, increasing positively as we move away.
    // Clamp out any values less than zero.
    Vector2 outside = Max(Abs(offset) - size/2, Vector2.zero);

    // Return the length of this vector, which is our distance to the closest point
    // on the square's edges or corners, and zero inside the square.
    return Length(outside);
}