var tang[3];
vec_diff(temp, player.x, my.x);
vec_set(tang, my.pan);
vec_inverse(tang);
vec_rotate(temp, tang);
//
// Now temp holds the offset of the object in local coordinates.
//
if (temp.x > my.min_x && temp.x < my.max_x) &&
   (temp.y > my.min_y && temp.y < my.max_y)
{
    // I'm inside the XY plane!
}