### Boundaries for parametric_plot3d?

Is it possible to specify the bounding box of the output of a call to parametric_plot3d?

I am trying to produce animations of Frenet frames. The following code 'works' more or less, but in each frame sage changes the bounding box slightly, producing less than ideal results. If I have to resort to plotting some kind of bounding object with opacity set to 0, then so be it, but I am hoping for a more elegant solution.

```
t = var('t')
x(t) = (cos(3*t)+2)*cos(2*t)
y(t) = (cos(3*t)+2)*sin(2*t)
z(t) = -sin(3*t)
gamma = vector([x,y,z])
def length(v):
return sqrt(v[0]^2+v[1]^2+v[2]^2)
a = 0
b = 2*pi
P = parametric_plot3d(gamma(t),(t,a,b),frame=False)
T = gamma.diff(t)/length(gamma.diff(t))
N = T.diff(t)/length(T.diff(t))
B = T.cross_product(N)
def frame(t):
return P + arrow(gamma(t),gamma(t)+T(t),color='green',frame=False) + arrow(gamma(t),gamma(t)+N(t),color='red',frame=False) + arrow(gamma(t),gamma(t)+B(t),color='purple',frame=False)
num_frames = 10
frenet_movie = animate([frame(tt) for tt in srange(a,b,(b-a)/num_frames)])
frenet_movie.show()
```

(You can up num_frames if you want a smoother movie, though these take a while to render. The length function is defined because the `normalized()`

method was producing lots of strange errors, namely with sage claiming it could not produce numerical approximations of the vectors involved....)