Clipping Asymptote 3D images

Question

I am using Asymptote to generate 3D images which I embed in .pdf's. Is there a way to clip these 3D images, much like you can clip 2D images to a path? I'd be happy to clip to the interior of a rectangular solid.

The reason I ask is that using Spline to graph some functions creates some wavy, undesired end behavior. I've given an example below of a hyperboloid of one sheet. With spline, there is a warble at the top. I've tried to cut it off with a bool condition, but that gives very undesirable results.

Here is an example; note where you can comment out lines to see different results.

 import graph3;

 size(200,200,IgnoreAspect); currentprojection=orthographic(4,4,0);
 defaultrender.merge=true;

 defaultpen(0.5mm);

 //Draw the surface z^2 - x^2 - y^2=1 
 triple f(pair t) {return (cos(t.y)*tan(t.x), sin(t.y)*tan(t.x),1/cos(t.x));
 }

 bool cond(pair t) {return 1/cos(t.x) <1.5;}

 //The following plots with just Spline. Note the warble.
 surface s=surface(f,(-1,0),(1,2*pi),32,16,Spline); 

 // Comment above and uncomment below to use the boolean cond. 
 //surface s=surface(f,(-1,0),(1,2*pi),32,16,Spline,cond); 
 pen p=rgb(0,0,.7); 
 draw(s,rgb(.6,.6,1)+opacity(.7),meshpen=p);

I could remove the Spline option but I like how it smooths things otherwise.

Here's the warble:

And here's what happens if you try to cut it off with the bool option:

Answer 1

This does not answer your question, but it does solve your problem here. You can eliminate the waviness in this case by telling the spline interpolator that z is monotonically increasing with respect to t.x (which is the u parameter). You do this by playing with the spline type parameter:

surface s=surface(f,(-1,0),(1,2*pi),32,16,
          usplinetype=new splinetype[] {notaknot,notaknot,monotonic},
          vsplinetype=Spline);

Code in full:

 settings.outformat="png";
 settings.render=4;
 import graph3;

 size(400,400,IgnoreAspect); currentprojection=orthographic(4,4.1,2);
 defaultrender.merge=true;

 defaultpen(0.5mm);

 //Draw the surface z^2 - x^2 - y^2=1 
 triple f(pair t) {return (cos(t.y)*tan(t.x), sin(t.y)*tan(t.x),1/cos(t.x));
 }

 surface s=surface(f,(-1,0),(1,2*pi),32,16,
          usplinetype=new splinetype[] {notaknot,notaknot,monotonic},
          vsplinetype=Spline);

 pen p=rgb(0,0,.7); 
 draw(s,rgb(.6,.6,1)+opacity(.7),meshpen=p);

The result: