Compare these two frames from an animation of a roller coaster I'm 
working on.  Notice that in the second frame, the track spine has 
disappeared.

Throughout the animation, the track spine disappears randomly for 1-3 
frames at a time.  What would cause this?

The spine is a sphere_sweep generated through SDL using bezier data. 
Sphere_sweeps are generated by using some macros from transform.inc to 
orient the components of the sphere_sweep to the current track heading 
and banking.

I don't understand why it would disappear in some frames though, 
considering that NONE of the SDL for the sphere_sweep looks at the clock 
variable at all.  Its generated on every frame and should be the same on 
every frame.

The only thing I can possibly think of is rounding errors.  But if that 
was the case, then the other two rails should be messing up as well.  I 
suppose I could post the code:

#declare xx = 0;
#while (xx < NUMBEZIERS -1 )
#declare yy = 0;
#while (yy < POINTSPERBEZIER)
GetBezier(bez[xx],c2[xx],c1[xx+1],bez[xx+1], yy/POINTSPERBEZIER, ret)
#declare point1 = ret;
GetBezier(bez[xx],c2[xx],c1[xx+1],bez[xx+1], yy/POINTSPERBEZIER + 0.001, 
ret)
#declare point2 = ret;
#declare point3 = point2 - point1;
#declare il = int(xx);
#declare b = -((bank[xx+1] - bank[xx]) * (yy/POINTSPERBEZIER) + bank[xx]);
#declare raxis = vaxis_rotate(y,point3,b);
#declare bpsl[xx*POINTSPERBEZIER + yy] = VPerp_To_Plane(raxis,point3) * 
0.5 + point1;
#declare bpsr[xx*POINTSPERBEZIER + yy] = VPerp_To_Plane(raxis,point3) * 
-0.5 + point1;
#declare bpsc[xx*POINTSPERBEZIER + yy] = -0.2*raxis + point1;
#declare yy = yy + 1;
#end
#declare xx = xx + 1;
#end

sphere_sweep { linear_spline (NUMBEZIERS-1)*POINTSPERBEZIER - 1
#declare xx =0;
#while (xx < (NUMBEZIERS-1)*POINTSPERBEZIER -1 )
bpsl[xx], 0.0725
#declare xx=xx+1;
#end
pigment { color rgb x+y }
}

sphere_sweep { linear_spline (NUMBEZIERS-1)*POINTSPERBEZIER - 1
#declare xx =0;
#while (xx < (NUMBEZIERS-1)*POINTSPERBEZIER -1 )
bpsr[xx], 0.0725
#declare xx=xx+1;
#end
pigment { color rgb x+y }
}

sphere_sweep { linear_spline (NUMBEZIERS-1)*POINTSPERBEZIER - 1
#declare xx =0;
#while (xx < (NUMBEZIERS-1)*POINTSPERBEZIER -1 )
bpsc[xx], 0.2023
#declare xx=xx+1;
#end
pigment { color rgb x+y }
}

(END OF CODE)

bez, c1, and c2 are bezier data.  GetBezier is a macro for getting the 
coordinates in space of a point on a bezier at T.  bpsl, bpsr, and bpsc 
are of the course the generated sphere_sweep points for the left, right, 
and center rails respectively.

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