[Visualpython-users] How do I truncate a curve?

[Gary R. <ga...@em...>]

Hi fellow VPython Users,
A few times in my code I have wanted to show a trail behind some moving object.
For example, you might want to see the path of the free end of the
double pendulum in the doublependulum.py example. To do this, you could add the
following code:

<code>
trailLength = 10000 # no of points in trail
trail = curve(color=(0.5,0.5,0.5))

# the existing while loop
while 1:
	.
	. other code here
	.
    posnew = frame2.pos+frame2.axis*L2
    trail.append(pos=posnew)
	# Now keep the trail length finite
    if len(trail.pos) > trailLength:
        trail.pos = trail.pos[20:]

    t = t+dt

</code>

However, I have encountered a problem with doing this which I assume is to do
with the curve being held in a Numeric array structure. If I understand Numeric
correctly, trail.pos = trail.pos[20:] won't cause a copy or truncation, just a
change to the offset to the start of the array, so the array/curve object will
continue to grow despite only showing the last 10000 points (in this example).
If a copy could be forced, the curve object would have to be reassociated with
the new array object, so I don't think a copy is occurring. A copy would be
very inefficient anyway.
Actually, when I run this example, everything seems to work OK, but in another
example I have, if I leave it running, the vpython window eventually locks up
and the lockup is associated with the code I have shown. I'm running Win98.
So, has anyone got a more efficient way to limit the length of a curve to some
maximum? Do others ever do this sort of thing and if so, do they agree that it
would be nice to have a property such as maxlength added to the curve object?

Gary
-- 
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