visualpython-users

[Visualpython-users] disconnect between curve position and color

[Bruce P. <bap...@te...>]

The VPython documentation discusses using the curve object to
create trails for moving objects. The curve can also be used to record
the history of an intrinsic parameter (such as temperature represented
by object color) with code like:
         obj.pos=newpos
         obj.color=newcolor
         trail.append(pos=newpos, color=newcolor)
However if an abbreviated history is desired then the code
         obj.pos=newpos
         obj.color=newcolor
         trail.append(pos=newpos, color=newcolor)
         if len(trail.pos)>maxlen:
                 trail.pos=trail.pos[1:]

correctly truncates the curve length but incorrectly truncates the color.
Internally the curve object appears to implement:

                 if len(self.color)>len(self.pos):
                         self.color=self.color[0:len(self.pos)-1]

Is there any known way around this so that colors remain correctly associated
with position?

BTW storing position and color in a seperate list and replacing the entire
list on each iteration rapidly goes into memory management meltdown - it
works for small examples but rapidly dies with larger examples.


Bruce Peterson
Terastat, Inc
Information Access Systems
Voice (425) 466 7344
Fax (206) 350 3685

Re: [Visualpython-users] disconnect between curve position and color

[Bruce S. <bas...@un...>]

This looks like a bug to me, and I'll try to see whether it can be fixed 
(assuming it's a bug, not a feature!). Here is a workaround, though it is a 
workaround that will do the wrong thing in the future if the bug is fixed:

    trail.color[:-1] = trail.color[1:]  # move the colors down one slot to 
compensate
    trail.pos = trail.pos[1:]  # delete first slot in the position list

Bruce Sherwood

At 02:20 PM 02/07/01 -0700, Bruce Peterson wrote:
>The VPython documentation discusses using the curve object to
>create trails for moving objects. The curve can also be used to record
>the history of an intrinsic parameter (such as temperature represented
>by object color) with code like:
>         obj.pos=newpos
>         obj.color=newcolor
>         trail.append(pos=newpos, color=newcolor)
>However if an abbreviated history is desired then the code
>         obj.pos=newpos
>         obj.color=newcolor
>         trail.append(pos=newpos, color=newcolor)
>         if len(trail.pos)>maxlen:
>                 trail.pos=trail.pos[1:]
>
>correctly truncates the curve length but incorrectly truncates the color.
>Internally the curve object appears to implement:
>
>                 if len(self.color)>len(self.pos):
>                         self.color=self.color[0:len(self.pos)-1]
>
>Is there any known way around this so that colors remain correctly associated
>with position?
>
>BTW storing position and color in a seperate list and replacing the entire
>list on each iteration rapidly goes into memory management meltdown - it
>works for small examples but rapidly dies with larger examples.
>
>
>Bruce Peterson
>Terastat, Inc
>Information Access Systems
>Voice (425) 466 7344
>Fax (206) 350 3685
>
>
>
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Re: [Visualpython-users] disconnect between curve position and color

[Bruce S. <bas...@un...>]

On second thought, I guess it isn't a bug, though there might be a missing 
feature. Here's the issue:

When you assign a new set of points trail.pos = trail.pos[1:], as far as 
Visual is concerned you could be assigning any completely new list, as in 
trail.pos = mylist. When the new assignment is noticed, Visual has no 
choice but to adjust the length of the trail.color list to be the same as 
the length of the trail.pos list. Visual doesn't know anything about the 
"1", just that a shorter list is being assigned to trail.pos. So it doesn't 
and can't do an intelligent job and delete the first slot in the color 
list; it arbitrarily deletes the last slot in the color list. You could 
argue that a better arbitrary guess would be to delete the first slot 
rather than the last, and I don't know where that would take us.

What this does suggest is that what is missing is a method for the curve 
object which would let you specify how to slice the pos and color lists in 
the same way at the same time.

Bruce Sherwood

At 10:36 PM 02/07/01 -0400, Bruce Sherwood wrote:
>This looks like a bug to me, and I'll try to see whether it can be fixed 
>(assuming it's a bug, not a feature!). Here is a workaround, though it is 
>a workaround that will do the wrong thing in the future if the bug is fixed:
>
>    trail.color[:-1] = trail.color[1:]  # move the colors down one slot to 
> compensate
>    trail.pos = trail.pos[1:]  # delete first slot in the position list
>
>Bruce Sherwood

RE: [Visualpython-users] disconnect between curve position and color

[David S. <dsc...@vi...>]

> What this does suggest is that what is missing is a method 
> for the curve 
> object which would let you specify how to slice the pos and 
> color lists in 
> the same way at the same time.

Maybe slice deletion?

c = curve(...)
del c[:3]

RE: [Visualpython-users] disconnect between curve position and color

[Bruce S. <bas...@un...>]

Sounds like a good suggestion. One oddity is that curve.pos and curve.color 
are very similar to Numeric arrarys (but not the same as?). Numeric arrays 
don't permit slice deletion. But presumably Visual can do whatever it wants to?

Bruce Sherwood

At 05:29 PM 02/07/02 -0400, David Scherer wrote:
>Maybe slice deletion?
>
>c = curve(...)
>del c[:3]