[Visualpython-users] a display problem with my Vpython program

[Ting L. <dri...@gm...>]

Hello,

I have a display problem with my Vpython program.
In the scene, I need to draw thousands of spheres and cylinders, and I
also need to switch the visibility of some groups of them on and off,
fast sometimes. I used a list to store all the objects and then loop
over each object to change its visible property.
The problem is
When I set visible to 1 for a group of objects by loop, then in the
following code I immediately change visible to 0 for a sub-group of
them, I found the whole group first displayed then the objects in the
sub-group disappeared (because I set them to be invisible). When I
repeat this procedure quickly, this problem cause ugly 'flashing'.
(some object appear then disappear.)

...
for o in objects:
	o.visible = 1
...
for o in objects[start:end]:
	o.visible = 0
...


Is there a better way to set the visibility of a group of objects more
efficiently?
According to the VPython doc, I can group some objects into one frame,
then I can move or rotate them as a whole. But to change the visible
property, I still need loop, right?

One guess of mine is this problem is related to the display mechanism
of Vpython.
VPython doesn't have a 'redraw' or 'UpdateView' or something like that
exposed to the user. Once you draw an object, it call the internal
'redraw' method implicitly.
If this is true, then I think we need an explicit 'redraw' method to
control the display of the scene we don't want them to display
immediately. This is in fact the intention of the 'double buffer' in
OpenGL.

I know I can combine the two loops into one if I can know the
visibility of all the objects in a prior. Thing maybe becomes better.
But the problem I explained above remains, because during the looping,
if an object will show up immediately after setting the 'visible=1',
the objects in the scene will look like cabbages emerging or growing
from underground!
I do prefer to hide them first and display all of them at the same time.

I have to say I'm not very familiar with Vpython. So if somebody can
help me to clarify these problems, it would be very appreciated!

Ting