Masking data

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Hello,
I'm displaying a cloud through a 3D display:
something like ( index -> (x,y,z,color) ) in Visad notation.

When you have more than 20000 vertices displayed , operations like=20
rescaling an axis are very slow. To deal with this issue, I had the idea=20
to mask some vertices. For example, with 50000 vertices, I would=20
temporarily (during the rescaling operation) display  only 1 out of 4=20
vertices. Does someone know an efficient way to do that, i.e. without=20
removing the reference data, and creating a new one with only 12500=20
vertices ?
Thanks by advance.

Renaud DA=CFAN
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