A weightmap is per-point, but it’s per-point on the emitting geometry, not the point cloud. So you can’t just do a plain “get weights” if you want to use the weightmap to control particle values like Velocity or Speed.
Instead, you use get a location, like the particle emit location, and then get the weightmap value at that location. Then you’ll have a particle per-point context to work with.
When you get the weight at a location, you get an interpolated weight value.
Courtesy of Vladimir Jankijevic on the XSI mailing list, here’s how to accumulate values in a weightmap:
The nice thing about his tree is how it sets things into per-point context so nicely. My own attempt seems crude in comparison:
The thing about setting weight map values is that you can’t do it like this:
ICE will evaluate this tree just once, even if it is in the Simulation stack. Perhaps this is a case of mistaken optimization? (branch is a scalar constant, so no need to reeval?)
If you replace the scalar node with any of these branches, then the weightmap values will change as your play through the timeline.
eX-SI 