[Sidefx-houdini-list] advect along surface

Gary Jaeger gary at corestudio.com
Mon Sep 12 01:20:10 EDT 2016


OK, so I’ve actually been able to sort of get what I want by using VDB combines which reveals the points over time as the two vdv’s intersect. I’m using the result of the combine into a convert to polygons as the scatter source. This works for the scatter, and the points appear as they should. The problem is in the solver part. The solver which advects the points on the VDB only seems to take into account the point positions on the first frame. So even though my combine is yielding an emerging volume, and the scatter points are scattered properly over time, the trails from the solver only seem to be emerging from the points that existed at frame 1. Does that make sense? Even though more points are appearing over time, they aren’t getting advected or moved by the solver. Actually they don’t even show up in the solver, just the points at the first frame. I’m sure it’s something obvious...

Gary Jaeger / 650.728.7957 direct / 415.518.1419 mobile
http://corestudio.com <http://corestudio.com/>
> On Sep 11, 2016, at 5:25 PM, Gary Jaeger <gary at corestudio.com> wrote:
> 
> Thanks guys. I’ll take a look and see if we can sort it out. We’re basically combining https://www.sidefx.com/tutorials/curl-noise-flow/ <https://www.sidefx.com/tutorials/curl-noise-flow/> and https://www.sidefx.com/tutorials/houdini-boolean-volume-denting/ <https://www.sidefx.com/tutorials/houdini-boolean-volume-denting/> but having our points (and trails) slide out of the intersection cavities i.e when the poly mesh breaks the surface we want to allow the points to come out and flow along the surface. Fun problem to solve. 
> 
> Gary Jaeger / 650.728.7957 direct / 415.518.1419 mobile
> http://corestudio.com <http://corestudio.com/>
>> On Sep 11, 2016, at 3:54 PM, Olex P <hoknamahn at gmail.com <mailto:hoknamahn at gmail.com>> wrote:
>> 
>> Hi Gary,
>> 
>> If I understand you correct, I would sample velocity from a volume, then
>> transform it into a tangent (uv / whatever) space of the "mesh", find new
>> position in the same space and finally transform that position from uv /
>> tangent space into 3d space.
>> How exactly you do that is another question. It really depends what are you
>> after.
>> Another method would be computing position as p' = p + dv, where dv is
>> velocity per frame and then doing correction of that new position p' by
>> volume gradient (i.e. pushing position along the normal to the surface).
>> This one is fully 3d, but you need a wide enough band in the volume.
>> 
>> On Sun, Sep 11, 2016 at 11:17 PM, Gary Jaeger <gary at corestudio.com <mailto:gary at corestudio.com>> wrote:
>> 
>>> Hello. We’ve been experimenting with advecting points in VDB volumes.
>>> Cool, but is it possible to do something similar but only along the surface
>>> of a poly mesh, instead of inside the whole volume?
>>> 
>>> Gary Jaeger / 650.728.7957 direct / 415.518.1419 mobile
>>> http://corestudio.com <http://corestudio.com/> <http://corestudio.com/ <http://corestudio.com/>>
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