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Volumesample and Volumegradient

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I was following a tutorial on how to create a sun in houdini, and I came across a technique used to keep advected points bound to the original surface where they were scattered from (in this case, a sphere).
For the purpose of advecting the points, it was created a popnetwork with a curl noise affecting the velocity.

To keep the points moving on the surface, the instructor did 2 things:
-Turn the sphere into a mesh and convert it into a VDB.

-Create a POP VOP, with a Volumesample and a Volumegradient, reading from the vdb converted sphere. After this, the gradient was inverted and multiplied by the output of the Volumesample node and added to the original positions, inputing into the new positions.

This last point got me lost. The instructor referenced in some abstract drawings the utility of both this nodes, the Volumesample and Volumegradient and I tried to cross check with Sidefx's Houdini documentation but it didn't have that much info on the subject.

Can some one briefly explain what is happening inside both this nodes, and their respective outputs?


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have a look on this tutorial it explains quite well as how the volumesample and volumegradient work


Edited by Entropy
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@Entropy Will do!

Have seen many tutorials by them, but never this one in particular.

Thanks alot.

EDIT: Just watched the explanation part in the tutorial and everything is clear now. 

Edited by diogomgf
Updated situation

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