Concave Hull in Houdini

Forums Technical Discussion Concave Hull in Houdini

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Samuel G-C: Member; 10 posts; Joined: June 2015; Offline

June 19, 2017 10:22 a.m.

Hi,

I am trying to find the concave hull of a set of points in Houdini.
I noticed that the bullet solver has a concave representation for colliding objects.
Is it possible to retrieve that geometry for use elsewhere?
And does the algorithm used to perform Concave representations there also work for a set of disconnected points?

Thanks


Nima: Member; 471 posts; Joined: Nov. 2013; Offline

June 19, 2017 1:46 p.m.

Hello.
Use Shrinkwrap node in SOP level to create concave representation of your object.
This node works with just points as well.


Samuel G-C: Member; 10 posts; Joined: June 2015; Offline

June 21, 2017 1:11 p.m.

The Shrinkwrap node seems to only create convex hulls.
I need something that will conform more closely to the “surface” of a set of points.


Nima: Member; 471 posts; Joined: Nov. 2013; Offline

June 22, 2017 6:04 a.m.

If you wanna have better collision representation of your objects in simulation:
1.Convert your model to VDB via VDB from polygon node.
2.Use VDB to sphere to create set of spheres which representing your model.
3.Use this geo in your dynamics simulation.

I've created a simple HIP file.
Hope this helps.

Edited by Nima - June 22, 2017 06:05:29

Attachments:
better collision representation.hip (214.0 KB)


Samuel G-C: Member; 10 posts; Joined: June 2015; Offline

Oct. 23, 2017 5:40 p.m.

I have also tried working with VDB's and this does give a closer representation. However, the conversion from point to VDB and then VDB to polygon seems to create an overhead I'd rather avoid.
Is there a way to bypass VDB and quickly get to the “surface.”

Surface here would mean also that the points are on that surface.
Any thoughts?

Thanks


cgbeige: Member; 99 posts; Joined: March 2009; Offline

Oct. 23, 2017 6:42 p.m.

this is what I do:

http://polygonspixelsandpaint.tumblr.com/post/157710723694 [polygonspixelsandpaint.tumblr.com]

It works a lot better for collision since you can have things skate over the surface naturally that wouldn't work for vdb method

Edited by cgbeige - Oct. 23, 2017 18:42:59


Samuel G-C: Member; 10 posts; Joined: June 2015; Offline

Oct. 24, 2017 1:01 p.m.

A good suggestion thanks. Unfortunately I can't use voronoi fracture in my case because I don't have a geometry to break. I am working off a particle system and trying to find a concave representation of the points in said system.


Olaf Finkbeiner: Member; 323 posts; Joined: Jan. 2015; Offline

Oct. 24, 2017 4:35 p.m.

hi there,
just add any normal to your points and use plointcloudiso

I also like: https://vimeo.com/228620264 [vimeo.com]

greetings

Olaf


bonsak: Member; 459 posts; Joined: Oct. 2011; Offline

Oct. 24, 2017 5:39 p.m.

If you first turn your source_particles node into a Deforming Collision object (collision tab -> deforming object) and then replace the collisionsource inside the source_particles with a vdbfromparticlefluid. Then you get a reasonable fast and accurate concave collision volume.

-b

Edited by bonsak - Oct. 24, 2017 17:41:06

Attachments:
collide.hipnc (468.2 KB)

http://www.racecar.no [www.racecar.no]


Samuel G-C: Member; 10 posts; Joined: June 2015; Offline

Oct. 25, 2017 1:43 p.m.

Thanks. Those are all good suggestions. I will look into them.

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