[Sidefx-houdini-list] basic instancing question

Remba, David (Space Technology) David.Remba at ngc.com
Fri Oct 24 13:10:52 EDT 2008

I've been working on clouds, and had good results
using simple geometry (spheres, boxed shapes) then
subdividing to round the shapes, then iso-offset to
create a low density volume (20x20x20).

The good stuff happens in a custom material:

1. displacement VOP SHOP uses various noise functions
to push P around (for clouds: only in the up +Y direction
to simulate convection, and using an abs() to make it
puffy looking).  For nebula I would try the built-in
aanoise VOP.

2. surface VOP SHOP based from the VEX Volumecloud shader,
and added extra features for clouds to cut off and
darken the bottom.  Higher density makes the gas more
chunky looking, lower values make it more wispy looking
(good for nebulas).


Very important to adjust the Volume Step size to as large
a value as possible (just below the noticeable speckle artifact).

For fast rendering: try only one or two lights; if you need
more, do multiple render passes and comp them.  Use Ray Trace
Engine, lower opacity limit to 0.9 if the nebula/clouds are

For nebula you might try some glow effects (see the emit
in the VEX Super Material shader).

p.s. For nebulas, you might try separating very far objects
which are more of a 2D problem, from closer 3D objects.

p.p.s. I work at a space technology company so I might get
to work on nebulas soon also!  I'd like to hear you things
go for you.  My email:  daveremba at yahoo.com

I hope this helps,


David Remba 
Tel 310 813 5899 Cel 310 809 6375 
Northrop Grumman Space Technology
IMAGE Center

-----Original Message-----
From: sidefx-houdini-list-bounces at sidefx.com on behalf of jon parker
Sent: Wed 10/22/2008 9:13 PM
To: sidefx-houdini-list at sidefx.com
Subject: [Sidefx-houdini-list] basic instancing question
Greetings esteemed Houdini masters!
I've been tasked to create a planetary nebula by some very demanding
scientists... I've done some lighting for studios using Houdini in the
past, but now the holes in my knowledge about Houdini are showing up,
so I am looking for some basic workflow questions to be cleared up.
For the nebula, the basic gas volume is looking great, however there
are a large number of 'knots' in the nebula that need to be quite
detailed.  I tried using volumetric metaballs first, but these don't
seem to shade as nicely as the volume primitives... the density
gradients don't seem as smooth, and the edge glows just aren't there
like they are with volumes.  Converting them to i3d files gave me a
much better look, but that's quite time consuming during look
development, since I need at least 1024^3 i3d files to maintain the
detail.  So I've tried instancing volumes to each knot, and this
potentially looks best, except I have two questions about it...
firstly, what is the best way to transfer point attributes to the
volume?  Secondly, how can I use world space displacement on an
instance?  On the first attempt I discovered that instances themselves
are not displaced; instead they pick up the displacement of the source
geo before they are instanced.   Aside from this question, which
approach makes the most sense?  There are obviously a lot of potential
paths to take in Houdini.
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