[Sidefx-houdini-list] units in houdini

Jeff Wagner jeff at sidefx.com
Fri Feb 2 12:32:19 EST 2007

I would assume that a Houdini unit is whatever you want it to be and
build your tools accordingly. Steven refers to the units that are
critical to DOPs when he mentions that a unit = 1 metre. Yes it is in
the Preferences and was added for DOPs but that's all. A nice guideline
but certainly not gospel.

A houdini unit is whatever you want it to be. 

Nothing is guaranteed with universal units. Units are cheated all the
time. Many times in many facilities modelling many props with all kinds
of different modelling tools, I have observed modellers delivering
props, characters and sets in vastly different scales/units on the same
job. It's a fact of life. Drives some TD's nuts. Others take it in
stride and build the support needed and move on. 
Tweaking scales of props, sets and other parts of a scene is an on-going
issue. Units are also changed if the scene is too large or too small so
you can work with it. Steven also mentioned this.

You should design your effects tools in such a way that you have a
single global scale parameter that properly scales the effect up and
down keeping both the behaviour and any shader patterns and other
scale-dependent parts properly aligned.

It is doable but requires some management as you build your effects
tools. If this is for general consumption, it is worth it.

In shaders that use procedural patterns, you can control globally within
the shader how to control the scale up and down. Helps to use "rest"
attributes. Same with SOPs.

Particle sims are trickier but you can with trial and error get the same
behaviour as you scale the environment that the sim is in.

DOPs isn't so easy. You pretty much have to scale things down then up
again going in to DOPs. Then again I haven't tried to build scalable
sims in DOPs yet. I would imagine that RBD sims would be easier to work
around than Cloth, wires, etc.


On Fri, 2007-02-02 at 06:10 -0800, steven ong wrote:
> Hi Luca,
> A general rule of thumb is 1 m/unit in Houdini. When you read in
> geometry from Maya, they will be hundreds of units in Houdini because
> Maya is using 1cm/unit.
> Unless the scale is so big that you get into precision errors, no need
> to rescale either way, just work with the geometry as it is.
> Cheers!
> steven
> > Hello,
> >
> > I'm planning to create a library of effects all based on the same
> > scene
> > scale to be used in different shots (wispy snow, snowstorm, rain etc.
> > etc.).
> > I receive the camera and the geometry cache in worldspace from maya
> > for
> > collisions emitters etc. etc.
> > Is there a rule of thumb on what units use in houdini?
> > Also do you rescale the maya assets down to a more convenient houdini
> > scale, or do you do the other way around?
> >
> > thanks
> > Luca
> >
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Jeff Wagner, jeff at sidefx.com, Senior Technology Consultant
Side Effects Software, www.sidefx.com, Toronto, Ontario, M5V 3E7
tel: (416) 504-9876, fax: (416) 504-6648

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