Hi guys,

I'm curious if anyone has had to deal with receiving a scene where geometry/assets/cameras/lights that are in US customary scale(e.g. feet),
but you had to bring it into houdini for simulation and then later export it back out in the same US customary scale?

So far, I've been eyeballing my simulation number's to get sims that look and feel correct for the scale, but I was wondering if there was any more scientific and methodical system of doing this. Perhaps, some way to convert the entire scene from feet into metres when first ingested and then some way to convert the entire scene back into feet when exporting the results? Wanted to see if any of you folks out there have had to deal with this problem so far and what did you find helpful?

Thanks!

you could try Edit > Preferences > Hip File Options
and change Unit Length to Feet

or parent everything to nulls at the object level and uniform scale by 0.3048

You just convert everything to metric. ;-)

Thanks for all the replies.

1. Change the Unit Length to Feet. That seems to be a good way to do this.

I noticed that the dop gravity force will automatically have it's unit scaled from -9.81 to -32.174. And other solvers have similar conversions also if weights was changed from kg to pounds for instance.

This seems the better approach then parenting and scaling everything down as I'll need to scale everything back up to feet again if I need to export the geometry out to other packages.

Thanks!

A follow up question to this.

Is there way to set the default length units to feet for a facility. Such as an environment variable?

Thanks

I'd be very careful with changing the units. In more recent versions of houdini, parameters have been flag to represent a certain unit, but many are not. I doubt all the parameters will scale properly.

For example, viscosity, is this a “normalized” value, or the SI units of m^2/s.

So far the only really reliable way I have found is to scale things on import and on export so that in Houdini a unit always represents a meter. In the past I found the wire solver for example to be really unstable, no matter how I tweaked the numbers, until I converted all the assets to meters, and it became solid.

it's a tricky problem in any pipeline, but I think you'll just have to give in to what the programmers assume you are working in or you end up fighting it all the way.

Cheers,
koen

Hey Koen,

Thanks for that tip.

I was actually wondering how such a big change in the scale would affect the solvers and it did worry me if certain parameters did not scale properly, I would only find out too late.

I wanted to avoid scaling everything down. But I guess that's the only way. As suggested by arctor, I would have to parent everything to null and scale it down. I'm going to have to test how cameras would react to this as well.

Thanks
Nelson

with regard to cameras…
DO NOT scale them.
get the transform of your original camera
scale it to whatever you need it to be to match Houdini's units - then make sure the object (null) has a scale of 1,1,1
then parent a new Houdini camera to that

this is why making a camera rig is such a good idea - you can get the transforms you need (that is to say the animation) and apply it to a camera that you know will be valid.

if you are using alembic you can get much of the needed info using alembicGetCameraDict - http://www.sidefx.com/docs/houdini14.0/hom/abc_extensions [sidefx.com]