[Sidefx-houdini-list] computing world space velocity

Michael O'Brien mobrien at pixar.com
Wed Jul 9 22:43:30 EDT 2008


> -----Original Message-----
> From: sidefx-houdini-list-bounces at sidefx.com [mailto:sidefx-houdini-list-
> bounces at sidefx.com] On Behalf Of Rangi Sutton
> Sent: Wednesday, July 09, 2008 7:17 PM
> To: sidefx-houdini-list at sidefx.com
> Subject: Re: [Sidefx-houdini-list] computing world space velocity
> Antoine Durr wrote:
> > There's a new option in the object_merge SOP in H9.1, where you can
> > transform the geometry into the space of a specified object.  I
> > haven't tried it, but if you make yourself a world_object that has
> > identity transforms, that /should/ work.
> >
> Just tried this, and you can object merge a sop back in to the same
> object with using a "world" null that's at the origin.
> But why do this!?! Perhaps you're only interested in the lenght of v?
> That's about all I can think of.

I want to use the velocity in a DOP simulation, or by a source POP; both
require a parameter "v". The velocity that is read from points by DOPs and
POPs isn't transformed, so velocity has to be computed in worldspace.

It seems strange to me that you can't compute the velocity of an object
without creating a scope outside of that object. To me, trail should have
some sort of option that tells it to compute velocity on something that is
manipulated by OBJ transforms.

This is one of those things that just works in Maya. In Maya, I tell it to
inherit velocity, and I don't have to go back and compute the velocity by
hand. In Maya most values are available in both local and world space. As
someone who is trying to transition to using Houdini, it's really difficult
to grok.



	(snip) 8<----

Michael K O'Brien                               Production Engineering
Pixar                                                mobrien at pixar.com

REMY: What is that?
EMILE: I don't really know.
REMY: You don't know... and you're eating it.

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