[Sidefx-houdini-list] computing world space velocity

Rangi Sutton rangi at kanuka.com.au
Wed Jul 9 23:21:56 EDT 2008

Michael O'Brien wrote:
> Hola~

>> -----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.

In both those cases I object merge into another static object with the 
transform object set to "..", and use that to calculate v /and/ provide 
the source for your simulations. That way the position and v attributes 
are both in world space and it will make sense. You would contain your 
POP network in this new object, or import back your dops here. You 
therefore wouldn't want this object to have the transforms on it, or you 
will get double transforms.

> 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.

I think that behaviour would encourage the mix of object/world space 
attributes described.

> 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.

The object / world space differentiation in Houdini is something that's 
a bit difficult to at first, and when I'm introducing someone to Houdini 
it's something I try to cover as early as possible. Just because it's 
difficult to grok doesn't make it a bad thing. Once you do grok it, it 
is both powerful and usable. In Maya I always found the lack of easily 
switching spaces do be problematic in complicated scenes. But then I 
haven't touched it for a while... maybe it is a better way to do things now.

Yes.. Houdini you often have to do some trivial steps by hand.. but I 
find that also gives you loads more control when what you're trying to 
achieve isn't trivial.


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