[Sidefx-houdini-list] computing world space velocity

Peter Bowmar pbowmar at gmail.com
Wed Jul 9 23:32:12 EDT 2008

I was also thinking that you might investigate the Object Position
DOP, to import the objects' positions directly into DOPs, then Apply
Data onto the "static" objects. That way the velocity is "native" in
DOPs rather than calculated on points. I have no idea if this is
appropriate for your scene though :)

I do what Rangi does, if I need world space velocities etc, Object
Merge them in then use that as the source for all subsequent work. I
usually hide the original objects too (just turn off the Display flag)
to avoid confusion.

This has some additional advantage in that if you change your source
objects, there is a single operator that needs updating (the Object
Merge) which can have a note attached for other artists etc.

I know little about Maya (except some of the API, strangely) so I
can't help with comparisons. I do agree with Rangi's observations in
this area too though.


Peter B

2008/7/9 Rangi Sutton <rangi at kanuka.com.au>:
> Michael O'Brien wrote:
>> Hola~
> Gudday.
>>> -----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.
> r.
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