[Sidefx-houdini-list] computing world space velocity

Mark Elendt mark at sidefx.com
Tue Jul 8 14:28:14 EDT 2008


On Tuesday Jul 08 at 11:14, Michael K O'Brien wrote:
> > -----Original Message-----
> > From: sidefx-houdini-list-bounces at sidefx.com [mailto:sidefx-houdini-list-
> > bounces at sidefx.com] On Behalf Of Antoine Durr
> > Sent: Tuesday, July 08, 2008 10:25 AM
> > To: sidefx-houdini-list at sidefx.com
> > Subject: Re: [Sidefx-houdini-list] computing world space velocity
> > 
> > If you do an object_merge, and set the 'transform space' to '.', the
> > geometry will now be in worldspace.  The trail SOP will work as
> > expected, now.
> 
> Sort of. The object merge needs to be in a network that has an identity OBJ
> (not an animated OBJ) for it to correctly import the points in world space.
> This is what led me to a separate SOP network to compute world-space
> velocity, as I described below... which seems wrong.
> 
> Is there any way to compute worldspace positions of points from within a
> SOP? Is there any reason why that can't be done?

I'm pretty sure the VEX ow_space() function(s) will transform from
"object" (SOP) space to "world" (world) space.

If you put down a

Global(P)->Transform->Output

Then set the parameters of the transform VOP to:
    space:world
    space:current

(Yes, the VOP lies, space:world seems to work in SOPs just fine).
you should get the world space location of the points.

You can also transform "direction vectors" (i.e. velocity) the same
way.  You may also be able to specify the full paths to objects, but
don't quote me on that.

But, to be honest, I'm not 100% sure this answers the original question.

If you want, there's also the "optransform" hscript expression
function, but that way seems ridiculously difficult.

> > > I'm sure I'm missing something fairly easy here.
> > >
> > > We have geometry that is imported by nodes that animate at the
> > > OBJ level. I need to compute the velocity for use in a DOP. The
> > > trail SOP computes velocity based on the object space positions.
> > > How does one compute the world-space velocity?
> > >
> > > Right now, I'm creating another SOP network that imports the
> > > original object, uses a trail on that node, and then uses a copy
> > > attribute to apply it back to the original. This seems overly
> > > clunky.

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