[Sidefx-houdini-list] Quaternons and Lag CHOP

Pablo Giménez pablogipi at gmail.com
Thu Sep 1 19:49:21 EDT 2011


2011/9/1 zhang xiao <zhangxiao6776 at gmail.com>

> hi , Pablo, check out this thread, it has a spherical interpolation
> setup for the quaternion, the reason i bring that up is the component
> of quaternion is not the same as vector3 component.
>
> http://forums.odforce.net/index.php?/topic/12707-learning-about-quaternions/

Well this in case you need to interpolate two quaternions, which is the only
way to interpolate rotations properly, linear algebra doesnt work here :(
 What I am trying is to apply is a lag, which I believe is a different
operation. I am not trying to interpolate between two rotations I am trying
to sample some points rotations attributes along my frame range and then
"lag" them to get a kind of delay.
Basically this rotations are modified using AttribReorient SOP, to reorient
my pieces, represented as points, to the new positions, this is the rotation
I am trying  to "lag", so it  looks like the points reacts to the new
position in a more natural way.
And this is the reason of my question, how to make Lag CHOP to work properly
with rotations.
Thanks!

>
>
> 2011/9/1 Pablo Giménez <pablogipi at gmail.com>
>
> > 2011/9/1 Edward Lam <edward at sidefx.com>
> >
> > > Have you tried changing the Method to "Lag Magnitude" ?
> > >
> > Thanks Edward.
> > I tried it but I got the same result, so I guess the Lag is not detecting
> > my
> > vectors components channels.
> > My rotation channels are in the next form: rot[xyzw]*.
> > So I have one tuple of rot[xyzw] per point.
> > I suppose the lag to be able to threat it as a vector it needs to process
> > every tuple separately, I mean it needs to detect every tuple of
> channels.
> > In the scope for th Lag CHOP I have put rot[xyzw]* to affect only
> rotations
> > chennels.
> > Looking at the help seems Lag Magnitude is designed to work with regular
> 3
> > dimensional vectors, not sure if it designed to work with quaternions.
> > In the case Lag is designed to work with 3 dimensional vectors and then
> it
> > expects something like r[xyz]*. Then I have to split my quaternions in
> > rotation axis and rotation angle and apply a Magnitude Lag to the r[xyz]*
> > channels and the same lag but changing the mode to Lag Value to my angle*
> > channels (being angle* the cahnnels with the rotation angle).
> > About the spherical interpolation zhang mentioned, not sure if it is what
> I
> > need to make a lag on my quaternions rotations.
> >
> > >
> > > -Edward
> > >
> > >
> > > On 01/09/2011 11:57 AM, Pablo Giménez wrote:
> > >
> > >> Hi all.
> > >> I am having some weird problems trying to apply some CHOPs to
> quaternion
> > >> channels.
> > >> I have a quaternion rot attribute (vector4).
> > >> I read it using a Geometry CHOP as rot[xyzw] channels.
> > >> Then I apply a Lag CHOP ( I have similar problems using Spring). Most
> of
> > >> the
> > >> rotations for my points works all right, except some of them, some
> > points
> > >> randomly at some frames just flip.
> > >> Like if I werent using quaternions and you get the typical rotations
> > >> flipping due to euler angles limitations.
> > >> If I pass through the channels and import my rot attribute again
> without
> > >> any
> > >> effect it works, but as soon I apply a lag it gives me this random
> > >> rotation
> > >> flipping.
> > >> If I change  the lag parameters then the flipping happens in other
> > points
> > >> at
> > >> different frames.
> > >> Really weird.
> > >> Has anybody tried to lag rotations using chops?
> > >> Thanks.
> > >>
> > >>
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> >
> >
> >
> > --
> > Un saludo
> > Best Regards
> > Pablo Giménez
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-- 
Un saludo
Best Regards
Pablo Giménez



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