RE: [Algorithms] RE: Using animation keys from 3DSMax

[Mike K. <Mik...@bl...>]

Here's some info that might help you out on this data conversion - It =
refers to something known in the trade as a 'similarity transform', ( =
Q'.R.Q )
Anyway, hope it helps
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Looking at a hierarchy and it's transforms

if m defines a vertex position
R[i] is a transform 4x4
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then inheriting transforms down a hierarchy can be defined as

 m' =3D R0*R1*R2*...Rn*m
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so, R0 is the root transform, Rn is the leaf transform.=20
(this is what you get with transforms at every node in your character =
skeleton)

m,R are all in world space.
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That's fine. Now, you have source m and all Rs defined in Max coords. =
So,  you want to transform to your own coordinate system (lets call that =
User). Ok, you work out the transform, lets call it Q.
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So applying Q to your mesh for instance transforms your Max mesh verts =
to User coords. Lovely. But what about those transforms?=20
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Well ok, here's one method that would work
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b' =3D Q*R0*R1*R2*....Rn*m
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meaning you've done all your rotations n stuff in max coords, then =
transformed the result to BB. That's not what you want, you want =
something more like
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b' =3D r0*r1*r2*....rn*(Q*m)
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i.e. your transformed mesh Q*m gives the mesh in b coordinates, and then =
r0,...rn rotate it about. So what are r0..rn?
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Well that's easy enough
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Take
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b' =3D Q*R0*R1*R2*....Rn*m
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To actually move Q along the line you need to introduce lots=20
of Q'.Q (where Q' =3D Inv(Q))
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So
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b' =3D Q*(Q'.Q )*R0*(Q'.Q)*R1*(Q'.Q)*R2*(Q'.Q)*m
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b' =3D (Q*R0*Q') * (Q*R1*Q') * (Q*R2*Q') * (Q*m)
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So we have our Qm now, and we know what r0,r1,r2..rn are,=20
they're just (Q.Ri.Q')
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Simple eh?
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- Mike

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PS
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They also distribute. We currently do this for getting the local =
transform from a node and its parent transforms
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  R1 * R0'
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Ok converting to User space that would be
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  Q*R1*Q' * ( Q*R0*Q' )'
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to invert reverse matrix order and invert components gives
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  Q*R1*Q' * ( Q* R0' * Q' )
 =3D
  Q* R1*R0' * Q'=20
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Hence=20
  Q*A*Q' * Q*B*Q' =3D Q*A*B*Q'
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Well alright!


-----Original Message-----
From: Ryan Greene [mailto:rg...@qu...]
Sent: 31 March 2004 20:47
To: gda...@li...
Subject: [Algorithms] RE: Using animation keys from 3DSMax


Okay, so my swap is incorrect... but that isn't the problem I'm dealing =
with. I can properly transform the mesh and skeleton, however since the =
rotation keys I'm getting are in Max's space I can't apply the rotations =
to the skeleton properly. It would seem that I would need to convert the =
quat from max's z-up space to my y-up space... however I am having =
difficulty doing that conversion. This is why I wanted to keep the mesh =
in max's z-up space, and then do the conversion when I build each bone's =
matrix (it's free).
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Swapping your Y and Z is not sufficient to change your coordinate =
system. And if you need to change handedness you have to do other =
things. Assuming your handedness is the same, you need to perform a =
clockwise 90 degree rotation about x.  (or swap y and z and negate one =
of them.)

For example, if your engine is the same handedness as MAX swap the y and =
the z and negate the Y.  This works for me as the engine that I'm =
working on right now has the same handedness as MAX.

swap =3D3D -y;
y =3D3D z;
z =3D3D swap;

As a side note, are you doing the swizzle at run-time?  If so why?  Why =
not take care of this at export time, then you never have to worry about =
using MAX's coordinate system to calculate the parent matrices using =
your coordinate system.

--Pete

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