[Plib-users] 'World' to 'Local' body velocity transform

[McEvoy, N. <nic...@ds...>]

I finally have to admit my knowledge of matrix math is very bad !!!  :-(

I have been struggling with this 'simple' problem for a few days now ... and
it is driving me crazy !

Say, I have a 'Body' travelling with a given 'world' velocity and a HPR. As
the 'Body' changes HPR I want to know what the velocity is in relation to
the 'bodies local axis' (by this I mean assume the body is facing +ve Y
direction, +ve Z up, +ve X right ... local coords not world coords) ... (I
hope this makes sense).

-----------------------------------
Example with body at some hpr (assume the body is facing +ve Y direction,
+ve Z up, +ve X right)
@=World body velocity (XYZ)
*=Local body velocity (xyz)
-----------------------------------

+Z +Y      +y
|  /      /
| /      *-- +z
|/        \
@----+X    +x

I have written some code to rotate the 'world' velocity by the bodies HPR
... but I think this is giving me incorrect results (maybe its the Eulers
are Evil problem: http://www.sjbaker.org/steve/omniv/eulers_are_evil.html)
... but it is hard to say for sure ... (ODE physics engine is involved too
... and at certain HPR the forces I apply to the 'Body' cause it to
accelerate off the screen) ...

MORE EXPLANATION ... The whole reason why I want to do this is to apply a
damping force to my 'Body' in (xz) directions normal to its +y heading (at
some ratio to the velocity in these normal directions) ...

The reason I have not posted my code is so that I do not confuse the issue
... I'm just interested in how other people might suggest to write the code
(using PLIB routines of course) ...

HELP ANY SUGGESTIONS PLEASE (I wish I paid attention in high-school math)
!!!

Nick
_________________________________________

Nick McEvoy
WorkMail: mailto:nic...@as...
HomeMail: mailto:ndm...@oz...
HomePage: http://members.ozemail.com.au/~ndmcevoy/
_________________________________________