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

[Steve B. <sjb...@ai...>]

"McEvoy, Nick" wrote:
> 
> 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).

So (in a nutshell) you want to transform the world-coordinate velocity into
'body' coordinates.

You know (or can easily construct) the matrix that converts 'body' coordinates
into 'world' by creating a matrix from the current hpr.

All you need to do is to 'invert' that matrix which switches it from being
a "body-to-world" matrix into a "world-to-body" matrix which you can apply
to the world velocity to turn it into a body velocity.

> 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) ...

So, once you have the veloity in body coordinates, you can simply multiply
the X/Z coordinates by some number a little less than 1.0 every frame and that
will damp it nicely.
 
> HELP ANY SUGGESTIONS PLEASE (I wish I paid attention in high-school math)

Yes - me too.  Matrix math isn't hard though - did you read this yet?

  http://www.sjbaker.org/steve/omniv/matrices_can_be_your_friends.html


----------------------------- Steve Baker -------------------------------
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