Re: pfMakeRotOntoMat()

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Don Hatch (hatch++at++hell.asd.sgi.com)
Thu, 18 Jul 1996 13:11:00 -0700

On Jul 19, 10:13am, Nicolas Gauvin wrote:
> Subject: Re: pfMakeRotOntoMat()
> Hi Don.
>
> Thanks for keeping us up to date with fixed pfMatrix functions.
>
> One little question:
>
> On Jul 17, 7:18pm, Don Hatch wrote:
> >
> > // Check for colinear vectors
> > if (!PF_ABSLT(sinTheta, pfEta))
> ^^^^^ Where does this value come from?
>
> I've checked all include files of my Performer 2.0 and I can't find it.

Oh, sorry about that...
it's the value pfGetFPConfig(PFFP_ZERO_THRESH), by default 1e-15.

One correction to my previous comment about the discontinuity
of makeVecRotVec() when the inputs are colinear....
I said this function will be discontinuous when the two input vectors
are almost equal, but this is not true: the axis of rotation,
defined as the cross product of the inputs, is discontinuous,
but the angle of rotation gets correspondingly small so it
doesn't matter-- the function approaches the identity matrix
as the angle between the vectors approaches zero.
The only discontinuity is where the two input vectors
are in opposite directions.

Don

-- 
Don Hatch  hatch++at++sgi.com  (415) 933-5150  Silicon Graphics, Inc.


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