[ODE] Vector that's perpendicular to two others

[Nate W]

On Thu, 4 Apr 2002, Martin C. Martin wrote:

>   dCROSS(p, =, ax1, ax2);
>   dNormalize3(p);
> 
> Which should work fine unless ax1 and ax2 are almost the same.  For
> example, if they're both (1, 0, 0), the p will end up being (1, 0, 0) as
> well.  In this case, any vector perpendicular to the two of them will
> do.

Suppose you first calculate the angle between ax1 and ax2...  If it's
above some threshold (say, greater than 0.01 radians?), use the cross
product as you decribed above.

Otherwise, find a vector that's perpendicular to just ax1, perhaps by
cooking up an arbitrary vector to use in place of ax2 in the same
cross-product code.  Like, change the sign on the largest element of ax1
and use that in place of ax2?

No warranty expressed or implied, I'm making this up as I go along. :-)

-- 

Nate Waddoups
Redmond WA USA
http://www.natew.com