Re: [Plib-users] Problem with rotations

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

Ben Woodhead wrote:

> Ya, I don't have a problem with that.. It seems to be sgMakeRotMat(mat,
> h,p,r) where pitch is set to something other then 0. Then I converted it
> back to sgCoord and used transform...

Could your confusion be this:

    * You take three angles (H,P,R) and convert them into a matrix M
      (using sgMakeRotMat...or whatever).

    * Then you convert M *BACK* into an sgCoord and extract (H',P',R').

Do you expect (or rely upon) H' == H,  P' == P and R' == R ???

If so, then that's not right.  There are an infinite number of values
for H,P,R that all generate the exact same rotation matrix.  Hence, when
you convert a matrix *BACK* into three Euler angles, you don't get the
same three angles back...although the three angles you get *DO* perform
the same effective rotation as the three you put in.

Hence, it's possible to say things like:

     H =   0
     P = 180
     R =   0

...then convert to a matrix...convert back again and get back:

     H' = 180
     P' =   0
     R' = 180

...totally different numbers - but having the same exact same effect.

If that is your problem, then it's not something that we can fix - it's
just the way math works.

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