Correctly combining two Quaternions

[Ed]

Hi folks,

I have a little problem which seems to be melting my brain. I have a (software) model which I wish to rotate based on the topography of the ground. I'm using two angles to represent the terrain - a "North" and "East" elevation (hopefully that's self explanatory).

Once I have the quaternions for these two angles, how to I *correctly* combine them? Unless I'm misunderstanding something, multiplying the quaternions would result in the same effect as performing one rotation and then the other - which I think would be wrong (they're non-commutative, being about perpendicular axes).

help? please?

 

[fresh_42]

Rotations aren't commutative either, which means the quaternions still represent what you want to do.