Re: rotation matrices

bob@xxxxxxxxxxxxxx wrote: 
i did try it.  it seems to work out.

Try again. It doesn't work out:

Assume that the two rotation have "generic" angles,
e.g. the amount of rotation is not an integer multiple
of 90 degrees or something special like that.

Let's look at a point P on the y-axis. If you first
rotate about the x-axis, then P moves to another point
P' on the yz-plane (its x-coordinate is still = 0),
Observe that P' is no longer on the y-axis, so
when you rotate about the y-axis, P' will then move
to a point P'' off the yz-plane.

If you reverse the order of rotations and first rotate
about the y-axis, the point P won't move at all (being
on the axis of rotation). When you follow up with the
rotation about the x-axis P will move to P' (= the same
point as above), but that is different from P''. Changing
the order of the two rotations thus makes a difference
for at least for the point P (it makes a difference for
most of the points actually).

Or did you mean something else?

Cheers,

Jyrki
.

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