ah I see where your going with this Andy, so how can I get a particles rotation into a current direction of rotation vector? say its spinning or 'tumbling' how to get the vector to rotate with it. I was thinking an 'up vector' but particles dont have them
On 5 March 2013 19:33, Grahame Fuller <grahame.ful...@autodesk.com> wrote: > A particle's orientation is its offset in rotation relative to the point > cloud's local reference frame. If as is typical, you leave the point cloud > untransformed, then this is the same as its offset in rotation relative to > the scene's global reference frame. > > The orientation is expressed as a rotation in axis and angle form, i.e., > [X, Y, Z] coordinates that define a vector relative to the point cloud, and > an angle of rotation around that vector. > > Axis and angle is just one of several ways to represent a rotation. The > other ways that are available in ICE are Euler angles, quaternions, and > matrices (but be aware that in general 3x3 matrices represent both rotation > and scaling, and 4x4 matrices represent rotation, scaling, and translation, > so be careful when using them in case you inadvertently apply more than > just rotation). Each of these formats has advantages and disadvantages in > different situations. There are nodes that can convert between them as > required. > > Axis and angle was chosen to represent orientation and other rotations > after some discussion on the Ariane beta list. It was felt to be fairly > intuitive for artists, and is fairly easy to use in computations (or at > least, easy to convert under the hood to a format that is useful for > computations). > > As with transforms in general, rotations can be used to manipulate vectors > and positions, or to convert between reference frames, among other things. > It depends on the problem you are trying to solve and how it is framed. > > gray > > From: softimage-boun...@listproc.autodesk.com [mailto: > softimage-boun...@listproc.autodesk.com] On Behalf Of Andy Moorer > Sent: Tuesday, March 05, 2013 12:58 PM > To: softimage@listproc.autodesk.com > Subject: Orientation and vectors, foundations > > Hi all, > > As I was working today I realized that despite doing operations on ice > "orientations" regularly I don't have a firm grasp on what they really are. > Orientation relative to what? And what form is this orientation in? > > Trying to phrase it differently... > > How is a particle's "orientation" different from a 3x3 matrix describing > the difference in rotation between it's local coordinate space and the > global frame of reference? Both are clearly descriptions of the same thing. > > I know this is kind of an abstract subject, and (being tired at the > moment) my question may not even be clear, but being self-taught and > lacking adequate formal math education I'd be very interested in your > answers and any discussion in general on how you all visualize rotations > and orientation, > > For some people I talk to about rotations (I'm the life of any party) it > seems to be all about manipulating vectors, others seem more comfortable > thinking about rotations as transformations between reference frames... and > I see a similar wide range of how people go about some of this stuff when > looking into various compounds. >
