Re: [Visualpython-users] Arbitrary frame transformation routine

[Jim T. <jt...@mi...>]

I don't have an easy code solution that I can share right now but I can 
give you some pointers.  Firstly quaternions can be used to do this type 
of thing so you might want to read up on that.  A quaternion can be 
constructed to represent the orientation of each frame and then you can 
do operations using them to perform rotations between frames.  This is 
how I normally handle a problem like yours.

Also visual has a rotate function which you could use.  You can 
transform the position of a point using a series of rotations (after 
first subtracting the difference of the frame origins).  The example you 
site shows how to get x,y, and z axes from the axis and up vectors that 
visual uses.  Then for each axis you perform a rotation (in series).  
The axis about which you rotate is computed using the cross product 
(cross(f1.x, f2.x)) and the angle is computed using the dot product 
(acos(dot(f1.x, f2.x)).  Rotate your point vector then repeat for y and 
z.  You must trap for the condition where the magnitude of the cross 
product is near zero, in which case you skip that rotation step since 
they are already aligned.

There is probably a shortcut along the lines of what the example does 
but I have not worked it out.  Also if you were working with something 
asymmetric rather than a sphere you would have to do transformations on 
axis and up for the object as well.

Hope this helps at least a little bit.  I have code that will do this 
but it is in no condition for me to publish at this time and I cannot 
extract a snippet that will help you, I'd have to release a whole lot of 
code.  I guess I need to work on getting my library for visual python 
out the door...

JT

PS There are a couple of quaternion functions in the odelib portion of 
my VisualPyODE library that I have released.  Unfortunately it is 
probably not enough to be a big help but if you are interested download 
the code at http://www.missioncognition.net.  For that library ODE is 
the owner of the coordinates/orientations and I just slave the visual 
objects off of them.  Still you might find some of code informative.

Tony Risinger wrote:
> I have been trying to no avail to write a routine to transform the coordinates of vector from the global frame to a local one; or even better would be a routine that transforms from one arbitrary frame to another (then global frame is just pos(0,0,0) and axis(1,0,0)).  the example in the documentation translates from local-to-global, but i cant seem to reverse it.  seems that my vector skills are weak...
>
> in this log file visualizer i am working on, i have a sphere traveling thru the global frame, but at a specified point, or at init, i want to inject it into a rotating local frame.  i want the switch to the localized frame to appear seamless, thus i need to get the local coordinates relative to the global ones the sphere currently has.
>
> is functionality of this type already present somewhere in the module but im missing it?  once i apply a frame to an object it immediately moves the coordinates it currently has but relative to the local frame, so it appears to jump thru space.  this is great and expected, i just need to get the switchover down now.
>
> does anyone have something whipped up already that you could kindly share?  otherwise direction of any kind would be greatly appreciated.
>
> Thanks,
>
> Tony Risinger
> Application Development Specialist
> Tel: 507-535-7563 | Fax: 507-292-5747 | Toll Free: 866-241-0639
>
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