Subject: Re: IK difficulties
From: Justin Barrett <[email-masked]>
To: Multiple recipients of list <[email-masked]>
Date: Fri, 5 Jan 1996 04:13:53 -0500

At 01:31 AM 1/5/96 -0500, you wrote:

>If you draw a square, and put a hinge on each corner, then the rotation of 
>the uprights can go through 180 degrees, yet opposite sides are always 
>parallel to each other, if you see what I mean...
>
>Now, if you build this in modeller out of four long thin boxes thus...
>
...<boxes thus snipped>.....
>
>I try parenting box 3 to 2, and 4 to 2, but what exactly do I do to box 1 
>to parent it to both 3 and 4?

Nothing.  An object can have only one parent, but that's all you
need....read on.

>Am I missing something really basic here? 

Possibly.  I would say you could parent box 1 to either 3 or 4.  Choose one,
and move box 1 in modeler so its origin is in the proper place for rotation.
Either way, calculating the angle of box 1 would be fairly easy.

I'm assuming the starting positions of the objects are the same as your
diagram, and you are looking "into" the middle of the parallelogram (down
the Z axis) with the camera, so all rotations will be done in the Bank
channel.  Whether the parent is box 3 or box 4, the objective is exactly the
same.  You basically want to rotate box 1 the same *amount* as its parent,
but in the opposite *direction*.

Here's an example:  From frame 0 to frame 30, let's say you rotate box three
-35 degrees, i.e. you *subtract* 35 degrees from its former angle.  For box
1 to still be parallel to box 2, just rotate it +35 degrees, or *add* 35
degrees to its former angle.  Whatever rotations you make for boxes three
and four, do the exact opposite for box one, and it will stay parallel with
box two.  Also make sure your keyframes all have the same spline settings.

          ---<   Justin Barrett - Free Spirit Productions   >---