One of the things I've always wanted to model is an armature system with a hydrolic "motor" that could move the arms, kind of like C3P0 on Star Wars (look at the little linkage between his upper arm and forearm):
I knew I wanted to use the LookAt feature of geometry. It took me about 30 or 40 minutes, but I came up with something kind of interesting. Here's what I did. First, I create an interesting model for one half of the armature, like this:
Note that the main axis of the armature extends up the y-axis, and the
center of the "pivot" is at (0,0,0). This isn't essential, but it made
it easier when I had to move things around later.
Then I went
back to the geometry level and duplicated that geometry (with a ^C ^V)
and parented the new geometry to the original. I rotated it 180 degrees
in the Y direction, and about 120 in Z, to give me:
Then I created two new geometry objects at the object level, filling their geometry with a single sphere SOP. I moved them so they were kind of embedded yet protruding from the individual armature. Each pivot is parented to each linkage, so any movement in the linkage moves the pivot along with it. The arm assembly with the pivots is show here:
After this, I created two "hydrolic" rods parented to the pivots. I used the Tube SOP for the rods, and made one slightly larger than the other, so that when they were "attached" it would look like one was sliding inside the other. For now, I created the tubes so that they extended in the positive direction down the Z axis. I did this because the LookAt transformation in Object points the Z-axis of the coordinate space towards the thing you are looking at:
At this point, note that the hydrolic rods are just extending in
opposite directions. This is because they are parented to the pivots,
the pivots are parented to the armature, and one of the armatures is
rotated in 180 degrees.
Next, we open hydrolic rod Objects and
select the LookAt from Transform tab. For each of the rods, we select
the LookAt object to be the pivot for the opposite rod. This will make
the rods always orient towards the opposing pivot, making them look
like they are always attached:
Now you can rotate the second armature (the "child" armature) and the hydrolic rods will be pinned to the pivots and will slide in and out of each other, like this:
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