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The Claw (Expressions)
Added on: Sat Jul 01 2000
Page: 2 3 4 

This tutorial will go through an expression for the opening/closing of a claw around a ball.Before we start get this file:, with the needed .MAX files for this exercise.

Open up the Pre-Expression.max file.

The scene has a simple floor with a ball sitting on a stand. "Floating" in the air is the
claw which will reach down and grab the ball. It is our job to write an expression for the
claw to open when it gets near the ball and close around it.

The claw has been setup with the following:

  • A Shaft (Cylinder)
  • A Connector (the box)
  • Fingers (more boxes with a bend modifier on them)
  • The 4 Fingers are linked to the Connector and the Connector is linked to the Shaft.
When we are all finished all we have to do is move the Shaft up and down, and it should open/close the claw around the ball.
Take a moment to go through the scene. Look at the pivot points for everything. These are key, since an object's position are determined with them. With good positions of pivot points it can make things a whole lot easier.

First lets figure out how we want this thing to open and close. We need to figure out when the claw starts to open then starts to close so it doesn't hit the ball. But to do that we need to decided what objects are going to be references. For example, how far from the ball will the claw begin to open.

Now since the Shaft is the parent for the rest of the claw we its position as a reference to determine the distance to the ball. So all of my measurements are going to be based on the "Z-axis" of these two objects. Most of this expression will be based on the distance between these two objects.
If you move the shaft up and down you will notice that at about 14 units away the claw begins to clip with the ball. So we will need the claw open at this point, and we would like the claw to be closed when the Shaft is about 7 units away from the center of the ball.

To make everything "semi" easy lets have the claw start to open at 21 units and be completely open at 14 units and then be completely closed at 7 units. (Note: I used the Measure Helper Object to do most of these measurements. I had one end at the center of the ball and the other at the end of the shaft. These measurements are pretty close -not exact- but close enough to fool the eye at 30 frames a second).

Also I need to pick how far the claw will open up. I picked -35 degrees since it gets the job done without looking all stupid. Now that we have a rough idea of what we want to happen lets setup the controllers for the fingers.

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