So while the puppet pins move the mesh around we have to remember that this is really just a deformer. It's moving the mesh around which moves the pixels around and bends them in a certain way. We can starch areas of the mesh. So they don't bend as much and we can also create overlap sections of the mesh, so that the pixels move over one another. But the pins really don't have any relationship to each other like Forward Kinematics or Inverse Kinematics do.

The idea of Forward Kinematics and Inverse Kinematics are probably easier to explain by actually seeing the principles in action. So let's take a look at Forward Kinematics first. On number 08 Simple IK we will start off very simply with a layer that has an elliptical mask on it. By selecting the layer we can see that the anchor point is over to the right-hand side. By opening the Rotate Parameter or typing R on the keyboard we can rotate the layer by clicking-and-dragging on the value. We can also do this in the Composition Window by simply using the W on the keyboard then rotating the layer by clicking-and-dragging here.

In order to get little more space for our Composition Window again we are going to close down the Information Panel and the Motion Sketch Panel. As we click-and-drag we reinforce the idea that the rotation is occurring around the anchor point. For this reason I want to move the anchor point down to a position inside our Mask Shape. I will type Y on the keyboard to select the Pan behind tool then simply click-and-drag on the anchor point and move it about here. This will define the joint for my Forward Kinematic movement. Again with rotation now simply by clicking W on the keyboard I can click-and-drag and see that the layer is now pivoting around the anchor point. With layer back at 0 degrees I will click-and-drag it over a bit then duplicate it using Command+D or Ctrl+D on the keyboard.

Notice that After Effects has automatically named this new layer part 2. I will move part 2 away from part 1 just so that it overlaps the previous layer where the anchor point is. If I rotate part 2 there is no affect on part 1. I will Undo that change. Then rotate part 1, again there is no influence from part 1 to part 2. I will Undo this as well. However, if I parent part 2 to part 1, we know that the relationship is set up. So that part 1's rotation will influence part 2. Clicking-and-dragging on part 1 we see that part 2 is moving around the anchor point for part 1. We will Undo this further then duplicate the layer part 2. I will use Command+D or Ctrl+D on a PC then move part 3 further out. With the anchor point of part 3 slightly overlapping the layer for part 2.

Go ahead and hold the Shift key down as you move it to constrain the movement along the X axis. Because I have duplicated the layer from part 2 to part 3 and because part 2 already had parenting set to the part 1 layer. What's going to happen is that part 1 has influenced over both of them. However when I rotate part 2 the influence from part 2 stops at that layer. I'll then do both of these movements, then parent part 3 to part 2.

Now we can see that each layer will influence the subsequent layer from part 1 to part 2 and from part 2 to part 3. However, it doesn't work in reverse, it's only forward. So while part 1's rotation will affect 2 and 3 and part 2's rotation will affect layer 2 the rotation for part 3 affects neither part 1 or part 2. It's only going to affect any layer that maybe parented to it. These are the basics of Forward Kinematics.

Let's take a look at the next composition in the Timeline called 08 Simple IK 2 (Brian). Here we have a very simple puppet that's been created. It's actually not so simple. Inside this composition there is precomp, and we can take a look at the precomp by holding down the Option key or the Alt key on a PC then double-clicking on the layer. This will open up the precomp and show all of the different layers that make up that composition. With the cursor hovering the Timeline I will use the Tilde key to reveal all of those different layers and their parent relationships. Here we can see that several of these are tied to the head, and the head is tied to the torso with the forearm and the hand and the bicep, all sharing similar relationship on the left side and the right side, and left and the right leg also are sharing relationship for Forward Kinematics up the chain until they get to the torso. At the end is the torso and that has no parent.

So every part of the puppet is ultimately linked back to the torso. However, if we take a deeper look at each one of these layers by clicking on the forearm or any layer that is parented to another we can type EE on the keyboard to reveal the expression. Now this expression is a little more advanced than other expressions we have seen previously. Here we can take a part of the expression backwards seeing that this is looking for an Angle Parameter for an effect called Left Elbow. The effect is actually on a layer called Brian_Puppet_setup. That layer is located is inside of a composition and that composition isn't this composition that we are currently in. That composition is the one that this entire composition is nested in. In order to see that comp we can use the small icon at the top of the Timeline which will allow us to navigate up into any composition that uses this precomp.

So I will select this and we will come back to that comp called 08 Simple IK 2 (Brian). Here we have that layer which is the entire puppet set up, and on this layer there are certain effects control which govern the rotation of each part of this puppet. So in this composition we have this entire precomp. So we can transform this entire precomp with its own values. But we can also modify an either rotation values for the parts of the puppet inside the precomp because we have got those expressions linking each one of those layers to the effects on this layer. If we open up anyone of these effects we can see that they are simply expression controls that govern the angle of the layer in the precomp. If we navigate up to the Effects pull-down Window we can add a new one here.

So under Expression Controls choose Angle Control, and this angle control was simply renamed to something else. Then that was linked to the expression in the precomp. We will delete that and because the expressions have already been assigned to all of the different parts of the puppet we can see now that by simply changing the value for the left shoulder we can see that because of the parenting it's also changing the rotation of the left elbow and the left wrist. We can open up the left elbow's rotation and change that as well.

Last, we will modify the angle for the left wrist and change the rotation for the hand as well. Note that these all work forward and by moving the wrist it's not going to move the elbow or the shoulder or even the torso.