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It is highly unlikely that a preset rig type will fit our character mesh perfectly upon creation. Oftentimes we will need to adjust the position and size of our bones so that they fit nicely inside our mesh, making the skinning process, of course, that much easier. Thankfully, CAT makes adjusting the bones of our rig a rather painless process. If we know we're going to be spending a lot of time simply selecting and manipulating only CAT bones in our scene, we might want to change our selection method over to CAT bones to make the selection process easier.
To do this, let's come up to the top of our Max UI and drop down our selection filter and just choose CAT Bone. This means if we click and drag in our scene, none of our geometry will be selected, and we can only select CAT bones. The simplest way to adjust the bones length and position is to select it and use the Move tool to translate it in 3D space. Let's adjust the position of our elbow to show how this works. I'll just press the W key to enter the Move tool.
And as we move it, you'll see that both bones-- according to CAT terminology--now stretch to accommodate the position in the scene of the pivot. If you would prefer only the parent bone to stretch, we can come over to the Hierarchy tab in the Command panel and then select the Link Info button. If we scroll down to the Setup mode section, we can uncheck the Manipulation Causes Stretching option. Now if we move the bone, you'll notice only the parent stretches so as to match the pivot location for the selected object. Let's just come back and turn that on.
Another way we can position bones in our mesh would be using CAT's Forward Kinematic or FK System. Before doing this, however, I'm just going to press the E key to activate my Rotate tool and then come up to my 3ds Max toolbar. I just want to switch over the Reference Coordinate System to Local. This will make our rotation axis obvious and just make our bone placement more predictable. Now we can select the upper arm bone and rotate it into place.
When we rotate our bone, all of the children follow along nicely. Once we have this bone into place, we can also rotate our forearm just to complete the positioning of this limb. Rotating our bones into place will help us avoid placing the bones at odd angles to each other, which can sometimes happen when using the earlier move method. Of course, which one we use is entirely up to us as both methods can produce the desired end result.
We can also adjust the length of our rig bones numerically. With our forearm still selected, let's come over to the Modify tab, and if we scroll down to the bottom of the Bone Setup rollout, you can see we have three options for controlling the Length, Width, and Depth of the bone. Let's just increase all three again until our bone better fits the mesh. I'm going to start with the Length to adjust the position of our wrist, and then I'll increase the Width, and finally the Depth.
Obviously, you would want to spend more time performing these operations, but as you can see, using these controls we can get our bone to better fit our character. Just above these three controls we also have an option that allows us to split a CAT bone into segments. These are oftentimes referred to as twisty bones. The most common example of such a setup is on the human forearm, which does indeed twist quite a bit. With our forearm still selected, let's increase the number of segments to three and then press Enter.
Now you'll notice in both viewports our bone is divided into three sections, and based on the orientation of the wrist or hand bone, we'll now pass an offset rotation up the Hierarchy to give us a twist effect. Understanding how we can adjust and shape our CAT rig is vitally important. If we get this path to the process right, chances are that both the skinning and animation phases of production will go that much smoother.
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