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Character Rigging

By Sergio Muciño
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Date Added: 9th December 2009
Software used:
3ds Max

ARM IK

Ok. First, before assigning any IK to the arms, we need to go over a few things. First, let me explain a bit about the architecture behind my rig. The arm rig uses some complex controls and behaviours to control both IK behaviour, and inheritances. Take for example, a character doing pushups. You'd like to animate the rotations on the hips and spine, without worrying about the hands sliding all over the place. I've set some inheritance controls so that the wrists can or cannot inherit the arms movement and rotations. It is, however, not as simple (I wish) as setting standard inheritances. I achieved this by using two goals in the arm, and using weighted constraints on the wrists, so that you could alternate between an 'inherit' object, and a 'no-inherit' object. The constraint's weights are controller through a custom attribute I set on my arm control object. This object, which is the one you'll use to move the arm, also has some other custom controls to drive finger curling, IK swivel angles, wrist rotations, etc. That way, I can animate almost anything in my arm rig from this object, without having to select other objects, animate them, re-select the arm control object, etc. This makes animating characters a breeze. Therefore, there are some things we need to do prior to creating anything here, so that the whole thing will work perfectly. Let's get to it.

First, select the last bone in the arm chain (in my case, it'd be the 'Bone_ikterminator_arm' small bone). In the hierarchy panel, pivot tab, turn on the 'Don't affect children' button. Move the bone so that it's pivot point meets exactly the pivot points of the bones that start the hand assembly. This is crucial, or the hand will slide when the torso moves. Ok.

Select the arm bone (not the forearm one). Now, in your perspective view, really zoom in onto the ikterminator bone. Switch to a wireframe view if necesary (the hand bones are most likely getting in the way). Now, with the arm bone still selected, go into the animation menu (I have a toolbar for most of these operations), and select IK Solvers - HI Solver. Now, select the ikterminator bone. You'll see a blue cross at the ikterminator's pivot, and a line running from the cross to the pivot of the arm bone. The line is the IK solver. The blue cross is the goal. Whenever you move the goal, the IK solver will try to rotate the bones encompassed by it in order to meet the goal. These rotations are dependant on something called the rotation plane. You can't see it, but it's there. It's a plane that serves as a 'grond plane' to the IK chain, and it's originally parallel to the bones in the chain (that's why it's very important that the bones are aligned). This plane is controller by something called the swivel angle. By modifying this angle, you rotate the plane, thus rotating the bone chain, using the goal as a pivot. It's very much like moving your elbows while having your hands placed at your waist.

Now that we've gone over some IK basic concepts. Let's rig our arm.
Let's create some nodes we'll need for our arm rig. I used one spline object (just like the rotator, but pointy), and two point objects. Align them to the ikterminator bone (pos and orientation).Ok. Here we go.

Select the rotator (mine is colored purple). Link it to the forearm bone (not the ikterminator bone). In the hierarchies panel, link info tab, turn off the rotate inheritances in all axes. Name it NoInherit_arm.
Select the first point helper. Name it HandOrient. Link it to the NoInherit_arm spline object. Turn off the rotation inheritances. Now, apply an orientation constraint to it. Selec the forearm bone as a first target, and the NoInherit spline obj as a second target. For now, set the forearm's weight at 100, and the NoInherit object's to 0.

Select the second point object. Name it WristControl.Link this one to the HandOrient point object. Now, select each root bone in the arm and link it to the WirstControl obj. Here's a shot of my schematic view.

1200_tid_image17.jpg
Click to enlarge
1200_tid_image18.jpg

Create a new point helper object. Name it Control_arm, and align it with the IK WirstControl obj. Now, add the following custom attributes to it:

Name: Thumb Curl. Type: Float spinner. From -15 to 70, default 0. Object's base level.
Name: Index Curl. Same params as before.
Name: Mid Curl. Same params as before.
Name: Pinky Curl. Same params as before.
Name: Finger spread. Type: Float spinner. From -15 to 15, default 0. Object's base level.
Name: Inherit Arm. Type: Float slider. From 0 to 100, default 0. 4 ticks. Object's base level.
Name: Swivel Angle. Type: Float spinner. From -360 to 360 default 0. Object's base level.

As you might have guessed, most of these are hand-related controls. You'll see how easy is to animate both the arm and hand with these controls. Link the IK chain you created to it.

1200_tid_image19.jpg

First, we'll wire the Swivel Angle parameter to the chain's swivel angle. Just select the point object (A_Control_arm), go to Animation/Wire Paramters/Wire Parameter, and select the IK chain (my point object has the cross option turned off so the IK chain underneath can be selected. I used instead the Axis Tripod option). The wire parameters dialogue will appear. Wire the point's object Swivel Angle Parameter to the IK chain's swivel angle parameter. Set the direction from the point object (driver) to the IK chain (driven). In the expression textbox, enclose the current expression in parenthesis, and add the following to the beggining of it... 'degtorad'. This tells the expression to convert the value from degrees to radians. This is necessary to have the results you expect.


1200_tid_image20.jpg
Proceed to the next page for more on the arms.





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(ID: 182453, pid: 0) Hassan on Tue, 19 February 2013 6:27am
REACTOR CONTROLLERS Reactor controllers have been available since R3, but I decided to include them here because we'll also be using them a lot. Basically, a reactor controller is a controller that will generate certain pre-set motion when something else happens in the scene
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