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Maya rigging: Introduction to rigging a human torso

By Jahirul Amin
Web: Open Site
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Date Added: 10th February 2014
Software used:
Maya

Adding the IK twist

Currently, we cannot twist the spine so let's quickly fix this. To do this, select the spine_IK and open up the Attribute Editor (Ctrl+A). Open up IK Solver Attributes and then Advanced Twist Controls. Turn on Enable Twist Controls. To control the twist through the hip_IK_ctrl and the chest_IK_ctrl, we need to set the World Up Type to Object Rotation Up (Start/End). Set the Up Axis to Negative Z as the back of our character is facing that direction in the Maya world. Now select any of the spine_IK joints and you will notice that the Z-axis is facing forwards so we need to set the Up Vector and the Up Vector 2 to 0, 0, -1 as our spine's Up is in the opposite direction. Now type hip_IK_ctrl into the World Up Object box and chest_IK_ctrl into the World Up Object 2. The last thing we need to do is set the Twist Value Type to Start/End.

You should now be able to twist the hip_IK_ctrl and the chest_IK_ctrl and get a nice falloff running down the spine.

1838_tid_step06.jpg
Using the Advanced Twist Controls to allow for twisting

Adding some stretch

Adding some stretch control will allow the animator to push the poses should he/she wish to or need to. To add the stretch we will divide the length of the spine_CRV by itself (which will give us an initial value of 1) through a multiply divide node and then feed the output of that value into the scale X of the spine_IK joints (which also has an initial value of 1). So every time the spine_CRV changes its length, the new length will divide itself by the old length and the offset value will be fed into our spine creating the squash and stretch.

Okay, so first we need to find out the length of our spine_CRV. To do so, select the curve and in a Mel window, type: arclen –ch 1;

This will make accessible the curveInfo1 node that houses the Arc Length (jot this number down). To find this, open up the Attribute Editor and you will find it as the right-most tab. You could also go into the Hypershade (Window > Rendering Editors) with the spine_CRV selected and go Graph > Input and Output connections. Rename the curveInfo1 node to spineInfo and then in the Hypershade, go Create > General Utilities > Multiply Divide. Double-click the Multiply Divide node to open it up in the Attribute Editor and set the Operation to Divide. Now in the Input 2 first window, pop in the value of the Arc Length that you jotted down earlier. Back in the Hypershade, Hold the right mouse button (RMB) over the Output arrow on the spineInfo node and select arcLength. You should get a stringy bit attached to the pointer. Now hold the LMB over the Input arrow of the Multiply Divide node and go input1 > input1X. Now, whenever the curve changes its length, the new length will be divided by the old length.

We now need to feed this into the scale of our joints. Select spineA_IK_jnt and in the Hypershade, go Graph > Add Selected to Graph. Now hold the RMB over the Output arrow of the Multiply Divide node and go output > output. Hold the LMB over the Input arrow of spineA_IK_jnt and go Scale > ScaleX. We need to do the same connection to the remainder of the spine_IK joints (minus spineEnd_IK_jnt). To quickly do this, go into the Script Editor and find and copy the following line from the History:

connectAttr – force multiplyDivide1.outputX spineA_IK_jnt.scaleX;

Open up a new Mel tab and then paste this line 5 times into the Input window (one line for each additional joint). Then simply, replace the name of the joints to correspond with the remaining joints; so it will read like so:

connectAttr – force multiplyDivide1.outputX spineB_IK_jnt.scaleX;
connectAttr – force multiplyDivide1.outputX spineC_IK_jnt.scaleX;
connectAttr – force multiplyDivide1.outputX spineD_IK_jnt.scaleX;
connectAttr – force multiplyDivide1.outputX spineE_IK_jnt.scaleX;
connectAttr – force multiplyDivide1.outputX spineF_IK_jnt.scaleX;

Go Command > Execute and abracadabra, we are done. Last thing to do is rename the Multiply Divide node to spine_md. If you wanted, you could add some volume preservation to the rig but for this character, I personally think that would be overkill.

1838_tid_step07.jpg
Adding squash and stretch to allow for more dynamic poses

FK joints and the controls

The IK setup will be driven by an FK joint chain so let's pop that in now. Turn on Point Snapping or hold down the V key on the keyboard and in the side view, use the Skeleton > Joint Tool to create a 4-joint chain. Start by point snapping to the root joint of the IK chain (make sure to click slightly away from the joint so it creates a new chain, rather than continuing the IK chain), then to every other joint and then to the end joint. Rename the joints from root to tip: spineA_FK_jnt, spineB_FK_jnt, spineC_FK_jnt and spineEnd_FK_jnt.

We now want to create flexion using the positive rotate X attribute, rather than the negative rotate Y that it is currently set to. To do this, select spineA_FK_jnt and go Skeleton > Orient Joint Tool (Options). We'll be coming in here often to set the orientation of our joints. For the FK spine, we can simply turn on Orient Joint to World and hit Apply. Try rotating the joints now and positive rotate X should give you flexion.

Now, let's add the controls. Edit the final line of the createControls.py script to read:
cmds.orientConstraint(ctrl, s, mo=0)

As the controls are to drive only the rotation of the joints, this constraint will be best. Now select all the FK joints (minus spineEnd_FK_jnt) and Execute the script. Once the controls have been created, edit the shape of the controls in component mode to make them easily selectable. Remember: do not edit the shape in object mode. Next thing we need to do is create an FK hierarchy for our controls to mimic the FK behavior of the joints. We will do this through parenting. First parent spineC_FK_ctrl_offset under spineB_FK_ctrl and then parent spineB_FK_ctrl_offset under spineA_FK_ctrl.

Finally, we can connect the IK setup to the FK setup. To do this, first parent chest_IK_ctrl_offset under spineC_FK_ctrl and then parent midSpine_IK_ctrl_offset under spineB_FK_ctrl.

1838_tid_step08.jpg
The final setup for the FK joints and the controls

Adding an extra hip control

Currently the hip control has an effect on the lower portion of the spine; let's add an extra control to only affect the hip itself. First duplicate hip_IK_jnt, rename it hip_FK_jnt and then delete the parentConstraint that lives underneath it in the Outliner. Duplicate the new joint, rename it hipEnd_FK_jnt, lower it down slightly and then parent it under hip_FK_jnt. This joint is not necessary, as all the rotations will happen from hip_FK_jnt, but I just like it visually to represent the hip. Now parent hip_fk_jnt under hip_IK_jnt. Next select hip_fk_jnt and run the createControls.py script to create an orient constraint between the control and the joint. Reshape the control to make it easily selectable and then parent hip_FK_ctrl_offset under hip_IK_ctrl. We can now use the hip_IK_ctrl as the main driver and then use the hip_FK_ctrl to add some offset if needed. This is just another way of adding extra flexibility to the rig and it gives more choices for the animator.

1838_tid_step09.jpg
Adding an extra hip control

Creating the COG control

The last control we need to add is the COG (Centre of Gravity) control; the control that carries the entire torso. In the top view, go Create > CV Curve Tool (Options) and set the Curve degree to 1 Linear. Now draw a unique shape of your choice around the world center, making sure it is a closed shape, and exit the tool when complete by hitting Enter on the keyboard. You can use grid snapping (hold X) or point snapping (hold V) to make it easier and more precise. Rename the curve COG_ctrl and then hit Ctrl+G twice to group it to itself twice. Rename the top-most group COG_ctrl_offset and the next group down COG_ctrl_auto.

Now we need to get the control into place. To do this, parent COG_ctrl_offset under hip_IK_ctrl and then zero out the translate and rotate values in the Channel box. This should snap the COG_ctrl_offset into place. Now, un-parent COG_ctrl_offset by hitting Shift+P. Reshape COG_ctrl so it is easily selectable, if needed, in component mode.

Let's piece this altogether now. First, select spineA_FK_ctrl, then Shift-select spineA_FK_jnt and go Constrain > Point. Then parent hip_IK_ctrl_offset and spineA_FK_CTRL_offset under COG_ctrl. Select spineA_IK_jnt, hip_IK_jnt, midSpine_IK_jnt, chest_IK_jnt and spineA_FK_jnt and hit Ctrl+G. Rename this new group spine_jnt_grp. Select spine_IK and spine_CRV and hit Ctrl+G. Rename this group spine_doNotTouch_grp. Now select spine_jnt_grp and spine_doNotTouch_grp and hit Ctrl+G. Rename this group spine_rig_grp. The last thing to do is to select spine_CRV, open up the Attribute Editor and disable Inherits Transform. This will make sense later on when we add the globalSRT control; the main control used to scale, rotate and translate the character into the starting position for the shot. For now, we will leave the rig here.

Next time, we'll create the rig for the neck and head and also add the globalSRT control. Happy rigging.

1838_tid_step10.jpg
Adding the COG_control to drive the entire torso and cleaning up the Outliner

Top Tip: Lock and hide attributes and color controls

A good idea is to lock and hide attributes that you wish the animator not to set animation keys on. For example, the FK controls of the spine are not to be translated or scaled, the IK controls are not to be scaled and so on. To stop the animator keying these attributes, we can lock and hide them. Simply, highlight the channels you wish to lock and hide in the Channel Box and hold down the RMB. When the menu window appears, drag-and-drop the cursor over Lock and Hide Selected. If you need to bring the attributes back, you can go Edit > Channel Control in the Channel Box menu or Window > Channel Control. To color the controls, select it and open up the Attribute Editor. Scroll down to Display > Drawing Overrides, turn on Enable Overrides and then use the Color slider.

1838_tid_toptip1.jpg
Hiding and locking attributes we wish the animator not to key, and color coding the controls


Related links
Check out Jahirul's previous tutorial on rigging: Introduction to rigging
See what else Jahirul has been up to on his site.
Jason Schleifer's Animator Friendly Rigging series has lots of useful information.




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Readers Comments (Newest on Top)
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(ID: 276734, pid: 0) Korosh on Thu, 22 May 2014 7:01am
The Jahirul's rigging series are definitely the best tutorials for not only Maya rigging but for rigging fundamental concepts I've ever read.
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(ID: 272288, pid: 0) KryX on Sun, 04 May 2014 5:49pm
I have been reading rigging tutorials for months and I have to say that this is probably the most straight forward one I've come across. Having it in both print and video helped to follow a lot. Thanks
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