Adding a Simple Automated Forearm Setup to the Marine Rig

  The earlier post finished up with a very simple ( and common ) 3 joint setup for the arm.  The downside to this setup is you get really poor twisting rotation in the Forearm and also the upper arm area ( this pictorial covers the lower arm ).  So some new things covered in this little rig mod:

 Add Influence - Allows adding new influences to an existing SkinCluster.

Multiply Divide Node - A Math utility node in Maya that can handle Multiplication/Division/Pow

Start by duplicating the Export Skeleton's Elbow joint, parent it under the Elbow.  Then offset translateX down to halfway distance between the Wrist/Elbow.  With the joint selected, add the mesh to the selection and navigate to Skin->Edit Smooth Skin - > Add Influence.

Start weighting the Forearm joint, from Elbow to mid way down the forearm should be just Elbow and Forearm influence.  Then from halfway down the forearm to the wrist should be NO Elbow, just Forearm and Wrist.

Open the Node Editor, right click and hold to open the Marking Menu that has "Create Node" as the North Option.  Create a new Multiply Divide node, named "md_LForearm_twist".  The graph shows some of Maya's unitConversion nodes that are automatically created, so just ignore them in the Image.  Plug the .rotateX value of the Wrist into the input1X of md_LForearm_twist", then send the outputX of md_LForearm_twist to the .rotateX of the LForearm.  Open the Attribute Editor and set the Input2X value to 0.5.  This setup takes half of the rotateX ( twist  value ) and applies it to the Forearm joint.

Polish the weighting.