Stage 4: Create and Rig the Pelvis and Backbone.
If you have done everything right so far you should have four complete legs rigged to create IK chains and FK chains for the tail, neck and head. All we need to do now is create the pelvis and backbone which are created separately and then linked. This part is quite tricky and requires a lot of concentration. The usual way to create a back bone is to start at the rear and work your way up to the lower neck and head but the method I am going to use allows for much more control because we can build an Isner spine set up which is IK controlled and allows for much more natural movement when fully rigged.
First of all, working in the right view port create a null and place it carefully where you want the bottom of the spine to begin and then a second null where the top of the spine will be. When you are happy with these positions select the Isner spine button and then click on the bottom and top nulls respectively. A dialogue box appears (Spine Maker) and you can choose the exact number of vertebrae that you require for your rig. Ten should be sufficient. Click OK. Ignore the spine control for scale. Now you will see a spine is created and there are two more control nulls, hip depth and chest depth which can be used to alter the curvature of the spine to create the profile of the resting position.
Move these until you are happy with the curvature.
The pelvis is cut from a standard XSi biped. Create a biped and delete all but the pelvis object bone. Move this into position at the rear of the spine. In point selection mode select the point in the pelvic recess and create a cluster. Then select the chain root at the top of each leg and Constrain Object to cluster. The legs will now move accordingly with the pelvis.
Now we will need four control objects to cover all bases and allow for the necessary translation and rotation control over the pelvis, spine and upper body. I used a circle and two squares which are from Get> Primitive> Curve. Reduce the subdivisions to the minimum number. Position the three objects at the base of the spine. make the square the smallest and each circle a unit larger than the previous one. Name these the Hips, Upper Body Control and the COG. The Chest control is created from the Library in Net View where you will find sixteen control icons. Position the chest control at the top of the vertebrae and make it the parent of it's “spacer”. The COG parents both the Upper Body and Hips control. The Upper Body parents the chest control. In the schematic view you will notice that the nulls we first created are parenting the hip and chest depth nulls You need to cut these free and then in COMP mode pose constrain chest depth to the chest control and hip depth to the pelvis object. TURN OFF COMP when done. The Chest control, Upper body and Hips are used for rotation only and the COG is for both translation and rotation.
Each separate hierarchy group for the four feet should be parented to the “Horse_SRT” as should the toes and front and rear body controls. Check out the diagram in the following paragraph. It is not advisable to parent the feet control groups to the front and rear body controls as this will create problems when keyframing your character later on.
Stage 5: Completing the Hierarchy.
Now we need to tie all the separate parts together into a complete hierarchy. In the schematic view select “vertebra” and use this to parent the roots of the shoulder bones and the neck bones. The Last bone in the Neck chain is the head bone and parents the Jaw and Left and Right Ear Roots. The Pelvis object should parent the roots of the rear leg bones and the root of the tail bone. The Chest control should parent the FK controls on the front legs. The Upper Body control should parent the FK controls on the upper rear legs. Use a four way arrow control object from the library and then create a Null which is placed under neath the horse in the centre. Another null should be placed a couple of units in front the first one and named “Action”. The centre Null should be called “Horse Controls” and is used to parent three groups of control objects together under one central null. Parent the Foot controls, UPV controls and the body controls to this. This null is then hidden after it is parented to the “Horse SRT” The “Action” null parents both the “Spine Curve” and the “Horse SRT” nodes. This allows everything to be selected if needed.
Click here for a close up of the control rig hierarchy in schematic view.
It is important to note that the controls are in their own hierarchy outside of the bones hierarchy. This will ensure that only the bones are enveloped and not the rig controls. If you branch select the “Spine Curve” null you will be selecting the entire bones hierarchy but not the IK controls. When you have created your character mesh you envelope it by selecting the mesh and then in Animate select Deform> Envelope> Set Envelope and branch select the “Spine Curve” null and then right click to exit this mode and the envelope is formed but the controls remain outside the envelope. It should be noted that any shape animations would need to be set up prior to any enveloping.
That just about concludes this tutorial. I hope you have found it helpful. No doubt their are many other ways to rig a quadruped and as I said at the beginning this set up forms the basis for rigging most quadrupeds. Horses are much more complicated in terms of creating the legs as they have to be divided into sections but the basic control rig is the same for a wild cat for example. Naturally I am grateful for any feedback regarding rigging four legged characters. No doubt you could go on to create claws on animals with paws and they could have custom control sliders for ease of use. Don't forget to hide the rotation controls on the feet and there is no reason why most of the roots and effectors can't be hidden as well.
Summary
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Source true and accurate reference material. Orthographic line drawings are best.
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Load into rotoscope and draw the bones over the real skeleton. Constrain and parent separate chains together.
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Duplicate the legs and constrain the controls onto them. Join the separate chains into an entire hierarchy. Name the bones to be animated by FK and the control objects on the IK chains.
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Test the rig for problems. Ensure that control objects are in their own separate hierarchy that will not be enveloped. Create scripted reset pose buttons at this stage.
For a picture of the complete rig click here.
To down load an alternative quadruped leg rig tutorial as a PDF click here.
To down load a quick and easy polygon mesh modelling tutorial as a PDF click here.
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