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Stage 3: Duplicate and rig the legs.
So far we have worked exclusively in the right view port to draw the bones for our character. Don't worry, later on we are going to create the pelvis and back bone. What we have to look at so far is enough to cope with for now. Take a look in the top and front view ports and you will see that all the bones are exactly central to the origin so we need to position the legs off centre by branch selecting the top root and moving in each leg chain in the X axis by about 1 or 1.5 units. Once you have done this you can in turn select each leg and duplicate it by selecting Create> Skeleton> Duplicate Symmetry. In the dialogue box select the YZ plane and click OK. Now we have four legs and the other chains remain in the centre. You are probably wondering why we don't rig the legs before duplication. The reason for this is that rigged bone chains tend to mess up when duplicated. Although it isn't entirely impossible to do it that way, rigging each one individually will get you plenty of practise at constraining the controls correctly.
Now for the hard part. You will need to follow the next steps very carefully. Below is an image that shows the the rig as it will look when you have finished the rigging the legs. As you can see, I have set things up so that the shoulder bone is animated independently by the FK rotation control and so is the following upper leg bone - the second in a two bone chain. The main leg is controlled by IK and the horse's foot is rotated directly by FK controls. It is the front legs of a horse that make it such a challenge to rig and animate.
Needless to say you should save your work periodically when you have completed each section.
Now to rig the legs. We shall begin at the front and I am going to use implicit cube objects as controls because I happen to like them. They are three dimensional and easy to select in all views. You can hide them when you come to playback and they will not render.
In model mode go to Get> Primitive> Implicit> Cube. Create three duplicates by pressing ctrl+d. Select the first cube and scale it to make it a manageable size. Position constrain the first cube to the legs end effector, then orientation constrain it to the the first foot bone. This aligns the box exactly as we want it ready for final constraining. Now relax those constraints and then make a orientation constraint from the foot bone to the box. This will control the foots rotation. You can scale this control down to be quite small as it will be hidden. Select the next box and position constrain it to the legs end effector and then relax this constraint. Now create the same constraint from the end effector to the control box and this becomes the IK control for the leg and also parents the first control.
The next box becomes the toe rotation control and this should be position constrained to the foot bones end effector and orientation constrained to the toe bone. Relax the second constraint and the reset it from the bone to the box. This now controls the toe rotation. Finally take the last box and scale it right down until it is quite small and place it line with the knee joint but not too close. Select the thigh bone and create an up vector constraint to the box be sure to select Skeleton> Chain Up Vector and not from the constrain panel. If the leg flips around then refer to the Animation properties from Edit> Properties on the right screen side. Refer to the kinematic joint tab and set roll to 180 on the Resolution plane option. This should rectify matters. This controls the legs angle of inclination so that it is not always straight and will prevent the leg from flipping around. This control should be parented to the foot control. Repeat these steps for the other three legs and name all the control boxes appropriately according to their function in the schematic window. Double click on the node and type the name into the box.
Foot control: will operate the foots position and rotation because it parents the rotation control.
Foot rotation: controls the foots rotation but can be hidden because it is parented to the above.
Toe control: controls the toes rotation.
Up vector control: controls the legs angle of inclination and prevents flipping.
You should always branch select these controls when animating.