When you start out animating most of the time it will be with biped characters and you have it all worked out!. Then one day you have in front of you a four-legged character and you are completely lost!. This little visual aid will hopefully help you out.
First off watch the little video below then I will walk you through each step that I use. Remember this is just to get you going you can add your own variations on top of this to suit your own particular character reflecting mood and personality.
STEP 01: My character is facing down the Z-axis I am using IK for the
legs, step 01 is just keyframing the maximum and minimum translate positions for the front
and back legs these may not be the same as there will probably be more reach in the back
legs. You should key Translate X, Y & Z and Rotate X. Go into the Graph Editor with all
your master foot controllers selected click on Curves and Pre Infinity/ Post Infinity and
for both select Cycle in the sub-selection menu. While you are in the editor in View check
the box infinity you should now be able to see your animation curves behind and in front of
your timeline length.
NOTE* on the timeline length this will define the speed of your walk cycle, 24 is a commonly used value but you could use 16 or 32 speeding up or slowing down your walk. This will also change with your choice of 24/ 30 fps in your global parameters.
In this example, I am using 24 so Key all for feet on frame 0 in their maximum extension position +Z and again on frame 24 (these should be exactly the same value) on frame 12 keyframe the opposite the feet in their maximum extension position but in -Z. You might need to do the front and back feet separately as their maximum reach might vary depending on your character and skeleton setup.
STEP 02: Yes It looks weird, the feet sliding backwards & forwards
along the floor!. Do regular playblasts just to make sure everything is cycling
correctly to use this as a testing environment triple the timeline length so in my case
3x24=72 and in the playblast setting for Time range use Time Slider. Doing this will show
you any abnormalities in your cycle if you forgot a curve for example it will not cycle and
become obvious and you can correct it. If you use just the 24 frame timeline length it
won't show you your errors. Back to business here we are adding the +Y position to all
the feet. This will definitely be different between front and back legs. Use the video and
pause it you will see the front legs are straight out from the human equivalent elbow joint
whereas the back legs are pushed up against the hips.
NOTE*: if you didn't already know the cat's leg skeleton setup when you are doing your rigging it is like they are always standing in (human terms) on tiptoes.
So make sure all feet are keyframed on frame 12 and then on frame 18 keyframe them all again in their +Y upmost position and the back, feet add Rotation in rotate X so as the toes point downwards.
STEP 03: Here we are adding the up & down movement (Globally) don't forget to add this control curve in the Graph Editor and do the same with the curve in Pre & Post as mentioned in more detail above. So the cat is in its lowest body position when the legs are at their maximum stride length, keyframe frame 0 at -2 in -Y same values on frames 12 and 24 on frames 6 and 18 keyframe them at 0 (or -0.5) helping you to avoid joint popping. If you are an experienced rigger you will already have built this into your rig counter reacting the max limits on joints, if not just do as mentioned above. Yes, it still does not look like a walk cycle with all the legs behaving in the same manner, don't panic the next step will sort all that out...
STEP 04: This is where it all starts to make sense and it's no secret the method being used is offsetting the keyframed keys. Simply select the front right foot control and SHIFT Select all the frames from 0 > 24 and drag them forwards in the timeline 12 frames, do the same gest with the opposite back foot so back left, with frames 0 > 24 selected drag it also 12 frames. The end result you are aiming for is both front/ back left side feet are apart and right side feet are together. This creates a triangle foot pattern viewed from the top and the animal will not fall over as its weight is evenly supported on both sides. Now do a quick playblast you should see you have a good starting point quadruped walk cycle animation.
STEP 05: This step is what is called a first pass (cleanup), the first thing you will probably notice is the back foot touches the front foot on the 12em frame. This is (1) because up until now there is no Translation in the X-axis on the feet (even though we included it in the keyframed keys) look in the Graph Editor Translate X is a straight line. so add this at frame 0 and 24 key Translate X to -2 and at frame 12 key it at 0. Think about a model doing a catwalk her feet cross over at the extremes of her walk and are at 0 on the passing point. (2) The offset is a perfect halfway mark for both feet this is easy to fix just drag your front feet offset forwards or backwards 2/ 3 frames. Anything else you do after this point is up to you, your personal interpretation of a catwalk. I have added a back hip and front chest Rotate X and alongside the global controller up/ down I added offset keys on frames 0, 6, 12, 18 and 24 this gives my catwalk a more bouncy feeling. I animated the neck/ head also this I did by keyframing frames 0, 12 and 24 all 3 FK neck controllers and the FK head controller (don't forget to add any extra curves you use to the Pre & Post curves in the Graph Editor). Then going up the hierarchy from neck > head SHIFT select all the keyframes and offset them by 2 frames repeat this each time move the frames forward 2 frames till you get to the head. This will result in a nice subtle little head movement. Finally, the tail is done in exactly the same way as the neck/ head apart from the tail up movement you see in the video this is keyframed outside of the cycle. Hope you have found this useful for a catwalk cycle for a cat run it's a whole new story...