– Surging


Question: If a cheetah is galloping at a constant speed but its spine is contracting and stretching, are the hips catching up to the chest or is the chest falling back towards the hips?

This an important question especially when creating cycles or animating quads with stretchy spines.

The answer.. well lets find out. My prediction is that the answer is ‘both’.. kind of.

Imagine this was a cheetah in space.. yes a SPACE CHEETAH and it was trying gallop back to its space den. The space cheetah would go nowhere.. but the spine would still stretch and contract. The chest and hips would move towards its center of mass.

– For a cheetah 55% of there mass sits on their front legs and 45% on their back legs. So the center of mass should sit somewhere just forward of its measured center

So back on earth this logic should should be the same in the cheetahs gallop. The cheetahs center of mass is the only part of the gallop that is truly moving at a constant speed.

If you are doing a cycle with your quad animated on the spot the center of your mass shouldn’t move back and forth. I know the temptation to surge the character forward as it leaves the ground is strong.. but consider how this will affect the cycle when you move it forward in space. For the animation to cycle the surge you put in will have to absorbed into the rest of the cycle. Meaning that in mid air the cheetah will slow down dramatically, which is… silly.

What does this mean for animation?

  • Don’t put ‘surges’ in your cycles. Find the center of mass of your quad and move the chest and hips towards it to create the stretching and contracting of the spine.
  • Consider your Full Body Controls pivot, place it in the center of mass.
  • Consider the trajectory of your center of mass, this trajectory will rarely have sharp moves or corners or slow down mid air.