This micro-article will not teach you how to model a character nor how to properly rig it. There are countless tutorials about that around these parts. Intermediate skill level is suggested but not required to fully enjoy the following.
To optimize is a philosophy. Sometimes a production is of dire need of optimization, other times it’s a matter of attitude.
What to cut away to make this rig simpler?
Bones which can be deleted from a rig without really hurting it’s plausibility is wrists (hands) and a couple of spine bones. Feet is hard to get rid of because they constantly have direct contact with the ground. To optimize further one could delete all (or almost all) spinal bones, clavicle bones and the neck. Many would stop there. Retaining the major and most important bones of the human body in the skeleton; Head, a couple of spinal bones, upper arms, lower arms, hip(root), upper legs, lower legs and feet. That would be a greatly optimized humanoid rig. However there is more to be gained.
What is the most needed features of the human skeleton? Well we, the human being, have two legs and two arms, each separated into two large bones. There is no way we can get rid of those. Feet and hands we might be able to cut away, since they are quite small and are not really used for unimportant characters. Of course it would be nice to know that the characters’ feet always visually nails it to the ground, but it is also two extra computational cycles of rigging and skinning per frame, and I reckon that those cycles are well saved to be well spend somewhere else. The neck is gone, and the spine is not needed either. Yes the character will be a bit stiff in the back ~ but long live optimization!
We’re down to this:
It looks weird I’ll admit that, but the functionality is outstanding. It now looks like a starfish as well. There is a Root bone in the middle of it all, that bone makes sure that the torso, shoulders and hip does not move out of place. Apart from the root bone there are 5 additional bones, one for each extremity. The bones in the arms take care of the elbow and out, including the hands. The same goes for the knee bones. The head has been spoiled with it’s own bone - this can be debated, but I’d argue that having no spine requires something on the spine to be animated to give the illusion that there is spinal activity. The head can fake that, not perfectly, but enough to keep the character alive.
To make the rig usable it is of course constrained and driven by a normal rig with inverse kinematics for both arms and legs, and all sorts of bajingle. Animation goes as smoothly as it would with other characters.
The head bone can be spared if you want a rig consisting of only 5 bones as stated in the headline. I chose to include the head bone since it effectively provides some sense of spinal activity. This specific Starfish Rig contains 6 bones in total.
Some shortcomings present themselves in the mesh. With no bones to keep track of the upper legs and arms, the mesh has a tendency to flatten out. Like a balloon being twisted.
The starfish rig is super simple to create and very easy to maintain. It might suit a flat shaded anesthetic more, where the dent in the knee and the elbow will not show at all. But for crowds a fully textured character would wear this fashionably.
Floating feet is a critical shortcoming, but acceptable when taking into account that two bones is saved. With some vertex shading, a clever shader might be able to take care of the feet. The artist will never miss the wrists in my experience - given that this starfish character is not used for complicated interactions. One will learn to live and overcome the stiff spine in clever ways. Head animations can fake allot of the liveliness originally provided by the spine.
Andreas N. Grontved
I did not invent this! Though I’ve never seen anyone ever do anything like this, I’m certain that all studios with the need of highly optimized characters has come across something similar if not identical.
Starfish Rig - 5 Bone Human Skeleton
A micro-article on how to severely optimize a humanoid rig - by only using 5 important bones.