We tend to think of real and virtual spaces as being worlds apart, so why is it that I can’t stop seeing an octopus arm in 2007’s spectacular Dead Space ‘Drag Tentacle,’ the alien appendage of developmental hell? Beyond surface xeno-weirdness, it’s what clever animation and the neural marvel have in common that has me interested. Since an octopus arm is infinitely flexible, it faces a unique challenge. How do you move an arm to set x,y,z coordinates and a certain orientation if it has infinite degrees of freedom in which to do it? How might the octopus arm tackle its virtual cousin’s task of going to grab the player when they could be anywhere in the room – free even to move as the animation is first playing?
You simplify. The former Dead Space developer and current senior core engineer at Sledgehammer Games, Michael Davies, took me through the likely digital solution. The drag tentacle is rigged with an animation skeleton – bones to twist and contort it so animation/code can bend it into different shapes. A trigger box is placed across the full width of the level Isaac needs to be grabbed from, with a pre-canned animation designed specifically to animate to the centre of it. Finally, to line up the animation to the player, inverse kinematic calculations are done on the last handful of tentacle bones to attach the tentacle pincer bone to the ankle bone of Isaac, while also blending the animation to look natural.
The octopus, conversely, constricts any of its flexible arms’ infinite degrees of freedom to three. Two degrees (x and y) in the direction of the arm and one degree (the speed) in the predictable unravelling of the arm. Unbelievably, to simplify fetching, the octopus turns an infinite limb into a human-like virtual joint by propagating neural activity concurrently from its ‘wrist’ (at the object) and central brain and forming an ‘elbow’ where they meet – i.e. exactly where it needs to be for the action.
