A crawler with no animations

The crawler is a one-second scare: it bursts out of the nurse station, scuttles across the hallway and slips through a door left ajar. It never happens again, even after loading a save. The problem was the model: a rigged mesh with 62 bones, numbered rather than named, and no animation clips at all.

Finding the legs without names

With bones called Bone_028 and Bone_035, I could not write code that said “the front left leg”. So the code does what I did by hand: every chain of bones from the body whose tip reaches the floor is a leg. Its side comes from its x position, and its pair from where it sits along the body. The bone names get logged as a check, but nothing depends on them.

The gait

The crawler walks in a tripod, the way six-legged animals run. Front-left, middle-right and rear-left swing together, then the other three. Each hip swings forward and back on a sine wave while the leg lifts during the forward swing.

The body follows its path with a small side-to-side wobble and quick hops timed to the steps, and the two whip-like tail pieces trail behind with a lag. All of it runs from one component, so real crawlers later can reuse it.

A scare that happens once

A trigger volume starts the scare on first entry, moves the crawler between two points over a set duration, plays its sounds and then destroys it. It writes a named flag to a small save file when it fires, and checks that flag first, so loading a save and walking through again does nothing.

The low thump that goes with it is generated in code: a 45 hertz tone for a quarter of a second with a fast decay. There is no audio file to find or license.

Decide first, build second

I wrote this as a plan before building it, with the open questions at the bottom: the size, which gait, where it appears, how a save behaves without a menu. Most of the questions were cheap to answer before and expensive after. The model is almost a metre long, and the plan proposed shrinking it to about 0.6 metres, which reads better as a crawler in a hallway.