Spine
Bodies and collision
What every thing in the world is made of, what shape it has, and how the engine stops things passing through each other.
- Sheet
- 3 of 18
- Latest release
- v0.42
- As of
- 2026-10-22
Before anything can be shot, climb a stair or fall in a heap, the engine has to know what it is and what shape it takes up. This sheet covers what every thing in the world is made of, the shapes bodies come in, and the handful of questions the simulation asks the world to keep things from passing through each other.
What a thing is made of
Every solid thing in the world wears a type that the game registers once and every copy shares. There are two tiers. A model type is any drawn, solid thing: a crate, a door, a gun, a rocket. A body type is a model type that is also flesh: the player, a soldier, a dog.
Because a type is shared, a hundred soldiers do not carry a hundred copies of what a soldier is: they all point at one. Tuning the soldier type tunes every soldier at once. When one of them needs to differ, say a captain who is bigger and bleeds a different colour, it states only its differences, and everything else still comes from the shared type.
A body can also change what it wears during play. At any moment, the type it wears is the top one of three:
| Rung | Where it comes from | Example |
|---|---|---|
| 1. A status | A status that changes what its victim is, for as long as it lasts | A creature turned into a spider sees, swims and breathes like a spider |
| 2. A swap | The body has been switched to another type | A transformation that sticks |
| 3. Born | The type the body was created with | An ordinary soldier |
An enemy that would grow by swapping needs room to grow into first, the same check it makes before standing up from a crouch. And a type with no shape is refused when a body is built from it, loudly, rather than shipping a creature that silently freezes where it spawned.
The shapes bodies come in
For moving and colliding, a body is not its detailed model but a simple solid around it. There are four kinds. The engine's standard human is a capsule, a pill shape 1.8 metres tall, but a game can give any body type any kind and size, and a separate shape for each stance: standing, crouching, swimming, or a stance of its own.
By default a shape stays square to the world as its body turns, as in the classic Quake games: only the drawn model and the aim turn with it. A body can opt in to turning its shape, or to tilting it to lie along a slope. All of this is plain geometry worked out the same way every time, so it never upsets a replay.
The questions the engine asks
The simulation never pokes at the level directly. It asks a small set of questions, and each question has its own rule about what may get in the way. That rule is what makes a crowd something a rocket hits but not a wall you walk into.
| Question | Asked for | Stopped by |
|---|---|---|
| Can I move this way? | Every step a body takes: its shape is slid along the move, and a short check looks for ground underfoot | The level only. Bodies keep apart by a separate spacing rule, so a crowd is never a wall |
| What does this shot hit? | Bullets, which travel instantly, and rockets or bolts, which fly a little each tick | The level and every body, down to which part was hit |
| Can I see that? | An enemy looking for you | The level and bodies that block sight; glass-like geometry can be see-through |
| Does the blast reach? | An explosion checking each thing in range | Walls, doors and lifts; never people or corpses |
| Is this line clear? | Gripping a ledge, checking a stair tread, finding a way out of water | The level only |
Moving is a sweep and a slide. The body's shape is pushed along the move until it would touch something, stopped a hair short, and the rest of the move is turned to run along the surface. It is the same sliding method the classic Quake games used, adapted for rounded bodies, and it is why you glide along a wall instead of sticking to it. When two ways out are equally good, the tie is broken by a fixed rule, never by whichever happened to come up first.
Steps and snags
Climbing a step
When a walking body meets a low ledge, it tries three moves in one tick: lift, move forward, set down. The step a body can climb is a fifth of its height unless the game says otherwise, so the standard human takes steps up to 36 centimetres.
Most of the work is in saying no. A step that would be wrong is refused, and the body slides as if the step were not there:
| The check | What it prevents |
|---|---|
| Nothing to land on | Stepping off a ledge as if it were a stair |
| Landing too steep | Standing on the rounded edge of a stair and sliding back for ever |
| No real progress | A body in a corner lifting itself for nothing |
| Higher than a step | Ratcheting up a tall wall one step at a time |
| Ending inside something | Climbing into a solid block taller than a step |
Only a body on the ground may step, so one in mid-air cannot climb a wall by pressing into it. A low ceiling does not refuse a step; it only shortens the lift.
Getting unstuck
Sometimes a body ends up inside something: a portal sets it down too close to a crate, a current carries it into a wall, a status makes it bigger in a tight corridor. A buried body is helpless, because every move it asks about answers "blocked". So the engine checks for it and digs the body out:
- NoticeAfter anything that moves a body by force, and every few ticks besides. Deeply buried is fixed at once; slightly buried only if it lasts.
- Press outMove the body the shortest way out of what it is inside, a few times over, since one push clears only the worst overlap.
- Search outwardIf that fails, try spots further and further away in each direction, up first.
- Or stay putIf nothing works, the body stays where it is. Moving it somewhere wrong would be worse than leaving it stuck.
Two kinds of contact are left alone. A body merely touching a wall is not stuck, so pressing against one never sets off a rescue. And a body a moving lift is holding in place is not nudged at all, because those nudges would walk it up through the lift a tick at a time.
Two collision worlds, one set of answers
The questions above have two different answerers. When the engine runs inside Unity, the adapter answers them with the physics engine Unity provides, using real solid shapes sized from the simulation's own numbers. In the automatic tests, a small, exact world made of boxes answers them, with no game engine at all. Both use the same sliding arithmetic, so what the tests prove about movement holds for the engine as it really runs.
The simulation's answers
- Movement, shots, sight, blasts and line checks
- Every body's position and health
- Decided inside the tick, the same way every time
The adapter's physics
- Ragdolls, severed pieces, tumbling props on screen
- Runs at the same gravity as the simulation
- For looks, with one exception: by default, where fallen bodies and severed pieces really lie is read back, so shots hit them there
Gravity is the same on both sides, about 2.3 times Earth's, which gives the snappy falls of a classic shooter. And every solid in the physics world is sorted by what it should stop, so an invisible wall can keep bodies out while letting fire through, and a severed piece only bounces off things.
Sizes and gravity as of 2026-10-22.
Related sheets
- Simulation and replayThe tick these questions are asked in.
- Player movementRunning, jumping and crouching on top of the slide.
- DamageWhat happens once a shot has hit.
- GoreCorpses, ragdolls and severed pieces.