introduction to ammo.js

david evans

@daredevildave

dave@playcanvas.com

what is ammo.js?

port of bullet, from C++ to javascript via emscripten

what is bullet?

open-source "full-fat" physics engine

why does physics matter?

games are about movement and interaction

physics engine essentials

or why you shouldn't write your own.

rigidbody

idealised representation of a body in which deformation is ignored.

but what is it?

a shape which doesn't change, and can be manipulated via certain properties.

position and orientation

linear velocity

angular velocity

forces and impulses

torques and torque impulses

rigidbody types

dynamic

static

kinematic

volumes and intersection

primitives

essential for:

collision detection

trigger volumes

crates

mesh

complex visuals, simple collision

essential for:

environment collision

broadphase

very quickly determine if two objects ARE NOT intersecting or colliding

narrowphase

accurately determine the nature of an intersection or collision

essential for:

speed!

ray-casting

collide a straight-line with a set of rigid-bodies, return the first or all intersections found.

essential for:

mouse interaction

AI vision

rail guns

trigger volumes

A hidden secret in ammo.js

btGhostObject - a collision object with no collision

stores intersecting pairs for efficient access

essential for:

triggers

goals

death pits

constraints

ball and socket

hinge

slider

essential for:

vehicles

physics-based puzzle games

altogether - ragdoll physics

special cases

character controller

dynamic character controller

move character indirectly by applying forces and impulses

physics engine resolves collision "realistically"

why not model a player with "real" physics?

often "real" physics is no fun

if (y_speed < 0 and y_speed is > -4 )
{
    if (abs(x_speed) >= 0.125)
    {
        x_speed = x_speed * 0.96875
    }
}

kinematic character controller

move the character by setting velocities directly

resolve collisions manually from penetration depths and position

designed to handle your game, e.g. auto-climb stairs

vehicles

simulated vehicle

4 wheels, hinge constraints, forces, torques

make sure you have plenty of time

raycast vehicle

var tuning = new Ammo.btVehicleTuning();
var vehicleRayCaster = new Ammo.btDefaultVehicleRaycaster(dynamicsWorld);
var vehicle = new Ammo.btRaycastVehicle(tuning, carChassis, vehicleRayCaster);

gotchas

stacking

lots of simultaneous collisions is hard.

don't expect a lot of stability.

penetration

fast-moving objects cause problems because of discrete collision detection

solution:

continuous collision detection

performance anxiety

asm.js - up to 1.3-1.7x native performance in firefox

no garbage collection in emscripten code

run in a web worker

thanks

dave@playcanvas.com

@daredevildave

http://playcanvas.com/

slides:

http://slid.es/davee/introduction-to-ammojs