introduction to ammo.js


david evans

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









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
http://playcanvas.com/








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Introduction to ammo.js

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