Box2D integration

This page documents the high-level structure of the webchurch-box2d translation layer. It (mostly) refers to code in physics2D.js.

Example usage of the code (with comparison to the canonical example from probmods.org) is here.

The story so far

A box2d world:

• gravity -- specified as a 2-vector
• entities -- list of structured 4-tuples
  • id -- name
  • shapeT -- shape and its dimensions
    • shape -- circle | rectangle | ngon [triangle, square, pentagon, hexagon]
    • dimensions -- 2D vector that holds radius | (width, height) | (radius, #vertices)
  • body -- bodyType, centroid position, and velocities if any
    • bodyType -- staticType | dynamicType | kinematicType
    • position -- 2D vector of (x, y)
    • velocity -- 2D vector of (x', _y')
    • angularVelocity -- scalar
  • fixture -- density, friction, and restitution (bounciness)
    • density -- scalar
    • friction -- scalar
    • restitution -- scalar

Upcoming Changes

1. More opened-up version of the translation layer create fixtures, bodies, and worlds directly with full webchurch <-> box2d translation, allowing for
   • sampling over many more free parameters (like static|dynamic) of fixtures and bodies
   • creation of compositional bodies -- multiple fixtures combined to form a single body (Josh Hartstone)
2. Collision listener and filter groups efficient spec and handling of collisions, allowing for
   • ghosting objects (useful for counterfactuals)
   • perturbation of particular collisions (either by flags or objects)