static void
add_box(cpSpace *space)
{
const cpFloat size = 10.0f;
const cpFloat mass = 1.0f;
cpVect verts[] = {
cpv(-size,-size),
cpv(-size, size),
cpv( size, size),
cpv( size,-size),
};
cpFloat radius = cpvlength(cpv(size, size));
cpVect pos = rand_pos(radius);
cpBody *body = cpSpaceAddBody(space, cpBodyNew(mass, cpMomentForPoly(mass, 4, verts, cpvzero, 0.0f)));
body->velocity_func = planetGravityVelocityFunc;
cpBodySetPosition(body, pos);
// Set the box's velocity to put it into a circular orbit from its
// starting position.
cpFloat r = cpvlength(pos);
cpFloat v = cpfsqrt(gravityStrength / r) / r;
cpBodySetVelocity(body, cpvmult(cpvperp(pos), v));
// Set the box's angular velocity to match its orbital period and
// align its initial angle with its position.
cpBodySetAngularVelocity(body, v);
cpBodySetAngle(body, cpfatan2(pos.y, pos.x));
cpShape *shape = cpSpaceAddShape(space, cpPolyShapeNew(body, 4, verts, cpTransformIdentity, 0.0));
cpShapeSetElasticity(shape, 0.0f);
cpShapeSetFriction(shape, 0.7f);
}
cpFloat
cpvtoangle(const cpVect v)
{
return cpfatan2(v.y, v.x);
}
