One-side collision with edge and chain shapes

This follows from Meredith's description here.

Ok, here's how I did it - and it relies on the relative position of scene object to collision fixture body - in Scene::BeginContact():

// Fetch scene objects.
    SceneObject* pSceneObjectA = static_cast<SceneObject*>(pPhysicsProxyA);
    SceneObject* pSceneObjectB = static_cast<SceneObject*>(pPhysicsProxyB);
    // ---------------------------------
    // inserted this

	if (pSceneObjectA->getOneWayCol() || pSceneObjectB->getOneWayCol())
	{
		// one way collision only applies to edge or chain shapes
		if (pSceneObjectA->getOneWayCol() && (pFixtureA->GetType() == b2Shape::Type::e_chain || pFixtureA->GetType() == b2Shape::Type::e_edge))
		{
			// one or both have one way collision set, so test our directions
			b2Manifold* manifold = pContact->GetManifold();

			Vector2 nPos = Vector2(pSceneObjectA->getPosition().x, pSceneObjectA->getPosition().y);
			nPos.Normalize();
			b2Vec2 vPos = b2Vec2(nPos.x, nPos.y);

			Vector2 nLPos = Vector2(pFixtureA->GetBody()->GetLocalCenter().x, pFixtureA->GetBody()->GetLocalCenter().y);
			nLPos.Normalize();
			b2Vec2 vLPos = b2Vec2(nLPos.x, nLPos.y);

			b2Vec2 nrml = vPos - vLPos;

			Vector2 nVel = Vector2(pSceneObjectB->getLinearVelocity().x, pSceneObjectB->getLinearVelocity().y);
			nVel.Normalize();
			b2Vec2 velB = b2Vec2(nVel.x, nVel.y);

			F32 prod = atan2(nrml.x, nrml.y) - atan2(velB.x, velB.y);
			if (prod < -0.5f)
			{
				pContact->SetEnabled(false);
			}
		}
		else if (pSceneObjectB->getOneWayCol() && (pFixtureB->GetType() == b2Shape::Type::e_chain || pFixtureB->GetType() == b2Shape::Type::e_edge))
		{
			// one or both have one way collision set, so test our directions
			b2Manifold* manifold = pContact->GetManifold();

			Vector2 nPos = Vector2(pSceneObjectB->getPosition().x, pSceneObjectB->getPosition().y);
			nPos.Normalize();
			b2Vec2 vPos = b2Vec2(nPos.x, nPos.y);

			Vector2 nLPos = Vector2(pFixtureB->GetBody()->GetLocalCenter().x, pFixtureB->GetBody()->GetLocalCenter().y);
			nLPos.Normalize();
			b2Vec2 vLPos = b2Vec2(nLPos.x, nLPos.y);

			b2Vec2 nrml = vPos - vLPos;

			Vector2 nVel = Vector2(pSceneObjectA->getLinearVelocity().x, pSceneObjectA->getLinearVelocity().y);
			nVel.Normalize();
			b2Vec2 velA = b2Vec2(nVel.x, nVel.y);

			F32 prod = atan2(nrml.x, nrml.y) - atan2(velA.x, velA.y);
			if (prod > 0.0f)
			{
				pContact->SetEnabled(false);
			}
		}
	}
    // -------------------------
    // Initialize the contact.
    TickContact tickContact;
    tickContact.initialize( pContact, pSceneObjectA, pSceneObjectB, pFixtureA, pFixtureB );

Modified SceneObject with a bool and get/set() for mColOneWay, modified b2Contact with m_reEnable and use that to gate the flag Meredith mentioned above.

In b2Contact::Update() right at the top of the method:

b2Manifold oldManifold = m_manifold;

	// Re-enable this contact.
	if (m_reEnable == true)
		m_flags |= e_enabledFlag;

The one-way part is determined by the positional relation between the fixture and the SceneObject - the "solid side" is the side facing away from the SceneObject - so the local center position of the fixture has to be off-center of the SceneObject. This is sub-optimal, but there seems to be a common theme on the Box2D forums suggesting that there isn't a good way to get the actual edge normal.

To test, I threw this in TruckToy:

// -----------------------------------------------------------------------------

function TruckToy::createOneWayWall( %this, %posX, %posY, %startX, %startY, %endX, %endY )
{
    %obj = new SceneObject();   
    %obj.setBodyType( "static" );
    %obj.setPosition( %posX, %posY );
    %obj.setSize( 0.25, 0.25 );
    %obj.setSceneLayer( TruckToy.ObstacleDomain );
    %obj.setSceneGroup( TruckToy.ObstacleDomain );
    %obj.setCollisionGroups( TruckToy.GroundDomain, TruckToy.ObstacleDomain );
    %obj.setDefaultFriction( TruckToy.ObstacleFriction );
    %obj.createEdgeCollisionShape( %startX, %startY, %endX, %endY );
    %obj.OneWayCollision = true;
    %obj.setAwake( false );
    SandboxScene.add( %obj );

    return %obj;
}

Then called it from ::reset():

%this.createOneWayWall(-92, TruckToy.FloorLevel + 0.75, 1, -3, 1, 3);

I should just make another branch on GitHub....