A change of approach/jabuka 0.6

As mentioned on the Jabuka website, I’m moving away from a tutorial-based approach, mainly because of lack of time. It’s much quicker to implement new code and then blog retrospectively, highlighting new features and interesting code/design decisions. In the tutorial based approach, making a change/bug fix in an earlier tutorial forces all later tutorials to be updated.

That being said, it’s worth discussing changes are in the latest version, 0.6, of Jabuka.
This code introduces a couple of simple steering behaviours, and flocking boids. Given the roadmap, why did I suddenly go off on a total tangent and implement boids? Well, apart from flocking behaviour being especially cool (look on youtube for the Carling Belong commercial, or starling flocks), I wanted to stress the engine (I use that term very loosely given the current functionality of Jabuka) as I’m itching to demonstrate multithreading and performance enhancements. This version of Jabuka has shown that it is still too early to look at performance, however, as the application still has some fundamental flaws to be addressed.

The flocking application implements a flock (based on the algorithms on Chris Reynolds website, and roughly based on the algorithms in the Killer Game Programming in Java book). The changes to Jabuka were that the force for a rigid body is obtained in the Update() method from introducing an IForceProvider interface, implemented by various behaviours.
If you look at the implementation of the steering behaviours, you can see that there is a large number of classes implementing this interface, each performing a simple operation such as aggregation/limitation. This does lead to a proliferation of classes, but I’m happy to live with it for now as it makes unit testing easier, and may lead to more easily changing behaviour dynamically at runtime.

The Update() method calling on the IForceProvider is not perfect in this case – the objects are iterated over, and each object’s Update() method calls on its IForceProvider. For the steering behaviours, the behaviour compares the body’s location to its neighbours to calculate its force and update its position. The problem is that the resulting locations are dependent on the order in which the bodies are updated. This has problems in measuring the flocking quality (more on this later), and it may be desired to update the steering forces independent of Update(), so we don’t do this in every time step (as it’s an expensive operation – O(2)?). Also, having the state of an object dependent on every other object on every timestep will get really messy when we introduce multithreading. Having each body’s Update() method independent of any other means that it’s much easier to parallelise/vectorise/split into multiple threads – it’s likely we’d want to avoid making every IEulerRigidBody thread safe, and would want to implement the thread safety at a higher level (though I won’t jump the gun, we’ll get there soon enough).

The sort of changes above hint that it may be better to have a Physics Manager class, that owns the Collision object, the rigid bodies, and also has knowledge of the IForceProviders and manages the overall sequence of operation.

The larger problem with having the currently flocking behaviour, including collision detection, is that I haven’t yet implemented resting contacts (well, any contact forces) apart from collisions, and as objects are forced towards each other, the simulation can halt. I’ve temporarily fixed this by introducing a scene where the boids don’t have any collision detection. I’ve also introduced a scene which demonstrates the halting, with only two spheres.

As to steering behaviours and flocking itself… I’ve introduced a scene for the seek behaviour, demonstrating that having the steering force directed towards the target is insufficient, the velocity tangential to the targed needs to be taken into account. The idea is to find the optimal trajectory towards the target. Reynolds copes with this by providing a force such that the body’s velocity is adjusted to be the maximum velocity towards the target. This is better, but does mean that the body needs a concept of its maximum velocity (this works OK on Earth, as air friction gives a terminal velocity, but in space this is arbitrary). Also, the maximum velocity may vary – the maximum velocity into the wind is less than having a following wind, and may depend on a body’s shape (or its orientation with respect to its velocity).

Looking at the arrive behaviour, where the speed is arbitrarily slowed down, the first thought that comes to mind is that a PID control towards the target would be desired. However, the problem is that the velocity tangential/perpendicular and distance from the target all need to be taken into account – a more complicated control scheme would need to be used, and that may be too computationally expensive.

The current boid implementation currently uses the same proximity distance for all individual behaviours (separation, alignment, cohesion). It may be that for separation, a smaller proximity should be used. Also, currently, I’m using a simple sphere without taking into account the blind spot behind the boid.

There are lots of tweaks to be made (and a Genetic Algorithm could be used to change these dynamically), but many of them are pointless without a method of measuring the quality of the flock. There is an interesting paper on that here. Some background on the flock theory / the underlying physics of flocking, can be found here, and here.