Cityscape – Update 12

Finally got round to adding some lights to the buildings to stop those pesky aeroplanes from crashing into them all the time:

Again, as with so many things, these are not so simple as they appear. Several steps were needed to get these little static lights in place – but hopefully, it should mean later things like streetlights, cars and, hey, maybe fireworks of something, should be much easier to implement.

First of all, we generate another texture – a simple circular fade from white to transparent that can be used as the basis for all lights. At the moment, the fade is linear, and does look a little artificial, but it’ll do for now.

Secondly, we need some way of rendering these: I’m using a billboarding technique that’s fairly widely used in particle systems in most games. Essentially, you render a quad that always points towards the camera, giving the illusion of a solid 3D object with rotational symmetry. The effect – especially at a distance – is quite convincing. The trouble is, we need to recalculate the position of these quads every frame, to ensure they’re always pointed towards the camera – and doing this on the CPU can get expensive. So, the obvious solution is to use a vertex shader: rather than pass in the pre-transformed coordinates, for each vertex in the quad, we pass in the location of the centre of the quad along with an offset to indicate which corner we’re talking about (handily, this also corresponds with the texture coordinates, so we can re-purpose the texture component of the vertex data for this). Then, in the vertex shader, we use this information along with the position of the camera to calculate the real quad vertices.

Lastly, we need something to manage all the particles, so I’ve added a ParticleBatch class, analogous to the BuildingBatch, that looks after all the particles, calculates the vert buffer, renders them and that sort of thing. It provides an IParticleService for other game components to grab and dump their particles into: The naming of some of the interfaces should give a hint as to one of the current restrictions – that is, we only support static particles at the moment. This is handy from a speed perspective, but means that nothing in our scene can move: this is something we’ll have to address a bit more when I want to add moving vehicles and things.

We’re up to revision 40 in the repository now. Next step – I’m going to have to delve into the murky world of city planning, and hopefully make things look a bit less like a random collection of buildings, and a bit more like a real city…

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