Cityscape – part 5

Get a load of this:

See that? That’s a 41×41 city, with 4 boxes per building – a whopping 80k polygons in total – running at our nice full 60fps. That’s over twice the polygon budget of the best of our previous versions, and dramatically more frames per second. How is such dark magic achieved?

Well, remember what we talked about last time? How graphics cards are much happier if you just give them a load of polygons and let them render them without interruptions? That, in a nutshell, is exactly what I’ve done here. Rather than rendering the buildings one at a time in a loop, we take advantage of the fact that buildings basically don’t move that often, and batch them all up into one big array as soon as they’re built. Then, instead of having to loop through a whole bunch of buildings, we just render the one buffer – much better.

Code-wise, we’ve had to do a bit of refactoring here – Buildings are no longer XNA GameComponents, and instead we’ve got a BuildingBatch DrawableGameComponent to handle that side of things for us. We create our buildings, add them to the BuildingBatch, then tell the BuildingBatch to update its geometry – whereupon it iterates through its collection of buildings, grabs their geometry and stuffs them all up in a single vertex/index buffer.

Inevitably, there’s some tradeoffs: by doing our heavy lifting upfront, we limit the amount of stuff we can do once the game is running. So, there’s no easy way to do, say, object culling or dynamic level of detail using this sort of scheme – the time taken to copy all the buildings into the batch buffer is about 150ms – not long, but much more than we’ve got available if we’re trying to render 60 frames a second. And as our buildings get more and more complex, we’re inevitably going to hit an upper limit for which this approach also no longer works – but I’ve got some plans for how to deal with that, too.

Additionally, our buildings are all looking very uniform – as there’s no way to change shader parameters between each building, there’s no way to, say, pass in a custom colour modifier to liven things up a bit – but again, there are ways around this that I’ll talk about at some point later on.

We’re up to bzr revision 14 now – and for some reason, this latest version doesn’t work on my NC10. I am investigating why and should hopefully have a fix soon.

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