So, yeah. I figured out why it didn’t work on my NC10, and it’s for reasons I’d hoped I wouldn’t have to worry about just yet.

See, XNA and DirectX like to pretend that all graphics hardware is equal and can all do the same things and that. And it does quite a good job, so long as all you want to do is draw a few boxes, which is, uh, what we’ve been doing so far. Unfortunately, not all graphics hardware is equal, and actually, there’s some fairly big differences between them.

For example, index buffers. Index buffers are simply a list of numbers that represent indexes into your vertex buffer. You can pick between two formats – 16-bit and 32-bit. With 16-bit, you’re restricted to a maximum vertex index of 2^15, or 32,768 (because, for some reason, they’re apparently signed values, even though a negative index makes no sense). A 32-bit index buffer gives you indices all the way up to 2^31 about 2 billion – far more than you’re likely to need. Previously, I’ve been using 16-bit index buffers, as my vertex buffers have been small and so I didn’t need to capacity of a 32-bit buffer. However, with the change to one single, huge vertex buffer, I needed more than 32,768 vertices and thus switched to a 32-bit index buffer.

Unfortunately, the Intel drivers (the vertex shadey part of the equation actually happens in software on Intel chipsets, although they do have hardware pixel shader support) don’t seem to support 32-bit index buffers. I can create it, stick my indices into it and even successfully call DrawUserIndexedPrimitives<> – but nothing appears onscreen. Culling the number of buildings and using a 16-bit vertex buffer means it works fine.

Again, there are things I can do that will help this – split across several 16-bit index buffers, for example – and it’s actually not necessarily the most efficient thing to render using a single, massive vertex buffer, either: but there’s some optimisations I want to do later than will probably allow me to switch back to using 16-bit index buffers anyway, so for the moment, I’m sadly just going to ignore my little NC10 and stick to developing on machines with more grunt. Ah well.