Rendering Galaxies

I haven't posted here much recently, so here are a few details on a fun project I worked on recently.

I spent the three weeks before NVIDIA's GPU Technology Conference working on the graphics for the galaxy simulation demo shown above, which was shown during the keynote presentation. You can watch the video here. The (ambitious) goal was to achieve something that looked like these classic Hubble telescope images.

Much of the look of these images is due to bright star light being scattered by the surrounding dust, so we spent a lot of time trying to get the dust to look right. For efficiency, the dust particles are simulated separately - they are affected by the stars' gravity, but don't interact with each other. The colour of the stars is mainly due to variance in temperature / age, but we took some artistic license here.

The rendering was done using OpenGL, using a variant of my favourite technique used in the old CUDA smoke particles demo (originally due to Joe Kniss). The particles are sorted from back-to-front, and rendered in slices, first to an off-screen light buffer, and then to the screen (sampling the indirect lighting from the light buffer). The light buffer is blurred after each slice to simulate scattering. Obviously this only simulates light scattering towards the camera, but this isn't a bad approximation in practice. The dust particles are drawn larger and less opaque than the stars.

I also added a lot of post-process glow (which makes everything look better), and a cheesy star filter (see below), which they made me remove in the end!

Anyway, most of the credit for the demo should go to Jeroen Bédorf and Evghenii Gaburov, who wrote the Bonsai simulation code, which you can read about here. Props also to Mark Harris (who did a lot of the optimization), and Stephen Jones, who did the CUDA dynamic parallelism implementation (which is pretty cool, by the way).

The biggest regret I have is not doing proper anti-aliasing for the stars that were smaller than a pixel. On a 60 x 20 foot screen, each pixel was about the size of a sugar cube and you could see them crawling from pixel to pixel!