Sunday, December 28, 2008

Interesting GDC talk #1: Light Pre-pass Renderer

The Light Pre-Pass Renderer: Renderer Design for Multiple Lights

GDC is fast approaching and I'll be attending this year. One of the talks I'm looking forward to is Wolfgang Engel's talk on a novel renderer design. I've been following his various entries on this topic for the last year. Additionally, if you have access to the PS3 dev site, the Uncharted guys gave a talk on a very similar renderer.

The technique is a hybrid between forward rendering techniques and deferred rendering techniques. In a forward renderer you are evaluating material and lighting properties simulatenously, and in a deferred renderer you evaluate material properties into a G-Buffer and then evalute each light sampling that buffer.

In the light pre-pass renderer, it is broken down further -evaluate the bare minimum material properties needed for the lighting equation, then evaluate the lights into an accumulation buffer, then apply the lighting in a final pass of geometry. While you still have one more pass than in a typical deferred setup, the light pre-pass renderer does give you a lot more flexibility for materials. 

I like this approach over forward rendering or deferred rendering because I think it fits the box today's consoles lay out very nicely. Deferred rendering requires a huge number of render targets. Forward rendering requires either huge complicated shaders or more rendering passes, and neither of these things scale very well as you add more lights or more material types. The light pre-pass approach gives you the linear scalability of deferred rendering while allowing the material variety of forward rendering. 

When it comes down to it, what is most interesting is the array of choices you have when implementing the technique. The technique can work well with statically computed lighting such as  lightmaps if you choose to go that way. You have different choices in how to approximate specular and accumulate it in either a separate or combined RT. You have a variety of choices of how far you want to implement MSAA. Particularly when you have a cross-platform renderer, having this sort of flexibility without massive rearchitecture on each platform can make things a lot easier. I plan to explore this technique further.

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