Sunday, June 21, 2009

Leaky abstractions in XNA

So continuing my exploration of XNA, this weekend I did some more work on my little toy project.

The first thing I did was get it running on 360. I was happy to see that XNA seems to be able to figure out how to deal with my various render targets, including one MRT, without too much trouble, and the performance was far superior on the 360 than on my laptop. I get about 200 fps on the 360 vs 60 on the laptop.

There was one issue worth noting.

First, some background on the deferred lighting approach I am using:

  1. Render normal + depth into a G buffer for all primitives. Depth writes and tests are enabled in this step.
  2. Render the lights into a lighting buffer using the G buffer. Depth writes and tests are disabled for this pass.
  3. Apply the lighting to each primitive using the lighting from step 2 while computing albedo and (eventually) other material properties on the fly. Depth tests are enabled but not writes.

So the first problem on the 360 is XNA blows away the depth buffer I lay down in step 1 by the time I get to step 3. After some searching on the internets, I discovered this is expected behavior.

I tried setting my render targets to PreserveContents, which does work, but is completely wasteful since I don't give a hoot about restoring the actual color contents of any of these buffers. This dipped performance down to 150fps.

My next attempt was to restore the depth buffer manually from my G Buffer. But this was exhibiting z-fighting, possibly due to slightly different methods of Z calculation for my G-Buffer vs the depth-buffer leading to small differences in the computed Z values. I didn't feel that messing around with z biasing would be a robust solution, so I abandoned this effort.

The solution I ended up choosing was to just clear the z buffer again and reconstruct it during step #3. Since my scenes are so simple this gets me back to just slightly under 200 fps.

It's not an ideal solution, since I had in mind some uses for a stencil buffer laid down in step #1 that would accelerate step #2 (mainly, masking off unlit pixels for the skybox).

XNA's EDRAM handling is a great example of a leaky abstraction. Only having a 10 MB EDRAM buffer does make render target management trickier, but in Microsoft's attempt to completely hide it from XNA programmers, I think they've just made things more frustrating. The concept of a limited buffer for render targets is not that hard to get your head around. You have to understand EDRAM anyway since techniques in XNA that work perfectly on Windows (like what I was doing) will break on the 360. Even worse, you have no real good idea *why* it's breaking unless you understand the limitations of EDRAM and take a guess at what Microsoft is doing under the hood. So what is really being saved here? Just let me deal with EDRAM myself.

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