Jason Gedge

End of Summer Updates

Posted on September 01, 2014

So it’s been a couple of weeks since my last update. I definitely haven’t had a lot of time to work on things, but there are a few updates and thoughts that I’d like to share.

Updates

First, the project updates. After refactoring my meshing code I was able to easily add in some performance tests to test my meshing code. What I found is that the greedy mesher was pretty slow, and after a bit of profiling I found out that it was due to poor access characteristics; we need to iterate over two-dimensional slices of the 3D chunk, instead of whatever iteration has the best performance. To improve on this I actually cache the most important information — the visibility information — of the chunk data in the greedy mesher and use this instead of the chunk’s interface. This allowed me to go from about 4-5ms per meshing operation to about 1.5ms. My goal is to eventually drive this down to under 1ms, but for now I’m happy with 1.5ms.

A small but helpful update was front-to-back sorting of chunks as a pre-render step. Quite trivial to implement and saved me about 0.5ms at peak. This gives you a performance boost because the fragment shader will not be executed for any fragment that fails the depth test, so we try to order our triangles in a way such that the depth buffer gets filled with the final values as quickly as possible. My fragment shader is fairly simple right now, so the gains could be much higher if the complexity rises.

The biggest update though, is moving all of my chunk generation and rendering code off the main thread. To some extent I was doing this before, but I had to collaborate with a single std::future. It was much easier to just have a worker thread and let it do as much work as possible on its own rather than maintaining a bunch of std::futures. The only thing that happens on the main thread right now is OpenGL code (i.e., fill a vertex buffer with the mesh). This didn’t improve performance in the sense of seconds per frame, but it did allow me to show the world much more quickly.

Other small notes:

  • Added code to display the number of triangles that are loaded. Right now I’m at ~1 million triangles and 2.9 seconds per frame to render. My goal is to reduce both of those numbers. Reducing the former requires either improved culling and/or “level-of-detail meshing”.
  • Significantly decrease my memory footprint by using 8-bit integers for the texture indices instead of 32-bit integers. This is mostly temporary though; as my voxel data gets more complex, I’ll likely have to take advantage of the flyweight pattern. Another two tasks that I have on my to-do list are implementing run-length encoding for voxel data storage in chunks and dropping the voxel data of distant chunks after they’ve been meshed.

C++11 Thoughts

I recently watched Eric Niebler’s BoostCon presentation title C++11 Library Design. I’ve embedded the video below for your viewing pleasure, but first I’ll summarize some of his key points:

  • Read-only arguments should be passed in as const references, sink arguments should be passed in by value (avoid rvalue references). The only time one should use rvalue references is when you’re looking to do perfect forwarding.
  • Make move operations noexcept, if possible.
  • Use versioned, inline namespaces from day one.
  • ADL can really bite you sometimes, so use constexpr functors instead of free functions since global functors are never found by ADL. Note that if you really want a free function to be extensible (think, std::swap) then stick to a free function.

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