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Real-time Global Illumination: Background Updating




Previously I wrote about introducing latency to the voxel cone step tracing realtime global illumination system. The idea here is to improve performance and quality, at the cost of a small delay when the GI calculation gets updated. The diffuse GI lighting gets cached so the final scene render is very fast.

Here's what a gradual GI update does. Of course, this will be running unseen in the background for the final version, but this shows what is actually happening:

Vulkan has a sophisticated system for supporting multiple device queues. I initially thought I could just run the GI update on a separate queue, like a low-priority CPU thread running in the background:


Unfortunately, this is not really possible on today's hardware. Even with the lowest queue priority setting, the GI queue hogs the whole GPU and causes the rendering queue to stall out. So I had to stick with doing everything in the main render queue.

The GI calculation only updates when the camera moves a certain distance. There is a latency setting that controls how many substeps the task is broken up into. High latency means many small steps, so the framerate will not dip much when GI is updating. Low latency means the GI will significantly decrease the framerate every time the camera triggers an update. It is possible to set up a combination of high resolution and low latency that will cause the render queue to stall out. If this happens the program will encounter a VK_ERROR_DEVICE_LOST error. I don't know how to prevent this for now, other than just don't use ridiculous settings.

Here you can see the GI updating seamlessly as the camera moves around. I actually have to use three sets of volume textures, one for rasterization and direct lighting, and then the final GI data is stored in a set of two textures that alternate back and forth. This allows me to display the previous results while the next update is still processing, and the two textures get swapped when the update is finished.

A couple of small issues remain.

  • The transition between textures needs to be smoothed, to handle changes to the environment.
  • I need a way to calculate the diffuse GI for dynamic objects that don't appear in the GI reflections. (I have an idea.)

The items are not too terribly difficult and they will be done in short order. I'm very happy with how this has turned out. It provides quality real-time global illumination without compromising performance, and it will work very well with VR.

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