Our latest and biggest (not to mention coolest) toy at the Interactive Media Lab Dresden is a ten square-meter interactive wall that’s fully touch-sensitive and supports markers and pens as well. It consists of twelve Full HD monitors hooked up to two Radeon 7970 graphics cards in a single dual-Xeon workstation. Since it’s driven by a single workstation, we can drive the complete wall with a single application, which is really cool and sets it apart from most similar setups. However, the dual-graphics-card setup causes issues: Under Linux, we have two separate desktops, and under Windows, applications that span the graphics card boundary are extremely slow.

To get full-screen rendering at interactive speeds, you basically have to open two borderless windows – each spanning 6 screens and pinned to one of the GPUs. Then you render the same scene with different viewports in each of the windows. That means that all context-specific data – textures, vertex buffers, shaders, framebuffer objects, and even caching of shader parameters – needs to be replicated across both contexts. Also, we can’t switch contexts too often, because that would make things slow.

libavg renders in two passes: The first (implemented in the Node.preRender() functions) prepares textures and vertex data. It also renders FX nodes. The second pass (implemented in Node.render()) actually sends render commands to the graphics card. The multi-context code changes a few things: While preRender() is still executed only once, render() is executed once per GPU. Uploads of data as well as effects that need to be rendered are scheduled in preRender and actually executed at the beginning of the render. In total, refactoring everything accordingly was (obviously) a lot of work that impacts lots of code all over the graphics engine, but the result is good rendering performance with 24 megapixels of resolution.

The code is still on a branch (The svn repository is at https://www.libavg.de/svn/branches/experiments/libavg_multicontext/), but it passes all tests, and I’ll merge it to trunk after we’ve used it a bit.