XRT is a raytracing based programmable rendering system for photo-realistic image synthesis built around a plug-in architecture design.


Ambient occlusion

Posted: 22 Jan 2010 22:40
Tags: ambient occlusion


Ambient occlusion measures how much of the hemisphere above each surface point is occluded by other objects in the scene. It is computed by tracing rays randomly distributed around the hemisphere above each shaded point. In the example picture, the shader colors pixels white if they have a completely unoccluded view of the hemisphere, and black if the view of the hemisphere is completely occluded: the result looks a bit washed out similar to the way an object would appear on an overcast day.
Because it takes into account other objects for lighting computations, it pretends to be a global illumination solution but is not physically based. Anyway, this is a much better solution for computing the ambient term compared to adding the usual constant to the rendering equation. The only drawback is that you need to fire lots of ray per pixel to get rid of the noise (random distribution of rays = noise).
Implementing ambient occlusion is as simple as its definition: if you already have shadow rays in your renderer, it takes a mere 20 lines of code.
Despite its simplicity, it gives surprisingly convincing results. Therefore, it is part of the toolbox of all major CGI studios: Pixar, DreamWorks, Sony Pictures and the likes. You will find more examples of this feature in the gallery (look here)

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OSL is out

Posted: 16 Jan 2010 18:12
Tags: compiler langage osl shading


Quoting Sony Pictures Imageworks Open Source Software site: «Open Shading Language (OSL) is a small but rich language for programmable shading in advanced renderers and other applications. OSL is similar to C, as well as other shading languages; however, it is specifically designed for advanced rendering algorithms with features such as radiance closures, BRDFs, and deferred ray tracing as first-class concepts.

The OSL project includes a complete language specification, a compiler from OSL to an intermediate assembly-like bytecode, an interpreter that executes OSL shaders on collections of points in a SIMD manner, and extensive standard shader function library.»

Although the project is not yet production-ready, browsing the source code is very instructive: aside from the fact that the project is a very good demonstration of how to design a langage compiler and byte code interpreter, it features things that I never seen before: automatic differentiation of expressions, symbolic computing. These features allow a raytracer to compute derivatives in a very efficient manner (no need to evaluate three times a shader like BMRT did in its time, once is enough) or to perform fast relighting (running a shader does not return a color but a symbolic expression that can be stored and evaluated later against a different set of light sources). There are many other aspects of OSL that are well thought out and will favor the implementation of global illumination renderers.

For all these reasons, OSL looks like a very worthwhile component to add to the XRT renderer. However, the langage specification is not yet complete and there is currently no way to define the equivalent of RenderMan shading langage "illuminance loops" (except by wiring them into the renderer). This is a long term goal for the project but is not due soon. The roadmap will probably be detailed in the forthcoming months.

One thing I am contemplating is that, given that RSL and OSL already share a lot (keywords, types, function library), it should be possible to transform the OSL compiler into a RSL one without too much hassle. This would allow me to reuse most of OSL.

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