Global Illumination

In the early days of computer graphics objects in a scene would only be illuminated if they were directly in line of sight of a light source. The real world is lit not just by light sources like the sun or a light bulb, but also indirectly from light bouncing off every surface and being scattered throughout the scene. Global Illumination (GI) is a term used to describe the effect of indirect illumination in computer graphics. Simulating indirect illumination is computationally more expensive than simple direct illumination, but it is invaluable in creating realistic interiors or enclosed spaces where the indirect illumination is a significant proportion of the overall lighting effect.

Conventional GI solutions tend to be complex to set up, requiring experience and deep understanding to set parameters “correctly” for the right effect. Shaderlight aims to simplify this process by using a new GI technique, Active Photon Tracing. This technique has been designed to be straight forward to set up. Active Photon Tracing also allows changes to light properties, so that the scene lighting can be changed after the render is complete and the indirect illumination updates accordingly. In future releases, this will extend to updating the indirect illumination as a result of changes to material properties.

Using GI

•	Check the Enable GI box in the Shaderlight Exporter.

•	If you want to tune lighting levels after you have rendered, check the Interactive GI box. (This will increase memory usage)

•	Click Export to render your scene.

Understanding GI controls

Shaderlight GI uses just three parameters:

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Photons. This value controls the accuracy of the GI by setting the number of Active Photons to use in the simulation. The more photons there are, the more likely that small detailed areas of the scene will be lit correctly. Note though that larger numbers of photons will render more slowly and use more memory. When they are created, the photons are distributed across all the lights in the scene according to how much energy each light source contributes, so a light that is twice as bright as its neighbor would emit twice as many photons.

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Samples. This controls how smooth the GI render will be. Active Photon Tracing takes into account the intensity of all sources of indirect light in the scene for each pixel in the final image. Decreasing the number of samples allows Shaderlight to put more weight on the closest or most intense indirect light sources, and makes the render more efficient. If there are too few samples, the render may have irregular light "hot spots" or banding in shadowed areas. Increasing the number of samples allows Shaderlight to take into account more of the indirect illumination in the scene, which will smooth out irregular lighting, but will take longer to render.

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Sub-sampling. This allows Shaderlight to take fewer GI samples where there is little change in the indirect lighting across a surface. Increasing this number will speed the render and reduce memory usage, but can introduce unwanted artifacts (such as brightly lit areas spilling into dark areas) if it is increased too far. Reducing the sub-sampling to 1 will ensure that samples are taken for every pixel in the scene, but will increase memory use and render time.

Interactive GI

Interactive GI allows you to tune light intensities and colors after a render is complete and see the indirect lighting update as a MELT change. Note that a lighting change in interactive GI will take longer than a regular MELT change.

If light intensities are changed significantly then the accuracy of the GI solution will degrade. This is because the number of photons sent from each light source in the scene is determined by their relative intensities (see "Understanding GI controls" above). If, for example, the brightest light in the scene is made the dimmest, and the dimmest turned up to be the brightest, then the indirect illumination will not be accurate. The reason for the inaccuracy is that the new brightest light has the fewest photons traced from it and the new dimmest light has the most photons and Shaderlight is no longer making best use of all the photons in the scene. After major lighting changes it is advisable to re-render the scene to get the most accurate render possible.

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