V-Ray for Rhino Manual - Rum

Secondary Engine: Light Cache
Light Cache is used for Secondary Engine to calculate light distribution in scenes. Its calculated in a
way that is very similar to Photon Mapping. With Photon Mapping the calculation starts from the
light source and collects light energy along way. Light Cache starts from the camera instead. Some
advantages to using Light Cache are that It doesn't have many settings to deal with and it renders
quite fast.
The image on the left is rendered with combination of Irradiance Map and QMC and the image on the
right is rendered with combination of Irradiance Map and Light Cache. The image on the left is
slightly brighter. This is due to the fact that Light Cache calculates an infinite number of secondary
bounces, where QMC only calculates a predetermined number of bounces. Although each of these
bounces individually is insignificant, their added affect increases the brightness of the image.
Subdivs is the most important factor for Light Cache. Subdivs is used to decide how many light
traced to use from Camera to calculate the light distribution. The actual number of traced is the
square of the number of Subdivs. As default of 1000 for example, the actual number of traced rays
will be 1,000,000.
When determining how many Subdivs will be sufficient for an image, the best way is to look at the
progress window, monitor the appearance of the image in the frame buffer, and approximate the
number of samples according to the progress and total number of samples. Say the Subdivs number
is 1000 for example, when the Subdivs is half way through its calculation, the rendering window's
black dots are almost gone, that means you only need to set the number between 500~600 and you
will get the correct render result. If the process is done but still have a lot of black dots in the
window, that means more subdivs are needed to produce an accurate result. Image below is showing
a Light Cache calculation which still has a large number of black spots.
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