V-Ray for Rhino Manual - Rum

Primary Engine: Quasi Monte-Carlo
Quasi Monte-Carlo is the most accurate method of light calculation in V-Ray. It is most useful for
scenes with a lot of small details. The downfall with this method is that it takes significantly
longer to render. There is also no prepass for this calculation as it is done as the image is rendered.
Image on the left is rendered with Irradiance Map. Image on the right is rendered with QMC.
Although the one on the right looks slightly grainy, the colors are reproduced much more accurately
with the QMC calculation.
QMC generally produces a slightly grainy result. One of the ways you can improve this is by using a
different image sampler. Open the Image Sampler tab in Render options and change the Sampler
from Adaptive Subdivision to Adaptive QMC. Although Adaptive Subdivision produces predictable
and fast results, Adaptive QMC works very well when QMC is used for primary bounces. Now change
the Max Subdivisions to a higher number such as 50. This will help decrease the grain in the image.
With QMC it is much easier to set up a
rendering as there are very few settings
that will need to be adjusted. Artifacts
such as light leaks and splotchiness will not
be a factor in QMC renders.
It is recommended that QMC only be used for final, or high quality test images due to the
amount of time required to complete the render. It is a good idea to use Irradiance Map or Light
cache for test images, then switch to QMC for final images. Results similar to QMC can be
obtained through Irradiance Maps, usually with less time than QMC, so it may not be completely
necessary to switch to QMC for final images depending on the situation.
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V-Ray for Rhino