rQMC (randomized Quasi Monte Carlo) sampling is employed throughout VRay
for every "blurry" value - antialiasing, depth of field, indirect
illumination, area lights, glossy reflection/refraction, translucency,
motion blur etc. rQMC sampling is used to determine what samples should be
taken and, ultimately, which rays to trace.
Instead of having separate methods for sampling each of those blurry
values, VRay has a single unified framework that determines how many and
what exactly samples to be taken for a particular value, depending on the
context in which that value is required. This framework is called the rQMC
sampler.
Note that although similar in concept, the sampling method employed by
VRay is different from strict QMC sampling in that the number sequences
that are used have higher discrepancy than pure QMC methods (but still lower
one compared to pure random Monte Carlo methods).
The actual number of samples for any blurry value is determined based on
three factors:
- The
subdivs value supplied by the user for a particular blurry
effect. This is multiplied by the Global subdivs
multiplier (see below).
- The importance of the value (for example, dark glossy reflections
can do with fewer samples than bright ones, since the effect of the
reflection on the final result is smaller; distant area lights require
fewer samples than closer ones etc). Basing the number of samples
allocated for a value on importance is called
importance sampling.
- The variance (think "noise") of the samples taken for a particular
value - if the samples are not very different from each other, then the
value can do with fewer samples; if the samples are very different, then
a larger number of them will be necessary to get a good result. This
basically works by looking at the samples as they are computed one by
one and deciding, after each new sample, if more samples are required.
This technique is called
early termination or adaptive sampling.
For more information on the relationship and effects of these parameters,
please refer to the
tutorials
section.
Amount - controls the extent to which the
number of samples depends on the importance of a blurry value. It also
controls the minimum number of samples that will be taken. A value of 1.0
means full adaptation; a value of 0.0 means no adaptation.
Min samples - determines the minimum
number of samples that must be made before the early termination algorithm
is used. Higher values will slow things down but will make the early
termination algorithm more reliable.
Noise threshold - controls VRay's
judgment of when a blurry value is "good enough" to be used. This directly
translates to noise in the result. Smaller values mean less noise, more
samples and higher quality. A value of 0.0 means that no adaptation will be
performed.
Global subdivs multiplier - this will
multiply all subdivs values everywhere during rendering; you can use this to
quickly increase/decrease sampling quality everywhere. This affects
everything, except for the lightmap, photon map, caustics and aa subdivs.
Everything else (dof, moblur, irradiance map, qmc GI, area lights, area
shadows, glossy reflections/refractions) is affected by this parameter.
Time independent - when this option is
On, the rQMC pattern will be the same from frame
to frame in an animation. Since this may be undesirable in some cases, you
can turn this option Off to make the rQMC pattern
change with time. Note that re-rendering the same frame will produce the
same result in both cases.
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