This page provides information on the DMC sampler options in the Sampler tab of the V-Ray Renderer.

Overview


The DMC Sampler is a framework within V-Ray for determining exactly which samples are to be taken around pixels involved in "blurry" effects such as anti-aliasing, depth of field, indirect illumination, area lights, glossy reflections/refractions, translucency, motion blur, and so on. DMC stands for Deterministic Monte Carlo.

 

UI  Path


 

||Parameters tab|| > Sampler tab 

(with the V-Ray Renderer node selected)

 

DMC Sampler Parameters


 

 


Animated Noise Pattern – When enabled, the sampling pattern changes with time. Disabling makes the sampling pattern be the same from frame to frame in an animation, which might be undesirable in some cases. Note that re-rendering the same frame produces the same result in both cases.

Use Local Subdivs – When disabled, V-Ray automatically determines subdivs values for sampling of materials, lights and other shading effects based on the Min. Shading Rate parameter for the Image sampler. When enabled, V-Ray uses the Subdivs Mult parameter and the Subdiv values for individual V-Ray lights and materials.

Subdivs Multiplier – Multiplies all subdivs values everywhere during rendering. This affects everything, except for the lightmap, photon map, caustics and anti-aliasing subdivisions. Everything else (dof, moblur, irradiance map, brute-force GI, area lights, area shadows, glossy reflections/refractions) is affected by this parameter. This option is useful for quickly increasing/decreasing sampling quality everywhere.

Adaptive 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 are taken. A value of 1.0 means full adaptation; a value of 0.0 means no adaptation. For most scenes, there is no need to adjust this parameter.

Noise Threshold – Controls V-Ray'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 is performed. For most scenes, there is no need to adjust this parameter.

Min Samples – Determines the minimum number of samples that must be made before the early termination algorithm is used. Higher values slow things down but make the early termination algorithm more reliable. For most scenes, there is no need to adjust this parameter.

Random Seed – If a non-zero value is specified, generates a random seed for the DMC sampler.

Divide Shading Subdivs – When enabled, allows V-Ray to divide the number of samples for lights, materials, etc. by the number of AA samples in order to achieve roughly the same quality and number of rays when changing AA settings.

Origin of DMC Sampler


Monte Carlo (MC) sampling is a method for evaluating "blurry" values. Monte Carlo sampling uses pseudo-random numbers which are different for each and every evaluation, so re-rendering a single image would always produce slightly different results in the noise. V-Ray uses a variant of Monte Carlo sampling called deterministic Monte Carlo (DMC) which uses a predefined set of samples, so re-rendering an image always produces the exact same result. These samples might also be further optimized to reduce noise.

 Instead of having separate sampling methods for each of the blurry values, V-Ray has a single unified framework that determines exactly which samples (and exactly how many) are to be taken for a particular value, depending on the context in which that value is required. This framework is the DMC Sampler.

 By default, the deterministic Monte Carlo method used by V-Ray is a modification of Schlick sampling, introduced by Christophe Schlick in 1991 (see the References section below).

 

Determining Values for DMC Sampler


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 Subdivs Multiplier parameter.
  • 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. 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 is 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.

 

 

References


More information on deterministic Monte Carlo sampling for computer graphics can be found from the sources listed below.

  • Schlick, C., 1991, An Adaptive Sampling Technique for Multidimensional Integration by Ray Tracing, in Second Eurographics Workshop on Rendering (Spain), pp. 48-56
  • Describes deterministic MC sampling for antialiasing, motion blur, depth of field, area light sampling and glossy reflections. 
  • Masaki Aono and Ryutarou Ohbuchi, November 25, 1996, Quasi-Monte Carlo Rendering with Adaptive Sampling, IBM Tokyo Research Laboratory Technical Report RT0167, pp.1-5;
    online version can be found here
    Describes an application of low discrepancy sequences to area light sampling and the global illumination problem.
  • Fajardo, M., August 13, 2001, Monte Carlo Raytracing in Action, in State of the Art in Monte Carlo Ray Tracing for Realistic Image Synthesis, SIGGRAPH 2001 Course 21, pp. 151-162;
    online version can be found here
    Describes the ARNOLD renderer employing randomized quasi-Monte Carlo sampling using low discrepancy sequences for pixel sampling, global illumination, area light sampling, motion blur, depth of field, etc.
  • Veach, E., December 1997, Robust Monte Carlo Methods for Light Transport Simulation, Ph. D. dissertation for Stanford University, pp. 58-65
    online version can be found here    
    Includes a description of low discrepancy sequences, quasi-Monte Carlo sampling and its application to solving the global illumination problem.
  • Szirmay-Kalos, L., 1998, Importance Driven Quasi-Monte Carlo Walk Solution of the Rendering Equation, Winter School of Computer Graphics Conf., 1998
    online version can be found here
    Describes a two-pass method for solving the global illumination problem employing quasi-Monte Carlo samp ling, as well as importance sampling using low discrepancy sequences.

 

 

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