This page provides information on the DMC Sampler rollout in the Render Settings.

 

Page Contents

 

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: ||Render Settings window|| > VRay tab > DMC sampler rollout




DMC Sampler Parameters 


 

 

 

Animated noise pattern – When enabled, the sampling pattern will change with time. Disabling will make 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 will produce 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 will use the Subdivs Mult parameter and the Subdiv values for individual V-Ray lights and materials.

Subdivs Mult – 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.

 

DMC Sampler - advanced


 

 

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 will be 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.

Adaptive 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 will be performed. For most scenes, there is no need to adjust this parameter.

Adaptive 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. For most scenes, there is no need to adjust this parameter.

 

 


 

Example: Noise vs Speed


You can use the DMC sampler for global quality control, for example:

For lower quality:

increase the Amount
increase  Noise
reduce  Min samples

For higher quality:

decrease the Amount
decrease  Noise
increase  Min samples 

These controls have an effect on everything: DOF, GI, glossy stuff, area lights etc.

Color mapping mode also affects render times and sampling quality, since V-Ray tries to distribute samples based on the final image result. 

The images below have the same subdivs for GI (Primary Engine Brute Force 38 subdivs, Secondary Engine Light Cache 1000 subdivs ). Notice how changes in the DMC sampler parameters affect the image quality and render time (click the images to enlarge them):


 Adaptive amountNoise thresholdMin samplesRender time

0.10.1643m 17.9s

0.850.001162m 30.2s

0.850.005162m 22.8s

0.950.015

2m 21.1s

0.950.0552m 29.7s

0.990.05

5

2m 26.6s

10.152m 31.9s





 

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 pre-defined 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 Mult 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 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. 

 

 

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 sampling, as well as importance sampling using low discrepancy sequences.