Image sampler (antialiasing)

General

In V-Ray, an image sampler refers to an algorithm for sampling and filtering the image function, and producing the final array of pixels that constitute the rendered image.

V-Ray implements several algorithms for sampling an image. You can choose between Fixed rate sampler, Adaptive DMC sampler and Adaptive subdivision sampler.

Parameters

Image sampler	Specifies the image sampler type.
Fixed	This sampler always takes the same number of samples per pixel.
Adaptive DMC	This sampler takes a variable number of samples per pixel depending on the difference in the intensity of the pixels.
Adaptive subdivision	This sampler divides the image into an adaptive grid-like structure and refines depending on the difference in pixel intensity.
Antialiasing filter	This section allows you to choose an antialiasing filter.

Fixed rate sampler

This is the simplest image sampler, and it takes a fixed number of samples for each pixel.

Subdivs

Determines number of samples per pixel. When this is set to 1, one sample at the center of each pixel is taken. If this is greater than 1, the samples are distributed within the pixel. The actual number of pixels is the square of this parameter (e.g. 4 subdivs produce 16 samples per pixel).

Adaptive DMC sampler

This sampler makes a variable number of samples per pixel based on the difference in intensity between the pixel and its neighbors.

This is the preferred sampler for images with lots of small details and/or blurry effects. It also takes up less RAM than the Adaptive subdivision sampler.

Min subdivs	Determines the initial (minimum) number of samples taken for each pixel. You will rarely need to set this to more than 1, except if you have very thin lines that are not captured correctly, or fast moving objects if you use motion blur. The actual number of pixels is the square of this number (e.g. 4 subdivs produce 16 samples per pixel).
Max subdivs	Determines the maximum number of samples for a pixel. The actual maximum number of sampler is the square of this number (e.g. 4 subdivs produces a maximum of 16 samples). Note that V-Ray may take less than the maximum number of samples, if the difference in intensity of the neighbouring pixels is small enough.
Thresh	The threshold that will be used to determine if a pixel needs more samples.

Adaptive subdivision sampler

This is an advanced image sampler capable of undersampling (taking less than one sample per pixel). In the absence of blurry effects (direct GI, DOF, glossy reflection/reftaction etc) this is the best preferred image sampler in V-Ray. On average it takes fewer samples (and thus less time) to achieve the same image quality as the other image samplers. However, with detailed textures and/or blurry effects, it can be slower and produce worse results than the other two methods.

Also note that this sampler takes up more RAM than the other two samplers - see the Notes below.

Min. rate	Controls minimum number of samples per pixel. A value of zero means one sample per pixel; -1 means one sample every two pixels; -2 means one sample every 4 pixels etc.
Max. rate	Controls maximum number of samples per pixel; zero means one sample per pixel, 1 means four samples, 2 means eight samples etc.
Thresh	Determines the sensitivity of the sampler to changes in pixel intensity. Lower values will produce better results, while higher values will be faster, but may leave some areas of similar intensity undersampled.
Edges	This will cause the image sampler to always supersample object edges (regardless of whether they actually need to be supersampled). This option has no effect if DOF or motion blur is enabled.
Normals	This will supersample areas with sharply varying normals. This option has no effect if DOF or motion blur is enabled.

Notes

Which sampler to use for a given scene? The answer is best found with experiments, but here are some tips:
- For smooth scenes with only a few blurry effects and smooth textures, the Adaptive subdivision sampler with its ability to undersample the image is unbeatable.
- For images with detailed textures or lots of geometry detail and only a few blurry effects, the Adaptive DMC sampler performs best. Also in the case of animations involving detailed textures, the Adaptive subdivision sampler might produce jittering which the Adaptive DMC sampler avoids.
- For complex scenes with lots of blurry effects and/or detailed textures, the Fixed rate sampler performs best and is very predictable with regards to the quality and render time.
A note on RAM usage: image samplers require substantial amount of RAM to store information about each bucket. Using large bucket sizes may take a lot of RAM. This is especially true for the Adaptive subdivision sampler, which stores all individual sub-samples taken within a bucket. The Adaptive DMC sampler and the Fixed rate sampler on the other hand only store the summed result of all sub-samples for a pixel and so usually require less RAM.