Image Sampler Overview
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. All image samplers support Max's standard antialiasing filters, although, at the cost of increased rendering time. You can choose between Fixed rate sampler, Adaptive DMC sampler, and Adaptive subdivision sampler.
Example: What is antialiasing?
The following example shows the basic difference between an image with antialiasing, and one without:
No antialiasing (Fixed rate sampler, 1 subdiv)
Antialiasing on (Adaptive subdivision sampler, rate -1/2)
No antialiasing (Fixed rate sampler, 1 subdiv)
Antialiasing on (Adaptive subdivision sampler, rate -1/2)
Image Sampler
Type - specifies the image sampler type:
- Fixed - this sampler always takes the same number of samples per pixel;
- Adaptive - 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 it depending on the difference in pixel intensity.
- Progressive - this sampler progressively samples the entire image.
Examples: Image Sampler Comparisons
Here are some examples demonstrating the quality vs. speed of the image samplers. All the samplers were set to produce approximately the same image quality.The first example is a normal smooth image (no blurry effects), click the images for a larger view:
Fixed rate (4 subdivs)
Render time: 36.1s
Adaptive (subdivs 1/4)
Render time: 4.0s
Adaptive subdivision (rate -1/2)
Render time: 1.8s
Both the adaptive and the adaptive subdivision samplers are substantially faster than the fixed rate sampler.
Here is a more complex example with some fine textures (lots of fine bump mapping) and an area light; this example uses a precalculated irradiance map - the render times are for the final rendering only. Click the images for a larger view.
Note: the Sponza Atrium model was created by Marko Dabrovic (http://www.rna.hr) and was one of the models for the CGTechniques Radiosity competition. The Athene model was a free model from the DeEspona Infografica model bank.
Fixed rate
(4 subdivs)
Adaptive
(subdivs 1/4)
Adaptive subdivision
(rate 0/2, threshold 0.05)
In this case, the Adaptive sampler performed best, and the Adaptive subdivision - worst. Why is that? Here is the non-antialiased image, to give an idea of what the image samplers had to deal with.
Some parts of the image are quite "noisy" because of the fine bump map. Lots of image samples were required to smooth this out. Furthermore, each image sample was quite costly to compute - there is an irradiance map and an area light, which (especially the area light) need a lot of computations. With the Fixed and Adaptive samplers, V-Ray knows in advance how many image samples will be taken for a pixel; therefore, it can optimize the computation of some values (the area light for example) so that the final image result is similar, while actually those values are computed with lower accuracy (i.e. tracing fewer shadow rays) for the individual image samples. This cannot be done for the Adaptive subdivision sampler - it does not know in advance how many samples will be computed for a pixel; therefore it needs to maintain a constant (high) accuracy. Constant accuracy is also required in order for the sampler to adapt correctly to the image. This is why, in this example, the Adaptive subdivision sampler performed worse than the other two methods.
Note: the dragon model is from one of the example scene files of 3ds Max 4
The third example is an image with direct GI and motion blur:
Fixed rate
(4 subdivs)
Adaptive
(subdivs 1/4)
Adaptive subdivision
(rate 0/2, threshold 0.1)
In this case, the Fixed rate sampler was the fastest and the Adaptive subdivision sampler was the slowest of all (admittedly, the image computed with adaptive subdivision is very smooth). This is because the cost of supersampling a pixel for the Adaptive and the Adaptive subdivision samplers becomes too great.
Here is the scene without motion blur, with irradiance map, and with the Adaptive subdivision sampler (render time includes GI calculations):
Min shading rate - this option allows you to control the number of rays shot for Antialiasing (AA) versus rays for other effects like glossy reflections, GI, area shadows, etc. It is especially useful with the Progressive image sampler. Higher values mean that less time will be spent on AA, and more effort will be put in the sampling of shading effects.
When loading scenes saved with V-Ray 2.x, the Min shading rate parameter is set to 1 in order to produce the same results as the previous V-Ray versions.
Example: Min Shading Rate
Shading rate = 1
Shading rate = 16
Both of the images above rendered with the Progressive image sampler for the same amount of time. With Shading rate set to 16, at least 16 GI rays are shot for each AA ray, which produces smoother GI, but overall leads to less samples left for the DOF effect.
Filter - specifies the filter type. All standard 3ds Max filters are supported with the exception of the Plate Matchfilter. See below for more information on antialiasing filters. Avoid using filters with negative components (sharpening filters) like Catmull-Rom or Mitchell-Netravali when using the Progressive sampler as this may increase the render times quite a bit - the sampler needs to take additional image samples to resolve the filter.
Example: Antialiasing Filters
Here is an example briefly demonstrating the effect of different antialiasing filters on the final result.
Note that rendering with a particular filter is not the same as rendering without a filter and then blurring the image in a post-processing program like Adobe Photoshop. Filters are applied on a sub-pixel level, over the individual sub-pixel samples. Therefore, applying the filter at render time produces a much more accurate and subtle result than applying it as a post effect. V-Ray can use all standard 3ds Max filters (with the exception of the Plate match filter) and produces similar results to the scanline renderer.
The Adaptive image sampler was used for the images below, with Min/Max rate of -1/3 and the Rand option on.
| Filter | Image | Zoomed-in image | Comments |
| Filtering is off | Applies an internal 1x1 pixel box filter. | ||
| Area filter, size 1.5 (default setting) | Slightly blurs the image, visually more pleasing than the box filter. | ||
| Area filter, size 4.0 | More blurring. | ||
Blend filter | Combination of a sharp and a soft filter, kind of dreamy effect. | ||
Catmull-Rom | Edge-enhancing filter, often used for architectural visualizations. Note that edge enhancing can produce "moire" effects on detailed geometry. | ||
Mitchell-Netravali | Allows control between edge-enhancement and blurring. | ||
MItchell-Netravali, ringing=1.5 | Strong edge-enhancement. | ||
Mitchell-Netravali, ringing=2.0 | Even more edge enhancement; kind of cartoon-style effect. | ||
Soften | Gaussian blur. |
Example: Antialiasing Filters and Moire Effects
This example demonstrates the effect antialiasing filters have on moire effects in your images. Sharpening filters (Mitchell-Netralavli, Catmull-Rom) may enhance moire effects, even if your image sampling rate is very high. Blurring filters (Area, Quadratic, Cubic) reduce moire effects.
Note that moire effects are not necessarily a result of poor image sampling. In general, moire effects appear simply because the image is discretized into square pixels. As such, they are inherent to digital images. The effect can be reduced through the usage of different antialiasing filters, but is not completely avoidable.
The scene is very simple: a sphere with a very fine checker map applied, texture filtering is off. The images were rendered with a very high sampling rate (15 subdivs, or 225 rays/pixel). This is enough to produce quite an accurate approximation to the pixel values. Note that the image looks quite different depending on the filter:
No Filter
Area filter, size = 1.5
Area filter, size = 4.0
Quadratic filter
Sharp quadratic filter
Cubic filter
Video filter
Soften filter,size = 6.0
Cook variable, size = 2.5
Blend, size = 8.0, blend = 0.3
Blackman
Mitchell-Netravali, blur = 0.333, ringing = 0.333
Catmull-Rom
Fixed Sampler
This is the simplest image sampler, and it takes a fixed number of samples for each pixel.
Subdivs - determines the 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).
Per Pixel Filtering - enables the antialiasing filter
Adaptive Sampler
This sampler makes a variable number of samples per pixel based on the difference in intensity between the pixel and its neighbors.
The following diagram shows visually the way V-Ray is placing samples when using the Adaptive sampler. The black squares represent the pixels of the image while the dots represent the individual samples. In the first pass, V-Ray always places the minimum number of samples determined by the Min subdivs parameter. Then, the color of samples is compared and more are added where needed in the following passes.
This is the preferred sampler for images with lots of small details (like VRayFur, for example) and/or blurry effects (DOF, motion blur, glossy reflections etc). 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 neighboring pixels is small enough.
Show Samples - enable ti
Per Pixel Filtering - enables the antialiasing filter
Threshold - the threshold that will be used to determine if a pixel needs more samples. This is ignored if the Use DMC sampler threshold option is on.
Use DMC sampler threshold - when this is on (the default), V-Ray will use the threshold specified in the DMC sampler to determine if more samples are needed for a pixel. When this is off, the Color threshold parameter will be used instead.
Show samples - if this is on, V-Ray will show an image where the pixel brightness is directly proportional to the number of samples taken at this pixel. This is useful for fine-tuning the antialiasing of the image.
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/refraction, 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.
The following diagram shows visually the way V-Ray works when using the Adaptive subdivision image sampler. With this mode V-Ray creates a secondary grid on top of the pixel grid and uses this grid to position the samples. This allows it to use less than a sample per pixel. After the first pass, the samples are compared and, if the difference between two samples is bigger than the values in the thresholds, the grid is subdivided and more samples are added. During the whole time, V-Ray has to keep the whole grid in the memory, which makes this method less memory efficient compared to the other two methods - 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.
Threshold - 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.
Normals threshold - determines the sensitivity of the sampler to the variation of normals.
Show samples - if this is on, V-Ray will show an image where the pixel brightness is directly proportional to the number of samples taken at this pixel. This is useful for fine-tuning the antialiasing of the image.
Jitter - displaces the samples slightly to produce better antialiasing of nearly horizontal or vertical lines.
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.
Example: Randomized Antialiasing
Normally V-Ray places the image sample in a strict grid-like pattern. This may cause unwanted banding of edges that are nearly horizontal or nearly vertical. You can use the Jitter option to avoid that. Here is a comparison of an image rendered with and without the Jitter option:
Jitter off
Jitter on
Here are close-ups of the two images:
Jitter off
Jitter on
The Jitter option can be very useful for images with long thin lines as well.
Example: The Edges Parameter
In the Threshold Example, the edges of the object are always sharp, regardless of the value of the Threshold. This is because the Edges option is turned on. Here are the first and the last of the images from the earlier example, rendered with Edges off:
Threshold 10.0, Edges off
Threshold 0.1, Edges off
Now the antialiasing of edges depends only on the Threshold. By default, the Edges option is on, meaning that the outlines of objects are always antialiased. If there are many small objects in the scene, this may slow the rendering. In that case, it's better to turn this off and use only the Threshold to control image quality.
On the other hand, if there are lots of fine textures in the image, which you don't want supersampled, you can simply turn up the Threshold. In order to still keep object edges sharp, you'll need the Edges option.
Example: The Normals Parameter
The Normals option allows you to always antialias internal object edges, in addition to the object outline, as shown in the example below (min/max rate -3/2, Threshold 10.0, Edges on):
Normals off
Normals on
Progressive Sampler
The Progressive sampler is similar to the Adaptive sampler, but instead of rendering the image in buckets, it renders the entire image progressively in passes.
The advantage of this sampler is that you can see an image very quickly, and then let it refine for as long as necessary as additional passes are being computed. This is contrast to the bucket-based image samplers, where the image is not complete until the final bucket is done.
A disadvantage is that more data needs to be kept in memory, especially when working with render elements. Also, when using distributed rendering, because of the continuous refinement, frequent communication between the client machine and the render servers is required, which may reduce the CPU utilization on the render slaves. This effect can be controlled to some extent using the Ray bundle size parameter.
Min. subdivs - controls the minimum number of samples that each pixel in the image will receive. The actual number of the samples is the square of the subdivs.
Max. subdivs - controls the maximum number of samples that each pixel in the image will receive. The actual number of the samples is the square of the subdivs. If zero, the number of samples is not limited.
Threshold - the desired noise level in the image. If this is 0.0, the entire image is sampled uniformly until either the Max. subdivs value is reached or the Render time limit is reached.
Max time - the maximum render time in minutes. This is the render time for the final pixels only; it does not include any GI prepasses like light cache, irradiance map etc. If this is 0.0, the render is not limited in time.
Bundle Size - this option is useful for distributed rendering to control the size of the chunk of work that is handed to each machine. When using distributed rendering, higher values may help to utilize CPUs on the render servers better.
Show mask - this option allows the user to see a mask of the pixels that remain to be sampled.
Example: Stages of Rendering with the Progressive Sampler
Image after 1 pass
Image after 16 passes
Image after 64 passes
Image after 256 passes
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.