This page provides information on the Image Sampler rollout in the Render Settings.
In V-Ray, an image sampler refers to an algorithm for calculating a pixel's color based on the colors within and around it.
Each pixel in a rendering can have only one color. To get the color of a pixel, V-Ray calculates it based on the object's material, direct light striking the object, and indirect lighting in the scene. But within a single pixel there might be multiple colors, which may come from multiple objects whose edges intersect at the same pixel, or even difference in brightness on the same object due to changes in object shape or falloff and/or shadowing of light sources.
To determine the right color for such a pixel, V-Ray looks at (or samples) colors from different parts of the pixel itself as well as the pixels around it. This process is called image sampling. V-Ray includes two main image samplers, each with its own approach to sampling and its own parameters: Progressive and Bucket.
Image courtesy of Tuna Unalan
Multiple colors within a single pixel. What color should the pixel be?
Image Sampler UI for V-Ray GPU
Image Sampler UI for V-Ray
What is Anti-aliasing?
One of the functions of image sampling is anti-aliasing, which is the reduction of jagged edges in a rendering. The following example shows the basic difference between an image with anti-aliasing, and one without.
If only one sample is taken for the each of the pixels around the edge of the sphere, the choice of pixel color is limited to the dark gray at the edge of the object, or the (white) background. Using one of these colors (i.e. taking only one sample) makes the image look jagged. This is the equivalent of no anti-aliasing at all.
If two or more samples are taken in each pixel, the colors are averaged, and pixels at the edge of the object end up being a color in between the dark gray of the sphere and the background. These in-between colors make the object appear smoother in the final rendering.
The sampling options for the V-Ray render engine and the V-Ray GPU render engine are different. Only the options for the V-Ray engine are on this page. Please visit the V-Ray GPU Setup page for its options.
Sampler type – Specifies the image sampler type. Specific parameters for the selected type appear at the bottom of this rollout.
Progressive – Progressively samples the entire image. See the Progressive Sampler for additional parameters.
Bucket – Takes a variable number of samples per pixel depending on the difference in the intensity of the pixels. See the Bucket Sampler section for additional parameters.
Render mask – Enables the render mask feature. The render mask allows you to define which pixels of the image are calculated. The rest of the pixels are left intact. This feature works best with the V-Ray frame buffer and the Bucket image sampler. The following types are available:
Disabled – The render mask is not used.
Texture – A texture map is used for the render mask. Black values in the map define pixels which are not rendered. Pixels with any other values are rendered.
Object Set – A custom set of objects to render can be defined.
Object IDs – Only objects with specified Object IDs will be rendered, you can list more than one by using commas to separate them.
Isolate Select – Uses Maya's Isolate Select option to only update items included in that viewport selection feature to update when (re)rendering. It is activated from the Show > Isolate select > View selected (or Ctrl+1) in the viewport menu. It works with both V-Ray and V-Ray GPU in Production and IPR modes. Note: Lights cannot be isolated, they will always light the objects no matter if they are isolated in the viewport or not.
For more information, see the Render Mask example below.
Render mask clear – Available when the Render mask is enabled. When disabled, the masked region will be overlaid on top of the previous image in the VFB. This can be used for draft renders or fast shader previews provided that the camera does not move. Note: When rendering an animation, the image clearing is forced on.
AA filter – Enables AA filtering in the image. When disabled, V-Ray applies an internal 1x1 pixel box filter.
AA filter type – Specifies the filter type to be used for anti-aliasing. V-Ray supplies eight types of Anti-aliasing filters: Box, Area, Triangle, Lanczos, CatmullRom, Gaussian and Cook Variable. Each has advantages and disadvantages, which make them useful for different tasks. V-Ray GPU supports only Box, Area, Lanczos and Gaussian filter types.
Size – Determines the size of the filter in pixels. Higher values yield blurrier results. To produce physically accurate results, the minimum value of this parameter is 1.000 and the maximum value is 20.000. For more information, see the Anti-aliasing Filters example and the Anti-aliasing Filters and Moire Effects example below.
Example: Render Mask
The original image.
The same image, only objects with chrome material were re-rendered.
The actual render mask image - RGB channels.
The actual render mask image - Alpha channel.
Isolate Select Render Mask Video
The following short demonstration video helps explain how the new Isolate Select Render Mask type works.
Example: Anti-aliasing Filters
Here is an example briefly demonstrating the effect of different anti-aliasing 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. The zoomed in images below have been zoomed in and cropped 300%.
When the filtering is off, it internally applies 1x1 pixel box filter.
The Catmull-Rom filter is an edge-enhancing filter, often used for architectural visualizations. Note that edge enhancing can produce "moire" effects on detailed geometry.
Filtering is off.
Lanczos filter, size 2.5
Box Filter, size 2.5
Area filter, size 2.5
Cook Variable, size 2.5
Gaussian, size 2.5
Example: Anti-aliasing Filters and Moire Effects
This example demonstrates the effect anti-aliasing filters have on moire effects in your images. Sharpening filters may enhance moire effects, even if your image sampling rate is very high. Blurring filters 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 anti-aliasing filters, but is not completely avoidable.
The scene is very simple: a sphere with a very fine checker map applied. 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:
Area Filter, size 1.5
Area Filter, size 4.0
Triangle Filter, size 1.5
Gaussian Filter, size 1.5
Gaussian filter, size 6.0
Drag the slider to change the filter type.
The Progressive sampler renders the entire image progressively in passes, unlike the bucket method.
These options are for the V-Ray render engine. If you are using the V-Ray GPU render engine, please visit the GPU Setup page for its options.
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 servers. This effect can be controlled to some extent using the Ray bundle size parameter.
Avoid using the Progressive sampler with sharpening image filters (Catmull-Rom, Mitchell-Netravali) as this may slow down the rendering - additional image samples will be required to resolve sharpening filters properly. V-Ray will print a warning in this case in the V-Ray messages window.
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. Note: Not available when using a GPU-based Production engine.
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.
Noise 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. render time (min) – The maximum render time in minutes. When this number of minutes is reached, the renderer stops. This is the render time for the whole frame; it includes any GI prepasses like light cache, irradiance map, etc. If this is 0.0, the render is not limited in time.
Ray bundle size – 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.
Example: Stages of Rendering with the Progressive Sampler
Image after 1 pass
Image after 16 passes
Image after 64 passes
Image after 256 passes
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 (like VRayFur for example) and/or blurry effects (DOF, motion blur, glossy reflections etc).
The diagram above shows visually the way V-Ray is placing samples when using the Bucket 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 the samples is compared and more are added where needed in the following passes.
These options are for the V-Ray render engine. If you are using V-Ray GPU render engine, please visit the GPU Setup page for its options.
Lock subdivs – Sets a fixed number of samples taken for each pixel.
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 samples 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 samples 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. V-Ray GPU's default Max subdivs value is 48.
Threshold – The threshold that will be used to determine if a pixel needs more samples.