Specifies the pixel filter used to combine sub-pixel samples to generate the value for the single pixel. The filter is normally specified as a filter type (eg. gauss) followed by an x and y filter width in pixels. To blur the image, increase the filter width.

There are several different pixel filters available.

denoise optix [-a ‹AOV›]

Controls how transparent samples are combined to produce the color values for individual pixel samples. The sample filter is used to composite transparent surfaces before the pixel filter produces final pixel colors.

Full Opacity Filtering (fullopacity): When stochastic transparency is enabled, this option causes a channel to be evaluated and composited with every opacity evaluation - as opposed to only being composited with the samples that are selected for full shading. It can be used to produce smoother results for channels that are fast to evaluate such as Ce or direct_emission. When stochastic transparency is disabled, this option behaves the same way as Opacity Filtering.

The storage type for the main image. The type of quantization used will affect image quality and size. If you need to adjust the image’s dynamic range in compositing, you should normally leave this value at the default of 16-bit floating point.

The default is "float16" for the first plane, and "float" for secondary planes. You can override the first plane’s value with the -b command line argument to mantra.

Normally, sub-pixel samples are filtered using the pixel filter defined on an image plane. When this is turned on, each sub-pixel is output without any pixel filtering performed.

The image:resolution property will be scaled by the image:samples property to determine the actual output image resolution. For example, if image:resolution was (1024,512) and image:samples was (4,6), the image rendered would have a resolution of 4096 by 3072. Each pixel would represent a single unfiltered sub-pixel sample.

When you render a target node with this option on using HQueue, the server will split frames to render into separate tiles and render each tile as a separate job. When you render locally with this option on, Mantra will render a single tile instead of the entire frame.

Tiled render can also be enabled using -t command line option to mantra, which can be used to render a tile locally without having to generate an IFD for each tile with Tiled render enabled.

Split the frame into this number of tiles horizontally, when Tile render is on.

Split the frame into this number of tiles vertically, when Tile render is on.

Which tile to render, when rendering locally with Tile render on. Tile numbers start at 0 in the top left and increase left to right, top to bottom.

Renders an image from the viewing camera. Sometimes, it is useful to skip this render, for example, when rendering shadow maps.

Enable or disable shadow map generation. Each light also has its own controls to determine whether shadow maps will be generated.

Enable or disable environment map generation. Each object can be set up to generate an environment map of all the other objects in the scene.

Enable or disable photon map generation.

A whitespace-separated list of shading component names that will be computed for export. If you have defined new component labels in your materials, these can be added to the list so that they are exported for per-component export planes. If you are not using some components, remove them from the list to improve render efficiency.

PBR light exports assume that this list is complete - that is, all components created by shaders are listed. If there are unlisted components, light exports may be missing illumination from these components.

Add an extra image plane for the combined lighting (all components) using pre-defined settings. For finer control, add the image channel using the generic image plane interface.

These controls let you output VEX variables as auxiliary image planes, either as extra planes in the output file or extra files.

The Channel Name parameter lets you give the channel in the output file a different name than the default (the name of the VEX variable). For example, you want to send out the Of variable. If the Channel Name is left blank, the plane name in the .pic file will be Of. If you set Channel Name to Opacity, the plane in the .pic file will be called Opacity.

When generating an image, mantra runs the sample filter to composite samples to a single color. Mantra then runs the pixel filter to produce the final color for a pixel. A deep resolver is used to store information about each sample prior to sample filtering. This allows the image resolver to store information about each individual sample before compositing.

If you want to change the vm_deepresolver through Python, you have to specify one of the the three tokens to represent the available modes: null, shadow, camera.

The file to generate when the Deep Camera Map resolver is used.

The samples stored in the deep file can be stored as either uncomposited, meaning that each sample is independent of any other sample for the same pixel; or it can be stored pre-composited, meaning that each sample stores the accumulated opacity of the sample behind it and the opacity of the sample itself.

When rendering DCM, force regular/non-deep image planes to have the same pixel filtering as DCM’s (i.e. Unit Box filter).

The name of the image creator. By default uses the current user’s log in name.

Houdini, TIFF, PNG formats

A text comment to include in the output file.

Houdini, OpenEXR, PNG formats

The name of the computer where this image was created.

The direction in which MPlay renders the image. Possible values are "middle" (middle out), "top" (top down), or "bottom" (bottom up).

When rendering to MPlay, all Houdini sessions will send the output to the same MPlay flipbook. This can be problematic when running multiple Houdini sessions. The MPlay Label lets you specify a label for the MPlay associated with the output driver. Only renders which match the given label will be sent to that MPlay.

Display gamma for MPlay, from 0.0 to 4.0.

JPEG Quality, integer from 10 to 100.

Type of image compression to use in TIFF files. Possible values are "None", "LZW", "AdobeDeflate", "Deflate", "PackBits", "JPEG", "PixarLog", "SGILog", "SGILog24".

Compression type for EXR format images. Possible values are "none", "rle", "zips", "zip", "piz", "pix", "b44", "b44a", "dwaa", "dwab".

Controls the number of primary rays Mantra will use to sample your scene per pixel. The two numbers represent an arrangement of samples in the X and Y axis and are generally the same number. However, for non-square pixels it may be necessary to use different values in X and Y. Multiplying these two values together will give you the number of primary rays per pixel.

Increasing Pixel Samples will result in a cleaner, higher quality image. However, since all other sampling values are multiplied by the number of Pixel Samples, they should only be increased when necessary. For more details on when to increase Pixel Samples, see the “Removing Noise” section.

Sampling color space for variance antialiasing. Setting this to Gamma 2.2 will cause darker parts of the image to receive more samples.

When enabled, this parameter will cause Mantra to use ray variance antialiasing when determining the number of Secondary Rays to send for every Primary Ray.

This means that rather than using a specific number of rays, Mantra will first send out a small number of rays and use this sample set to evaluate the Variance. Depending on the amount of variance, Mantra will continue to send more rays up to the Max Ray Samples value. Ray Variance Antialiasing is useful for optimizing your render by sending more rays only in the areas they are needed.

In cases where the minimum number of rays to remove noise is equal to the maximum number of rays, you may save a small amount of render time by disabling Ray Variance Antialiasing.

This value is the minimum number of secondary rays to use for each BSDF type when generating an image. When Ray Variance anti-aliasing is disabled, this number represents the number of secondary rays to send regardless of the Noise Level.

Remember, this number is multiplied by the current number of Pixel Samples and the number of BSDF types on the material being evaluated.

When Ray Variance anti-aliasing is enabled, this parameter represents the maximum number of secondary rays allowed for each BSDF type even if the Noise Level is never reached. This parameter, along with Min Ray Samples, essentially allows you to create a range of acceptable sampling for your image. Carefully controlling the total number of potential rays is the best way to optimize your renders.

Remember, this number is multiplied by the current number of Pixel Samples and the number of BSDF types on the material being evaluated. For example, if it’s a purely diffuse material, and Pixel Samples are set to 3×3, and the Max Ray Samples is set to 1, then it will cast up to 9 secondary rays (9 diffuse rays). If the material is both reflective and refractive, then it will cast up to 18 secondary rays (9 reflection and 9 refraction rays).

For more details on when to increase Max Ray Samples, see removing noise.

Represents a threshold in the amount of variance allowed before mantra will send more secondary rays. Variance essentially represents how “spread out” the values in a set of samples are. For instance, a set of samples that were all the same would have a variance of 0. It is generally a good idea to keep this value as high as possible so that rays are sent only into those areas where an unacceptable amount of noise is present.

Adding “direct samples” and “indirect samples” image planes can help you track how many samples are being sent and to which parts of the image. For more information about sampling, see the “Sampling and Noise” section.

If you find that certain objects in your scene require substantially more samples than other parts of your image and you are unable to “target” those objects using the Noise Level parameter, it may be a better idea to add per-object sampling parameters to the problem areas. See removing noise for more details.

Controls the quality of indirect diffuse sampling (for information on the difference between direct and indirect rays, see sampling and noise). Often, indirect sources of light (such as the surfaces of other objects, and light scattered inside of a volume) will be a significant cause of noise in your renders. Turning this up should decrease this type of noise, at the cost of slowing down rendering.

This parameter acts as a multiplier on Min Ray Samples and Max Ray Samples and also as a divisor for Noise Level. For example, if you have Min Ray Samples set to 1, Max Ray Samples set to 8 and your Noise Level to 0.1, then set Diffuse Quality to 2, Mantra will send between 2 and 16 secondary diffuse ray samples based on a Noise Level of 0.05. Remember these numbers apply only to the indirect samples. Mantra uses the original values for all direct sampling.

To find how much noise is in your indirect diffuse component, add the “Indirect Lighting (per-component)” image plane in the Extra Image Planes tab. This lets you check each indirect component individually. For this parameter, you should check the Indirect Diffuse component.

Controls the quality of indirect reflection sampling (for information on the difference between direct and indirect rays, see sampling and noise). Indirect reflections (reflections of other objects in the scene) can sometimes add noise to the render. Turning this up should decrease this type of noise, at the cost of slowing down rendering.

This parameter acts as a multiplier on Min Ray Samples and Max Ray Samples and also as a divisor for Noise Level. For example, if you have Min Ray Samples set to 1, May Ray Samples set to 8 and your Noise Level to 0.1, then set Reflect Quality to 2, Mantra will send between 2 and 16 secondary reflection ray samples based on a Noise Level of 0.05. Remember these numbers apply only to the indirect samples. Mantra uses the original values for all direct sampling.

To find how much noise is in your indirect reflection component, add the “Indirect Lighting (per-component)” image plane in the Extra Image Planes tab. This lets you check each indirect component individually. For this parameter, you should check the Indirect Reflection component.

Controls the quality of indirect refraction sampling (for information on the difference between direct and indirect rays, see sampling and noise). Indirect refractions (refracted light from of other objects in the scene, such as when viewing an object through glass) can sometimes add noise to the render. Turning this up should decrease this type of noise, at the cost of slowing down rendering.

This parameter acts as a multiplier on Min Ray Samples and Max Ray Samples and also as a divisor for Noise Level. For example, if you have Min Ray Samples set to 1, May Ray Samples set to 8 and your Noise Level to 0.1, then set Refract Quality to 2, Mantra will send between 2 and 16 secondary refraction ray samples based on a Noise Level of 0.05. Remember these numbers apply only to the indirect samples. Mantra uses the original values for all direct sampling.

To find how much noise is in your indirect refraction component, add the “Indirect Lighting (per-component)” image plane in the Extra Image Planes tab. This lets you check each indirect component individually. For this parameter, you should check the Indirect Refraction component.

Decouples the Direct from Indirect Rays, allowing for different sampling rates as well as separate noise levels. Generally speaking, it will only be necessary to enable this parameter if you find that too many samples are being sent as Direct Rays.

Adding “direct samples” and “indirect samples” image planes can help you track how many samples are being sent and to which parts of the image. For more information about sampling, see the “Sampling and Noise” section.

This parameter will enable three new parameters: Min Indirect Ray Samples, Max Indirect Ray Samples, and Indirect Noise Level.

To understand the difference between Direct and Indirect rays, see sampling and noise.

How finely or coarsely a volume is sampled as a ray travels through it. Volumetric objects are made up of 3d structures called Voxels, the value of this parameter represents the number of voxels a ray will travel through before performing another sample.

The default value is 0.25, which means that every one of every four voxels will be sampled. A value of 1 would mean that all voxels are sampled and a value of 2 would mean that all voxels are sampled twice. This means that the volume step rate value behaves in a similar way to pixel samples, acting as a multiplier on the total number of samples for volumetric objects.

For volumes that aren’t voxel based, like CVEX procedural volumes, Mantra will divide the bounding box of the volume into roughly 100 “virtual” voxels. In these cases, setting the Volume Step Rate correctly is essential to maintaining the correct level of detail.

Keep in mind that increasing the volume step rate can dramatically increase render times, so it should only be adjusted when necessary. Also, while increasing the default from 0.25 can reduce volumetric noise, increasing the value beyond 1 will rarely see similar results.

For more information about volume sampling, see sampling and noise.

A factor to proportionally decrease the volume step rate only for shadows, relative to the volume step rate. Smaller values will cause mantra to use a larger ray march step size for shadow rays than other shading rays. A value of 1 will produce equal quality for shadow rays and shading rays.

The number of transparent samples to be shaded as a ray travels through translucent objects. Increasing this value will result in less noise in translucent objects and is generally less costly than increasing Pixel samples, Volume Step Rate, or Min and Max ray samples. Stochastic Sampling will not have any effect on noise from Indirect Sources however.

This may make the image noisier than without stochastic transparency, so you may need to compensate by, for example, increasing the pixel samples. You should generally leave this option on.

The renderer ignores this option for micropolygon rendering (except for secondary ray tracing) and for renders that only generate opacity (such as deep shadow maps). In those cases it is more efficient to composite all the transparent shading results.

Added in Houdini 12.

The number of transparent samples to shade when Stochastic Transparency is on. Higher values improve shading quality for volumes and transparent surfaces, but are slower to render.

Sampling generally occurs in random patterns which change on every frame of an animation. This can cause a distracting “buzz” when there is a significant amount of noise in your images which can make evaluation of other aspects of the scene difficult. Enabling this parameter will “lock” the sampling patterns so that the noise remains the same on every frame.

Also, in some cases where the final rendered images will be sent through a post-render de- noise process, it can be useful to have the noise remain constant frame to frame. Consistent sampling patterns can help when analyzing the noise.

It defaults to “off” because it is generally unacceptable to have a locked sampling pattern for final sequences.

Adjusting this parameter will cause the pixel sampling patterns used by Mantra to be regenerated in different configurations. By default, the patterns change on every frame, so manually changing this value is not necessary.

Occasionally, when motion blur is going to be added to an image as a post-process or for other compositing operations, it is necessary to calculate the motion blur but not include it in the final rendered image. In these cases, Allow Image Motion Blur should be disabled.

This means that the blurred positions necessary for Motion Blur can be exported as a custom Motion Vector Image Plane from within a shader using the GetBlurP() function without the small overhead of doing the actual shading in the render.

This parameter is related to the motion blur parameters which are available only when Motion Blur is enabled. Disabling this option will cause motion blur to be removed from the final rendered image, however the blurred Position will still be calculated, allowing for custom motion vector image planes to be created.

The number of times a ray can be reflected in your scene.

This example shows a classic “Hall of Mirrors” scenario with the subject placed between two mirrors.

This effectively creates an infinite series of reflections.

From this camera angle the reflection limits are very obvious and have a large impact on the accuracy of the final image. However, in most cases the reflection limit will be more subtle, allowing you to reduce the number of reflections in your scene and optimize the time it takes to render them.

Remember that the first time a light source is reflected in an object, it is considered a direct reflection. Therefore, even with Reflect Limit set to 0, you will still see specular reflections of light sources.

To control what happens when the maximum number of reflections is exceeded, use At Ray Limit.

This parameter control the number of times a ray be refracted in your scene.

This example shows a simple scene with ten grids all in a row.

By applying a refractive shader, we will be able see through the grids to an image of a sunset in the background.

From this camera angle, in order for the image to be accurate, the refraction limit must match the number of grids that that are in the scene. However, most scenes will not have this number of refractive objects all in a row and so it is possible to reduce the refract limit without affecting the final image while also reducing the time it takes to render them.

Keep in mind that this Refract Limit refers to the number of surfaces that the ray must travel through, not the number of objects.

Remember that the first time a light source is refracted through a surface, it is considered a direct refraction. Therefore, even with Refract Limit set to 0, you will see refractions of Light Sources. However, since most objects in your scene will have at least two surfaces between it and the light source, direct refractions are often not evident in your final render.

To control what happens when the maximum number of refraction is exceeded, use At Ray Limit.

The number of times diffuse rays can propagate through your scene.

Unlike the Reflect and Refract Limits, this parameter will increase the overall amount of light in your scene and contribute to the majority of global illumination. With this parameter set above zero diffuse surfaces will accumulate light from other objects in addition to direct light sources.

In this example, increasing the Diffuse Limit has a dramatic effect on the appearance of the final image. To replicate realistic lighting conditions, it is often necessary to increase the Diffuse Limit. However, since the amount of light contribution usually decreases with each diffuse bounce, increasing the Diffuse Limit beyond 4 does little to improve the visual fidelity of a scene. Additionally, increasing the Diffuse Limit can dramatically increase noise levels and render times.

The number of times a volumetric ray can propagate through a scene. It functions in a similar fashion to the Diffuse Limit parameter.

Increasing the Volume Limit parameter will result in much more realistic volumetric effects. This is especially noticeable in situations where only part of a volume is receiving direct lighting. Also, in order for a volumetric object to receive indirect light from other objects, the Volume Limit parameter must be set above 0.

With the Volume Limit set to values above zero, the fog volume takes on the characteristic light scattering you would expect from light traveling through a volume. However, as with the Diffuse Limit, the light contribution generally decreases with each bounced ray and therefore using values above 4 does not necessarily result in a noticeably more realistic image.

Also, increasing the value of this parameter can dramatically increase the amount of time spent rendering volumetric images.

Controls how Mantra deals with rays that reach the ray tracing limit (For example the Reflect Limit or Refract Limit).

In this example, the refract Limit has been set to 2.

Setting At Ray Limit to Use Black Background will simply render black once the limits are reached. This is the default setting and will work in most scenes since the Reflect or Refract Limit is unlikely to be reached. However, in scenes where the limit is noticeable in the rendered image, the black color can be quite noticeable and stand out against the colors in the scene.

In this case, increase the limit until the effect is avoided or use the Use Direct Lighting as Background Color option. This will replace the black color with whichever color or image is used in your direct lighting, for instance an Environment Light.

For More Information about how the settings on an Environment Light affect this parameter see lighting.

Global raytracing bias to be used for PBR rendering, specified in world space units. Increase the raytracing bias only if doing so eliminates rendering artifacts in your render. For really small scenes (1 unit in size), it is sometimes necessary to decrease the raytracing bias.

When biases are used in VEX shaders, the bias can either be performed along the ray direction or along the surface normal. If this parameter is turned on, biasing will be along the surface normal - using the Ng VEX variable.

If the ray direction and normal point in different directions, the normal will first be flipped and then biasing will be performed in the direction of the flipped normal. This setting is particularly useful when ray traced surfaces that are seen edge-on.

To save on computation, mantra will only compute exports on surface shaders if the variable needs to be saved to an image plane. This means that some surface exports may not be available in fog shaders for the simport() function. However, to properly render advanced features like absorption and nested dielectrics, additional exports from surface shaders are required. This enables on these special exports.

The type of path tracing to perform in PBR mode.

Roughness parameter in GGX BSDFs are clamped by the maximum roughness value propagated down the ray chain in pathtracing. Enabling this option can cut out a lot of noise in indirect specular (in particular, cases where glossy surface is reflected by a rough specular surface) at the cost of a bit of accuracy.

A space-separated list of component types that will behave like refract bounces. This will affect which reflection scope is used based on the ray type and also which bounce limit to use. Uncategorized component types are assumed to be reflections.

A space-separated list of component types that will behave like diffuse bounces. This will affect which reflection scope is used based on the ray type and also which bounce limit to use. Uncategorized component types are assumed to be reflections.

A space-separated list of component types that will behave like volume bounces. This will affect which reflection scope is used based on the ray type and also which bounce limit to use. Uncategorized component types are assumed to be reflections.

A space-separated list of component types that will behave like subsurface scatter bounces. This will affect which reflection scope is used based on the ray type and also which bounce limit to use. Uncategorized component types are assumed to be reflections.

The size (in pixels) of the tiles rendered by mantra. Larger tile sizes may consume more memory.

When enabled, automatically set the thread count (renderer:threadcount IFD property) to the number of CPUs of the rendering machine.

When Use Max Processors (renderer:usemaxthreads IFD property) is disabled, sets the number of threads Mantra uses for rendering.

Whether to use a fixed size cache (vm_cachesize) or whether to use a proportion of physical memory (vm_cacheratio)

The proportion of physical memory Mantra will use for its unified cache.

For example, with the default vm_cacheratio of 0.25 and 16 Gb of physical memory, Mantra will use 4 Gb for its unified cache.

The unified cache stores dynamic, unloadable data used by the render including the following:

• 2D .rat texture tiles

• 3D .i3d texture tiles

• 3D .pc point cloud pages (when not preloaded into memory)

• Tessellated meshes required by ray tracing:

  • Displacements

  • Subdivision surfaces

  • Bezier and NURBS primitives

Controls the type of ray tracing accelerator used by mantra. A ray tracing accelerator is a spatial data structure used to optimize the performance of ray intersection tests against complex geometry.

Change the memory/performance tradeoff used when constructing KD-Tree acceleration data structures. Values larger than 1 will cause mantra to use proportionally more memory and take longer to optimize the tree in an attempt to make ray tracing faster. Smaller values will cause mantra to use proportionally less memory and take less time to optimize the tree, while possibly compromising ray tracing performance. The default value of 1 will try to balance the amount of memory used by ray tracing data structures with the amount of memory used by geometry.

If you are noticing long tree construction times, try decreasing the KD memory factor to 0.1. If your render is too slow after tree construction, increase the value until you find a good balance of tree construction time vs. render performance.

When texture baking to UDIM images, this is the name of the texture coordinate attribute used for unwrapping.

Perform hidden surface removal. When hidden surface removal is disabled, all surfaces in the camera’s frustum will be rendered, regardless of whether they are occluded. This can impact render time significantly.

Enabling this checkbox will expand any variables in OTL paths, breaking the dependency on Houdini environment variables, but possibly making the IFD less portable.

Mantra is able to load the shader directly from the OTL when Houdini uses a shader defined in an OTL. When shaders are built using VOPs, the shader must be embedded in the IFD. Enabling this option will force Houdini to embed the shaders defined by OTLs.

This option makes the IFD more self-contained so that machines which don’t have the OTL installed (or a different version of the OTL) are able to evaluate the shaders correctly.

However, if you have complicated shaders, embedding them will bloat the size of the IFD significantly.

Enabling this checkbox will expand any variables in OTL paths, breaking the dependency on Houdini environment variables, but possibly making the IFD less portable.

Controls which style sheets defined in the hip file are embedded into the IFD. Standard Houdini pattern matching is used on each embedded style sheet name.

Specifies which style sheets mantra should apply during rendering. This is a space separated list of either names of style sheets embedded in the hip file, or external JSON files on disk. As with individual styles within a single style sheet, style sheets later in the list take precedence over style sheets earlier in the list.

Controls which SHOPs are embedded in the generated IFD. This parameter can be used to force all SHOPs or all Material SHOPs to be embedded even if Houdini does not find explicit references to those SHOPs on the output objects and geometry.

A global multiplier on all per-object shading quality (vm_shadingquality) parameters in the scene. This parameter can be used to globally increase or decrease shading quality. The shading quality used for an object is determined by…

shadingquality = object:shadingquality * renderer:shadingfactor

Fast moving objects which have significant amounts of motion blur are rendered with the same sampling quality as slow moving or static objects. However, in cases where objects are very blurry, small details are usually lost. In these cases, it is a useful optimization to reduce the shading quality on those objects which are moving quickly since the loss in detail is hidden in the motion blur.

Increasing the Motion Factor will dynamically reduce the shading quality of an object based on the rate of motion. This optimization is primarily useful for objects which are refined at render time like subdivision surfaces or objects with displacement-based shading.

In the above example, you can see that the motion factor does not have a large impact on the quality of the final render.

However, sometimes too much detail can be lost, especially in cases where much of the surface detail is generated by the shader. Objects whose shape is derived through significant amounts of displacement, for example.

In these cases, the Motion Factor value must be adjusted carefully to retain a believable amount of surface detail.

Automatically adjusts the shading quality for objects which are significantly blurred. Increasing the motion factor of an object will dynamically decrease the shading quality based on the rate of motion. This can significantly speed up renderings of rapid moving objects. It also affects depth of field and may improve speed of scenes with deep depth of focus.

Motion factor reduces shading quality using the following formula:

new_shading_quality = shading_quality / max(motion_factor * pixels_of_motion/16, 1)

This is only true if motion_factor > 0.

Objects traveling more than 16 pixels within the frame will have their shading quality reduced by the above factor. For example, an object blurred over 32 pixels with a shading quality of 1 will have the quality reduced to 0.5. You should not use very large values for this parameter. Values between 0 and 1 are reasonable.

When using the Ray Tracing or Physically Based Rendering rendering engine, motion factor will only affect the geometric subdivision for subdivision surfaces, NURBS/Beziers, or displacements and will not change the amount of shading.

When primitives are rendered in mantra, they are split into smaller primitives if they are too big to be rendered. The primitives are measured to determine if they are too big using the measurer.

There are several different measurers available, each which take some optional arguments.

This parameter controls the z-importance for nonraster measuring. See vm_measure above.

This parameter controls the shading quality scale factor for geometry that is not directly visible to the camera. For geometry that is outside the field of view (ie. visible only to secondary rays), mantra will smoothly reduce the shading quality based on the angle between the geometry and the edge of the viewing frustum. Smaller values can increase performance particularly in scenes where the camera is within the displacement bound of nearby geometry, where it permits the hidden primitives to be diced more coarsely than those that are directly visible.