This page provides information on the Render tab parameters of the V-Ray PhxShaderSim node.
Overview
The options in the Rendering tab specify how the V-Ray PhxShaderSim will be rendered, including options for displacement.
Parameters
Mode – Specifies the method for visualizing the grid content.
Volumetric – Visualizes the content similar to a V-Ray Environment Fog. This method is used mostly for fire and smoke.
Volumetric Geometry – Used to export deep images and render elements such as normals, velocity, multi matte, etc. It produces the same result as the Volumetric option by using procedural geometry made up of multiple transparent layers. Note that the Approximate and Approximate+Shadows options for the Scattering parameter in the Smoke tab are not supported in this mode.
Volumetric Heat Haze – Produces the same result as the Volumetric option, but adds a heat haze effect when used with the Heat Size parameter. Note that you might need to increase the Max Depth of a V-Ray Material with refraction in case it intersects with the Heat Haze shader.
Approximate and Approximate+Shadows options for the Scattering parameter in the Smoke tab are not supported in Volumetric Geometry mode.
For a complete list of the supported Render Elements in both Volumetric and Volumetric Geometry modes, please check the List of Supported Render Elements page.
Heat Size – When Mode is set to Volumetric Heat Haze, this value acts as a multiplier for the amount of haze. A value of 1 corresponds approximately to the normal heat haze in air caused by the temperature. If smoke or other channel is selected as source, a larger multiplier may be required to achieve a visible result.
Sampling
Step (%) – Specifies the ray marching step of the camera rays as a percentage of the cell size. As the renderer traces rays through the Volume Grid, this value controls how often to get information from the grid. If this value is more than 100, some cells start getting skipped and artifacts can result. If rendering volumetrics with a specific transparency curve, a lower percentage might be necessary so that fine details are not lost. On the other hand, increasing this value increases the rendering speed. This parameter is used when Mode is set to Volumetric, Volumetric Geometry, and Volumetric Heat Haze modes. See the Step % example below.
Shadow Step (%) – Specifies the ray marching step of the rays used to evaluate the lighting (shadow rays) as a percentage of the cell size. Usually, this value can be higher than Step %, as generally, shadows do not need so much detail. Increasing the Shadow Step % also speeds up rendering performance, particularly with dome and area lights.
Sampler Type – Determines the blending method between adjacent grid cells.
Box – Displays cells as cubes. There is no blending between neighbor cells. This is the fastest mode.
Linear – Linear blending occurs between neighbor cells to smooth out the fluid's look. Sometimes this mode may unveil the grid-like structure of the fluid. Up to 20-30% faster than the Spherical option.
Spherical – Uses special weight-based sampling for the smoothest looking fluid. With increasing resolution, the visual advantage of this method over the Linear method becomes less noticeable.
Optimize Big Volumetric Grids | trOptimizeVolumetrics – Helps render scenes with caches above 25 Million cells faster. This option requires additional grid memory during rendering, so you could disable it if you don't have enough RAM. If using Progressive rendering or there are a lot of Volume Grids in the scene, and you want your render to start without a pre-pass, you might want to disable this option.
This option is ignored when rendering with V-Ray GPU.
Motion Blur
Multiplier – Specifies a multiplier that affects the strength of the motion blur. May also be a negative value.
Type – Specifies the motion blur method from Forwards Trace and Backwards Trace.
Apply Velocity Smoothing – Enables frame smoothing for the specified input channel.
Smoothing Threshold – If this value is 0, the entire grid is smoothed evenly. The higher the threshold is raised, the less voxels are affected and only the sharpest gradients are smoothed.
Smoothing Similarity – Increasing this value allows you to smooth only the finer small-scale noise without changing the areas of the fluid which are already smooth. This option won't take effect if the Smoothing Threshold parameter is raised to the maximum.
To render your simulation with Motion Blur, make sure that your cache files contain a Velocity field and that it is properly remapped to the internal V-Ray Volume Grid Velocity channel in the Input → 3rd Party Channels Mapping dialog.
Global Illumination
Generate GI – Controls whether the object generates indirect illumination. Note that to obtain the caustics effect you must set the appropriate value for the Caustics Multiplier below as well as put some objects that generate caustics in the scene
Receive GI – Controls whether the object receives indirect illumination. A multiplier can be specified for the received indirect illumination.
Generate Caustics – When enabled, the selected objects refract the light coming from light sources that are caustics generators, so that caustics are produced. Note that in order to generate caustics, an object must have a reflective or refractive material.
Receive Caustics – When enabled, the selected objects become caustic receivers. When light is refracted by objects that generate caustics the resulting caustics are only visible when they are projected on caustics receivers.
Generate GI Multiplier – Specifies a multiplier for the amount of GI generated by the material.
Receive GI Multiplier – Specifies a multiplier for the amount of GI received by the material.
Caustics Multiplier – A multiplier for the caustics.
Example: Step Parameter
This example shows how the Step (%) parameter can be used to improve the quality of the ray-marching.
Step (%): 50%
Step (%): 150%
Notes
- V-Ray calculates volumetric motion blur in one of two ways, both of which differ from Mantra: 1) By tracing the velocity for a specific voxel backwards and stretching the content for the newly found voxel at position (current voxel + velocity*-1). This method is the closest to what Mantra seems to be doing, which is directly stretching the content for the currently shaded voxel. 2) The default option expects grid velocity information to be present around the content (e.g. density) of the blurred voxel. This works really well for simulations where the entire velocity grid's bounding box contains velocity information but may give you trouble if you've generated a VDB with a random velocity field right around the edges of the density. In such cases, the density is stretched as far as the velocity vectors go, and abruptly ends there.