This page provides general information about the Rendering tab of the V-Ray Volumetric Grid.
The V-Ray Volume Grid node works not only as an importer for the cache files but also has a built-in volumetric shader for rendering Fire / Smoke simulations.
The V-Ray Volume Grid is able to operate in multiple rendering modes that can be divided into two main branches: volumetric and surfaces. The volumetric modes are used for fire and smoke. The surface modes can be used for some interesting fire-related effects, like freezing flame or cartoon-style smoke. Unlike the volumetric modes, surface modes have no dedicated shader; they use the material applied to the Volume Grid.
Enable Rendering | render – Enables/disables the rendering of the Volume Grid.
Render Mode | rendMode – Specifies the method for visualizing the grid content. For fire/smoke, you will want to select a Volumetric method. For liquids, select a geometry method.
Volumetric – Visualizes the content similar to a VRayEnvironmentFog. 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 Color 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 Heathaze parameter.
Isosurface – Used mostly for liquid simulations. It produces a procedural Isosurface geometry using the Surface section options.
Mesh – The content is converted into a standard Maya mesh using the . This mode is mostly used for liquids but can be applied to thick smoke using a scattering material or to plumes of smoke to create effects such as large underwater bubbles.
Ocean Mesh – The grid content is extended to a flat area, fitting the camera's view. In most cases, this mode is used with displacement. Note that the ocean surface can be generated only when the liquid touches the sides and the bottom of the grid, which act as a container for the liquid. The detail of the mesh extension around the Volume Grid depends on the camera resolution – for each pixel of the viewport or the rendered image, one or several polygons are generated, depending on the Ocean Subdivisions option. This mode works best when using a Phoenix FD Ocean Texture as displacement.
Cap Mesh – Only the upper liquid surface is rendered. This mode can be used for swimming pools and other placid liquid surfaces.
Probabilistic Rendering | rendProbabilistic – A simplified mode of rendering in which a random point of the fluid is shaded instead of using the ray-marching technique. This greatly speeds up the rendering but produces more noisy results. With this option, you can control whether and when to use the probabilistic rendering mode.
Disable – Probabilistic rendering is disabled.
Always – Probabilistic rendering is used with both the V-Ray GPU and V-Ray production renderer.
V-Ray GPU Only – Probabilistic rendering is used only when rendering with V-Ray GPU.
Step (%) | rendStep – 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, Volumetric Heat Haze and Isosurface modes.
Shadow Step (%) | shadowStep – 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.
Fade Out | rendSoftBound – Makes the content near the grid's boundaries more transparent to prevent sharp edges from being rendered. This parameter controls how far from the boundaries the transparent effect should start, in voxels.
Motion Blur Multiplier | rendVelMult – Specifies a multiplier that affects the strength of the motion blur. May also be a negative value.
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.
Sampler Type | rendSamplerType – Used to interpolate the grid content at non-integer values.
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.
UVW Bias | rendUVWBias – Specifies a manual bias when sampling textures. The normal direction is used to offset the sampling position.
Example: Step %
This example shows how the Step % value can be used to improve the quality of the ray-marching.
Step %: 50
Step %: 150
This section controls the conversion of the grid content into geometry. If Mode is set to Mesh, Ocean Mesh, Cap Mesh or Isosurface, the Surface channel needs to be set and an appropriate Isosurface level must be chosen from this section.
The controls in this section also denote the surface used in Gradient and Surface-driven displacement. This way it can affect all render modes, not just the surface modes.
The technique for generating the surface is based on the Isosurface concept, which is the 3D version of the isoline, well known from topological maps and weather forecasts. The resulting surfaces are mostly used to render liquids but can be used for smoke and fire as well to create effects like underwater bubbles, freezing fire, etc.
Surface Channel | sarg – Specifies the channel that defines the surface of the fluid. It is used for solid rendering and displacement.
Isosurface Level | rendSurfLevel_x, rendSurfLevel_t, rendSurfLevel_s, rendSurfLevel_v, rendSurfLevel_f – Allows you to specify a threshold value for the generation of the geometry surface. Grid cells below this value are ignored. By default, the Isosurface Level is set to 0.5 and should only be modified if there is flickering in the generated geometry. Isosurface Level is used only in Isosurface, Mesh, Ocean Mesh and Cap Mesh Modes.
The proper value for the Isosurface level parameter depends on the numerical range of the surface channel. For example, Phoenix FD liquids are kept in the range of 0 to 1. A value of 0 means there is no liquid in a certain voxel, and a value of 1 means the cell is 100% full of liquid. Values in between indicate a certain mixture of air and liquid. For such cache files, an Isosurface level value of 0.5 is best for visualizing the surface between the air and liquid.
Imported caches from Houdini, on the other hand, use positive and negative values to indicate whether a voxel is inside or outside the liquid volume, so a correct "halfway" Isosurface level value would be 0.0.
For Phoenix FD Smoke, the proper value is about 0.01, and for Phoenix FD temperature, which is in Kelvins, the value is several hundred.
Invert Volume | rendInvertNorm – By default the values above the surface level are considered internal. When enabled, this option swaps the inside and outside.
Use Cutter | rendUseGizmo – When enabled, turns on the render cutter.
Set Selected Object as Render Cutter – When a polygon mesh and the Volume Grid are selected, the selected mesh is used as the render cutter for the geometry generated by the Volume Grid.
Invert Cutter | rendInvGizmo – When enabled, rendering occurs only outside of the render cutter. This is not the same as a cutter with inverted geometry because any rays that do not intersect the cutter are shaded as well.
Cutter Geometry | rendGizmo – Specifies a polygon geometry that clips the rendering only to inside its volume.
If using a Render Cutter for a liquid pouring into a glass or otherwise contained into another refractive object, you may need to set the Render Mode to Isosurface. By default, the mode is set to Mesh which may produce artifacts in the rendered image.
GPU Render Options
DI mode | GPUDIMode – Controls the direct illumination rendering method.
GI mode | GPUGIMode – Controls the global illumination rendering method.
Cache samples count | GPUSamplesCount – Specifies the number of samples to take when DI mode is set to Fast cache or Full cache.