This page provides general information about the Rendering tab of the V-Ray Volumetric Grid.

Overview

For convenience, the shading and rendering of Volumetric effects (Render Mode: Volumetric, Volumetric Geometry, and Volumetric Heat Haze) are handled by the built-in Volumetric shader of the V-Ray Volume Grid. All parameters necessary for rendering a Fire / Smoke simulation can be found under the Rendering roll-out and its sub-rollouts. 

The Volume Grid is able to operate in multiple rendering modes that can be divided into two main branches: volumetric and surfaces (Isosurface, Mesh, Ocean Mesh and Cap Mesh). The volumetric modes are used for fire and smoke. Cinema 4D materials can't be used to shade volumetrics, so their shading is described in the Fire , Smoke Color and Smoke Opacity rollouts. 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.

The V-Ray Volume Grid also provides you with the option to generate a mesh surface based on any of the supported channels. Thus, a Smoke simulation can be rendered as a polygon mesh with a custom material applied to it.

For a list of supported Render Elements, please check the Render Elements page.

You may also check the Volumetric Rendering In-Depth guide for tips on speeding up the rendering of volumetric effects with V-Ray.

 

 

Rendering

Enable Rendering – Enables/disables the rendering of the Volume Grid.

Render Mode – 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 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 Color tab are not supported in this mode. 

Approximate and Approximate+Shadows options for the Scattering parameter in the Smoke color window are not supported in Volumetric Geometry mode.

For a complete list of the supported Render Elements in both Volumetric and Volumetric Geometry mode, please check the Render Elements page.

Volumetric Heat Haze –  Produces the same result as the Volumetric option, but adds a heat haze effect when used with the Heathaze 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.
Isosurface – Used mostly for liquid simulations. It produces a procedural Isosurface geometry using the Surface section options. Compared to the Mesh mode, the result is always smooth but will take longer to render.
Mesh – The content is converted into a standard mesh using the Surface section options. 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 a displacement texture such as the Phoenix FD Ocean Texture.
Cap Mesh – Only the upper liquid surface is rendered. This mode can be used for swimming pools and other placid liquid surfaces.

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 simulator 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.

 

Rend. 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, Volumetric Heat Haze and Isosurface modes. See the Step % example below.

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.  

Border Fade – 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 scene units.

Sampler Type | rendSamplerType – 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.

Motion Blur Multiplier – 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.

When rendering regular meshes with motion blur, the entire mesh is moved along its transformation path back and forward in time, and so each individual vertex of the mesh follows this path. However, when rendering a meshed surface, for each rendered frame a new mesh must be built from the voxel grid, and so it usually has a different number of vertices than the previous and the next frame. Because of this, individual vertices can not be traced back or forward in time between frames. Instead, motion blur of fluid meshes uses the velocity of vertices which is recorded by the simulation, and moves each vertex back and forward in time along the vertex velocity. This is why the generated liquid mesh does not support frame sub-sampling for motion blur. This may cause a mis-match between the liquid and transforming/deforming objects in your scene that interact with it. The fluid mesh is generated from data at the exact rendered frame and fluid data for the preceding or following frames is not used, unlike regular deforming meshes. As a consequence, the liquid and the objects in your scene would synchronize best if those objects do not use additional geometry samples for motion blur.

Velocity Texture – Specifies a custom texture used for motion blur when rendering Fire/Smoke simulations. The colors of the texture represent the velocity direction for each cell, similar to how Vector Displacement textures are evaluated. Every simulator cell receives a certain color from the texture, treats it as a direction vector and stretches the contents of the simulator along to produce motion blur. This parameter can be used with a V-Ray User Color texture for motion blur in a single direction(e.g. by setting the color to (0, 1, 0) for motion blur in the +Z axis), or any other Maya texture applied as a 3D projection. The texture overrides the Velocity grid channel even if available in the cache files. Note that the Y and Z components of the velocity vectors are flipped - they are sampled respectively from the Blue and Green color values of the texture.

 

 

 

 
Example: Step %

This example shows how the Step % value can be used to improve the quality of the ray-marching.

 


Step %: 50

 


Step %: 150

 

 

 

 

 

 


Example: Heat Haze

 

This example shows how Heat Haze adds refraction at each ray-marching step through the volume. This only affects the camera's view. Heat haze will not affect shadows cast through a volume.

 


Heat haze: 0

 


Heat haze: 1

 

 

 

Surface

This section controls the conversion of the grid content into geometry. If Render 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. 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 – Specifies the channel that defines the surface of the fluid. It is used for solid rendering and displacement.

Texture - the values of a custom texture will define the liquid surface.
Liquid/Temperature - the Liquid/Temperature channel will define the liquid surface. Temperature is typically in the range 0-1 for Liquid simulations and 600-2000 for Fire / Smoke simulations.
Smoke - the Smoke channel will define the liquid surface. Smoke is typically in the range of 0-1 for Fire / Smoke simulations.
Speed - the Speed channel will define the liquid surface. Speed channel output has to be enabled for this to work. Speed is calculated as the length of the velocity vector for each voxel.  
Fuel - the Fuel channel will define the liquid surface. Fuel channel output has to be enabled for this to work. 

Surface Texture – If the Surface Channel is set to Texture, this slot specifies the texture. In Mesh, Ocean Mesh, Cap Mesh and Isosurface render modes, the selected map will completely replace the cache files that have been loaded, if any.

Isosurface Level –  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.

Invert Volume – By default the values above the surface level are considered internal. When enabled, this option swaps the inside and outside.

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.


Use Render Cutter – When enabled, rendering will occur only inside the selected geometric object's volume. If Fire Lights are enabled, only those inside the cutter will be rendered. Note that this will not work when the Render Mode is set to Volumetric Geometry.

Cutter Geometry – Specifies a polygon geometry that clips the rendering only to inside its volume.

Invert Cutter – 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.

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.

 

 

 

The examples below are shown in 3ds Max.