Smoke tab

This page provides information on the Smoke tab parameters of the V-Ray PhxShaderSim node.

Overview

This tab controls both the Diffuse color and Opacity of the Volumetric Shader. The Diffuse color can be either a mixture of colors dragged by the simulation (e.g. Cd) or a function of any grid channel. It is also affected by lights and shadows cast by objects in the scene.

The diffuse color needs external light to become visible if GI is enabled in the V-Ray Render Settings in Houdini (V-Ray Renderer node > Renderer tab > Global Illumination tab).

Light Scattering

Type – Controls how the light rays are scattered inside the volume.

Ray-traced (GI Only) – Enables physically accurate scattering of light rays. This mode produces the most realistic results but it's the slowest to render. It requires V-Ray with enabled Global Illumination, otherwise, the rendered result would be the same as if the option is Disabled. The Master Multiplier does not affect the rendering in this mode.
Disabled – Disables scattering. The Master Multiplier value can be used to correct the brightness because without light scattering the volume would generally render darker.
Approximate – Uses an approximate formula which is faster than Ray-Traced scattering and produces good-looking results. Brighter areas of the volume would transfer light farther than dark areas. This option is not supported by the Volumetric Geometry render mode in the Rendering tab.
Approximate +Shadows – Same as Approximate, but also affects the strength of shadows over the scene geometry. Approximate+Shadows is not supported by the Volumetric Geometry render mode in the Rendering tab.

Own Light Scatter Mult – [Valid only when Scattering is set to Approximate or Approximate+Shadows] This multiplier determines how far the light travels through the volumetric, based on the distance from the emissive light itself.

External Scatter Mult – [Valid only when Scattering is set to Approximate or Approximate+Shadows] This multiplier determines how far the light from all external light sources travels through the volumetric. Both Scatter Boost parameters don't alter the visual density of the smoke to the camera and only apply to the illumination.

Scatter Depth – Specifies the maximum depth of the GI rays.

Phase Function – Controls the direction in which the light scatters inside the volume. Accepts values in the interval [-1 , 1]. Negative values correspond to backward scattering, which mimics a volume made up of solid particles and produces denser and more detailed looks. Negative values are more suitable for smoke or dust effects. Positive values correspond to forward scattering, which mimics a volume made up of water droplets where light scatters more. Positive values are suitable for highly scattering volumes such as clouds. The default value of 0 scatters the light in all directions and create an even and (isotropic) diffuse look. Phase Function is ignored by the Approximate and Approximate+Shadows scattering modes. See the Phase Function examples for more information.

Phase Function works best when Volume Light Cache is disabled from Smoke tab > Volume Light Cache rollout.

Optimize Anisotropic Scattering – Controls the application of optimized anisotropic scattering. When enabled, the anisotropic scattering optimization can significantly reduce the render times for highly scattering volumes such as clouds. Due to certain assumptions made for the optimizations, the optimized render result might produce different results compared to the exact brute-force anisotropic scattering. The option is ignored when the scattering is set to Approximate or Approximate+Shadows.

Volume Light Cache

Enable – Enables light caching for each voxel of the grid, which speeds up bucket rendering considerably when the voxel size of the grid is significantly larger than the rendering pixel. If the grid resolution is large enough compared to the rendering resolution, disabling this option speeds up rendering. The higher this option is set, the faster the rendering will be, but the lower the quality of the volumetric Light Cache. You can increase this parameter and gain render speed as long as you don't start getting dark cubic grid artifacts on the smoke.

Light Cache Speedup – Multiplier for Light Cache Speedup.

This option refers to the internal volumetric Light Cache, which is not related to the V-Ray Light Cache.
When using V-Ray Progressive Rendering, this option might slow down rendering startup or the overall render speed.
This feature might produce artifacts when the smoke is very dense.
This feature might produce artifacts when objects cast shadows through the smoke.
Rendering multiple copies or instances of the simulator node with this option turned on might lead to slow-downs.
This option will consume additional memory, so beware of high RAM usage when rendering very large cache files or a large number of copied or instanced volumes.
When rendering in Volumetric Geometry mode, for any of these Render Elements: [ Shadow ], [ Raw Shadow ], [ Global Illumination ], [ Raw Global Illumination ] , the Phoenix FD Light Cache will require additional memory to produce the correct result. Please be aware that these Render Elements (and most other V-Ray Render Elements except for [ Atmosphere ] and [ Self-Illumination ] ) do not work in Volumetric mode.
This option is ignored when using V-Ray GPU. It is a CPU-only feature that helps speed up Bucket rendering.

Opacity

The Smoke Opacity settings below affect the rendering of Fire if the Opacity Mode in the Fire tab is set to Use Smoke Opacity.

Absorption Color – Controls the color of the volumetric shadows and the tint for the objects seen through the volume. Brighter colors make the volume more transparent, while darker colors make it more opaque (denser).

Scale – Multiplier for the smoke density.

Source Range – Specifies the range of the float field used for computing the smoke opacity.

Opacity Ramp – The curve remaps grid simulation data to smoke opacity. The float simulation data channel specified under the Bindings tab → Smoke Opacity parameter is laid out horizontally along the X axis in the opacity curve. The opacity is mapped vertically on the Y axis.

Minimize/Maximize Ramp button – Expands or contracts the opacity ramp.

Simple Smoke Opacity – Controls the transparency/opacity of a cell with a Smoke value of 1.

Min. Visible Opacity – If the opacity of a voxel is below this specified value, the voxel is not shaded at all. Increase this value to speed up rendering. Decrease it if parts of the smoke are too transparent and start disappearing in the render.

Scale Opacity by Scene Units – When enabled, the opacity per unit length remains constant when changing the Grid resolution. Therefore, the larger the unit scale is, the denser the smoke appears in renders and vice versa. This option is Off for imported Field3D and OpenVDB cache files. Note that this option also affects Fire in Use Own Opacity mode. It does not take effect if there is no loaded cache file.

You can use the following controls in the curve:

Double click – Creates a new point or changes an existing one.
Left mouse button click – Selects a point.
Left mouse button drag over a point – Moves the point. If several points were selected beforehand, they move the same amount.
Middle mouse button drag – Drags the visible area.
Ctr + Left mouse button – Toggle selected.
Shift + Left mouse button – Select range.

Color

Smoke Brightness – Specifies a multiplier for the smoke brightness.

Cast Shadows Opacity – Specifies the strength of the shadow. Higher values make the smoke denser, lower values make it less dense.

Source Range – Specifies the range of the source field used to determine the smoke color.

Color Ramp – The curve remaps grid simulation data to smoke color. Ignored when a 3-float vector field (e.g. "Cd.*") is specified under the Bindings tab → Smoke Color parameter. Otherwise, the float simulation data channel specified under the Bindings tab → Smoke Color parameter is laid out horizontally along the X axis in the color gradient window and the resulting color is based on the specified color swatches.