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

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Overview


This dialog controls the transparency part of the volumetric shader. By default, it affects only the smoke. It can affect the fire as well if the Fire Opacity Mode in the Fire settings is set to Use Smoke Opacity.

 

Parameters




Based on | trSource – Specifies the channel that determines the smoke density.

SimpleSmoke – Uses directly the Smoke channel as opacity, without remapping it through the Opacity Curve. The opacity is controlled by the Simple Smoke Factor.
Temperature – Use the temperature channel and remap it through the Opacity Curve to get the rendered opacity. Can be used to produce smoke without needing the smoke channel in the simulation.
Smoke – Use the smoke channel and remap it through the Opacity Curve to get the rendered opacity.
Speed – Use the magnitude of the velocity channel and remap it through the Opacity Curve to get the rendered opacity.
Fuel – Use the fuel channel and remap it through the Opacity Curve to get the rendered opacity.
Texture – Use an external texture map as opacity, without remapping it through the Opacity Curve.

Texture | trTexture – If the Source parameter is set to Texture, or if Modulate is enabled, this slot specifies the texture to be used.

Reset to Defaults – Resets the settings to their default values.

Modulate | trModulate – When enabled and Source is not set to Texture, this option enables the Source channel to be multiplied by the texture in the Texture slot.

Simple Smoke Opacity trSimpleSmFactor – When the Based on parameter is set to Simple smoke, this value controls the transparency/opacity of a cell with a Smoke value of 1.

Minimum Visible Opacity | trSkipShLevel – If the opacity of a voxel is below this specified value, the voxel will not be 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 | trAlphaUnitScale – When enabled, the opacity per unit length will remain constant when changing the Grid resolution. Therefore, the larger the unit scale it, the denser the smoke will appear in renders and vice versa.

Optimize Big Volumetric Grids | trOptimizeVolumetrics – Helps render scenes with caches above 25 Million cells faster. 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.

Opacity diagram – This diagram specifies the opacity at given point as a function of the channel selected as the Based on parameter. The selected channel's data range will be highlighted in light blue.

 

You can use the following controls in the diagram:

Double click – Creates a new point or changes an existing one.
Left button drag over a point
– Moves the point. If several points were selected beforehand, they will move the same amount.
Left button drag over several points
– Selects several points.
Middle button drag over the background
– Drags the visible area. If the Shift key is pressed, scales the diagram in the corresponding direction.
Mouse wheel
– Zooms in/out.
Right-click
– Displays a drop-down menu where you can add a point, edit or delete a selected point, and fit the entire diagram into the view. If multiple points are selected, they can be edited simultaneously. Multiple selected points can also be scaled and flipped with the Scale Selection option as seen below.