||Select V-Ray Volume Grid|| > Modify panel > Rendering rollout > Volumetric Options... button > Volumetric Render Settings window
Based on on | targ – The channel that determines the smoke density. The default option is simple smoke, which produces the smoke channel without curve transformation.
Simple smoke – Uses the smoke channel with a predefined analytical formula.
Temperature – Uses the temperature channel.
Smoke – Uses the smoke channel.
Speed – Uses the magnitude of the velocity channel.
Texture – Uses an external texture map.
Fuel – Uses the fuel channel.
Texture Texture | ttxt – If the Based on parameter is set to Texture, or if Modulate is enabled, this slot specifies the texture to be used.
Modulate | mod_t – When Based on is not set to Texture and this option is enabled, the Texture slot is enabled and the color is multiplied by the texture in the Texture slot.
Simple Smoke Opacity | smoketransp – When the Based on parameter is set to Simple smoke, this value is used as an opacity multiplier.
Scale Opacity by Scene Units | alphaunitscale – 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. This option is Off for imported Field3D and OpenVDB cache files. Note: When disabled, Phoenix caches simulated at higher grid resolution will appear more opaque than the same simulation at a lower resolution.
Optimize Big Volumetric Grids | optvolpass – Helps render scenes with caches above 25 Million cells faster. If using Progressive rendering or if there are a lot of simulators in the scene, and you want your render to start without a pre-pass, you might want to disable this option.
Opacity Diagram | transp_t, transp_s, transp_v, transp_f – This diagram specifies the transparency/opacity at given point as a function of the channel selected in the Based on parameter. The selected channel's data range is denoted by a blue-green line.