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Based on | trSource – Allows you to select how the smoke opacity is computed. By default, this is set to the Simple Smoke option. It could also be based on a texture map, or read from the cache files - in that case, the corresponding grid channel must be enabled from the Output rollout before the simulation is run.

• Simple Smoke – Use directly the Smoke channel as opacity, without remapping it through the Opacity Curve. The opacity is controlled only by the Simple Smoke Opacity. If you want to get similar results to Simple Smoke mode using the Opacity Curve in Smoke mode, create a straight line in the Opacity Curve from coordinates [0; 0] to [1; Simple Smoke Opacity], and then bend the middle of the curve upwards. As Simple Smoke Opacity increases, the curve needs to be bent more strongly.

• 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.

• Texture – Use a texture map plugged in the Texture slot as opacity, without remapping it through the Opacity Curve.

• Fuel – Use the fuel channel and remap it through the Opacity Curve to get the rendered opacity.

Texture | trTexture – If the Based on parameter is set to Texture, or if Modulate is enabled, this slot specifies the texture to be used. For more information on texture mapping in Phoenix, please check the Texture mapping, moving textures with fire/smoke/liquid, and TexUVW page.

Modulate | trModulate – When enabled and Based on is not set to Texture, the Based on channel is multiplied by the texture in the Texture slot.

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

Absorption  – 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).

• Constant Color | trAbsorptionColor – Specifies the color that will be used for the absorption effect.

• Texture | trAbsorptionTexture – When a texture is connected in the slot the absorption constant color will be ignored and the texture color will be used instead. For more information on texture mapping in Phoenix, please check the Texture mapping, moving textures with fire/smoke/liquid, and TexUVW page.

Simple Smoke Opacity | trSimpleSmFactor – When the Based on parameter is set to Simple smoke, this value is used as opacity multiplier.

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 is, the denser the smoke will appear 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.

Optimize Big Volumetric Grids | trOptimizeVolumetrics – Helps render scenes with caches above 15 Million cells faster. This option requires additional grid memory during rendering, so you could disable it if you don't have enough RAM. If using Progressive rendering or there are a lot of Phoenix Simulators in the scene, and you want your render to start without a pre-pass, you might want to disable this option. 

Opacity Curve – When the Based on option is set to a grid channel, this curve remaps grid simulation data to smoke opacity. The sim data channel is laid out horizontally along the X axis in the opacity curve. The opacity is mapped vertically on the Y axis.

The channel data range is highlighted in the curve with a light-blue range. You can find out more about Phoenix Grid Channel Ranges here.