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This page provides general information about the Smoke Opacity sub-section of the Rendering rollout of Chaos Phoenix FD.

Overview

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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 rollout is set to Use Smoke Opacity.Opacity component of the Volumetric Shader.

In the volume shader, there are two types of content — diffuse and emissive. For simplicity's sake, we call the diffuse content Smoke and the emissive content Fire.

Diffuse (Smoke) needs external light in order to become visible, and also casts shadows. On the other hand, emissive (Fire) is visible even without being lit by lights. It also ignores their illumination, and does not cast shadows on itself.

Fire and smoke also have their own Color and Alpha. Alpha is a synonym for opacity, and transparency is the opposite of opacity. The Smoke Opacity, Smoke Color, as well as the Fire's Color and Opacity, can be mapped to a physical Grid Channel from the simulation, using the Based on parameter.

The Based on parameter specifies the source channel that will be rendered, and is set independently for the Fire, Smoke Color, and Smoke Opacity respectively. This makes shading very flexible, so that you can even render a simulation's fire as smoke, and smoke as fire, depending on the channel you set the Based on parameter to for each.

Parameters

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UI Text Box
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When there is both Smoke and Fire (diffuse and emissive) in the same voxel, you can use the Fire Opacity Mode option in the Fire sub-rollout, to determine the resulting color and alpha. If the Fire Opacity Mode is set to Use Smoke Opacity, then the Smoke Opacity settings below will affect the rendering of Fire. Otherwise, you can separate their opacities for additional control, using either the Fully Visible or Use Own Opacity modes in the Fire sub-rollout.

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Using the Based on parameter below, you can achieve a wide variety of results when shading Fire & Smoke. For example, the Fire (emissive content) is by default based on the Temperature Grid Channel. If instead it were based on Smoke Grid Channel data, then voxels containing Smoke values would be shaded as emissive (Fire). In other words, the Smoke would be rendered as Fire. On the other hand, if the Smoke Color (diffuse) & Smoke Opacity were based on Temperature, then voxels with Temperature values would be shaded as diffuse volumetrics (Smoke Color & Opacity). In other words, the Fire would be rendered as Smoke.

Section
Anchor BasedOn BasedOn Column width 40% Based on | trSource – Allows you to select how the smoke opacity is computed. By default, this is set to

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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

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the Smoke channel as opacity, without remapping it through the Opacity Curve. The opacity is controlled

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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

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Curve from coordinates [0; 0]

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to [1; Simple

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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

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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.

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Smoke – Use the smoke

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channel and remap it through the Opacity Curve to get the rendered opacity.

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Speed – Use the magnitude of the velocity

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channel and remap it through the Opacity Curve to get the rendered opacity.

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Texture – Use a texture map plugged in

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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

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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 |

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trModulate – When enabled

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and Based on is not set to Texture,

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the Based on channel

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is multiplied by the texture in the Texture slot. Reset to Defaults – Resets the settings to their default values.

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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. UI Text Box type info For more information, see the Absorption Constant Color examples below. Simple Smoke Opacity |

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trSimpleSmFactor – When the Based on parameter is set

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to Simple smoke, this value is used as opacity multiplier. Minimum Visible Opacity |

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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 |

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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. UI Text Box
	medium	type note When Scale Opacity by Scene Units is disabled, Phoenix caches simulated at higher grid resolution will appear more opaque than the same simulation at a lower resolution. Optimize Big Volumetric Grids |

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trOptimizeVolumetrics – Helps render scenes with caches above

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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

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Phoenix Simulators in the scene, and you want your render to start without a pre-pass, you might want to disable this option.  UI Text Box
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type warning This option is ignored when rendering with V-Ray GPU using OpenCL / CUDA. . 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.   Column width 60%  Image Added

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You can use the following controls in the diagramOpacity Curve: 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 curve in the corresponding direction. Mouse wheel – Zooms in/out. Mouse wheel near the borders of the control – Zooms in/out only along X or only along Y. Right-click – Displays a drop-down menu where you can add a point, edit or delete a selected point, and fit the entire diagram curve 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.

 

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AbsorptionExamples AbsorptionExamples

Example: Absorption Constant Color

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