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The Fire rollout controls the emissive color (fire) of the volumetric shader, and the light emitted by the VRayVolumeGrid. The color and intensity of the emission can be controlled and gradually transitioned between a physically correct appearance and an artistic appearance. Unlike the smoke color which needs an external light to become visible, fire will be visible immediately.

Unlike the smoke (Diffuse) color, which tells what fraction of the light is reflected by the volume and can't go above 1, the emissive color is an absolute value which can have values larger than 1. There are two parts to it - the color itself and its brightness (luminance). The final color is obtained by multiplying the two. Unlike the smoke color which needs an external light to become visible, fire will be visible immediately if you have set the color. 

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UI Path

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||Select V-Ray Volume Grid|| > Modify panel > Rendering rollout > Volumetric Options... button > Volumetric Render Settings window

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Fire

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Based on emSource – The source channel that will be rendered as fire. By default the Temperature option is used.

Disabled – Disables the emissive color.
Temperature – Uses the temperature channel.
Smoke – Uses the smoke channel.
Speed – Uses the magnitude of the velocity channel.
Texture – Uses an external texture map.
RGB – Uses a plain color.
Fuel – Uses the fuel channel.

Texture Texture  | emTexture – If Based on is set to Texture, this slot specifies which texture to use.

Modulate – When  | emModulate – When the fire is not based on Texture and this option is enabled, the selected channel is multiplied by the map in the Texture slot.

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Additional Parameters

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Fire Opacity Mode –  | emIgnoreAlpha While smoke has its own opacity in the Smoke Opacity rollout, fire's opacity can be determined in either of three ways:

Use Smoke Opacity – Fire will use the same opacity that is set to the smoke in the Smoke Opacity rollout. This way the fire will not be visible in cells where there is no smoke.
Fully Visible – Fire  – Fire will always use opacity of 1.0, regardless of the smoke's opacityrender as if it has full opacity, but will not produce alpha. This way fire will be visible even in cells that have no opacity. This mode is intended for use with Phoenix simulations, such as simulations with sources that emit Temperature but do not emit Smoke. During rendering, Phoenix internally composes the fire with the scene using additive blending, and such blending must be used when compositing fire manually. However, this mode is not suitable for compositing fire mixed with smoke.
Use Own Opacity – A varying opacity can be set for the fire using the Opacity diagram.

Physically Based –  | emBlackBody Transitions between artistic look of the fire (when set to 0) and realistic physically based Intensity (when set to 1). The realistic mode multiplies the fire intensity by the Black Body Radiation model, which gives strong brightness to the hot parts of the fire. The example below uses the settings from the miniature image on the right. For more information, see the Physically Based example below.

Fire Multiplier –  | emMult General multiplier for the fire intensity.

Opacity Multiplier –  | emAlphaMult General multiplier for the fire's opacity as set by the opacity function represented in the diagram. Available only when Fire Opacity Mode is set to Use Own Opacity.

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At the bottom of the Fire rollout is a gradient for setting the color, and a diagram for setting either the intensity or opacity of the fire depending on the Fire Opacity Mode selection. The selected channel's data range is denoted by a blue-green line.

 

Color –  | emRampColor_temRampColor_semRampColor_vemRampColor_f The color gradient represents the color of the light as a function of the selected channel's value. The selected channel's data range is denoted by a blue-green line.

Intensity –  | epower_t, epower_s, epower_v, epower_f The luminance of the emitted light is determined by the function represented in the diagram control below the color gradient when the Fire Opacity Mode is set to Use Smoke Opacity or Fully Visible. Along the X axis is the value of the selected source channel (TemperatureFuel, etc.). Values are multiplied by the colors and by the Fire Multiplier value to achieve the real value that will be used. Values are also affected by the Physically Based value if other than 0. 

OpacityOpacity | epower_t, epower_s, epower_v, epower_f – The opacity of the emitted light is determined by the function represented in the diagram control below the color gradient when the Fire Opacity Mode is set to Use Own Opacity. Along the X axis is the value of the selected source channel (TemperatureFuel, etc.). Values are multiplied by the colors and by the Opacity Multiplier value to achieve the real value that will be used. Values are also affected by the Physically Based value if other than 0. 

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