Fire

Based on – Specifies the source channel rendered as fire. By default, the Temperature channel is used.

Fire Texture – If Based on is set to Texture, this slot specifies the texture from which fire is rendered. This slot can also specify a texture to be used with the Modulate option.

Modulate by Texture – Enabling this option multiplies the Based on channel by the map in the Texture slot. This is only effective when Based on is not set to Texture.

Fire Opacity Mode – While smoke has its own opacity in the Smoke Opacity tab, the Fire opacity can be determined in either one of the following three ways:

Use Smoke Opacity – Fire uses the same opacity that is set to the smoke in the Smoke Opacity tab. This means the fire is not visible in cells where there is no smoke.
Fully Visible – Fire always renders as if it has full opacity but does not produce alpha. This way, fire is visible even in cells that have no opacity. This mode is intended for use with certain 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 – Custom varying opacity for the fire using the Opacity diagram and/or using a texture.

Physically Based – Transitions between an artistic look of the fire (when set to 0) and a 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. See the Physically Based example.

Fire Opacity Source –  Available only when the Fire Opacity Mode is set to Use Own Opacity.

Don't Use Opacity Texture – The Fire Opacity is based entirely on the Color and Opacity Curve below.
Multiply Opacity By Texture – The Color and Opacity Curve is multiplied by the specified texture to produce the final Fire Opacity.
Fire Opacity From Texture – The Fire Opacity is based entirely on the specified texture - the Color and Opacity Curve are ignored.

Fire Opacity Texture – If Opacity Texture Mode is set to Multiply Opacity By Texture or Fire Opacity From Texture, this slot specifies the texture used to modulate the Fire Opacity.

Fire Multiplier – General multiplier for the fire color's intensity.

Opacity Multiplier – Multiplier for the fire's own opacity when it is detached from the Smoke Opacity. Visible only when the Opacity Mode is set to Use Own Opacity.

When the Based on option is set to a grid channel, this grid simulation data must be remapped to render data. The sim data channel is laid horizontally along the X-axis in the color gradient and intensity/opacity curve. The color gradient remaps the sim data from the Based on channel to fire color. The curve remaps the sim data to fire opacity (in Use Own Opacity mode) or to fire intensity otherwise. The opacity/intensity is mapped vertically on the Y-axis.

This sim data has different ranges, depending on the solver it came from - Phoenix Smoke usually goes from 0-1; Fuel goes from 0-1; Temperature is in Kelvins, so it goes from 300-2000, or a few thousand; Speed is in Voxels/sec, so it depends on the grid size and resolution but usually goes from 0 to several hundred. Data from other solvers often differs a lot from Phoenix ranges. The cоlor gradients in Cinema 4D are using the 0 to 1 range. When the sim data goes outside the 0 to 1 range, the gradient needs to be remapped through the Gradient Min and Gradient Max values in order to match the grid channel range. You can find the exact range of your grid channels in the Frame Info tab.

Gradient Min – Remaps the minimum value used for the Gradient.

Gradient Max – Remaps the maximum value used for the Gradient.

Fire Color Gradient – Controls the color of the light as a function of the selected channel's value. This color is multiplied by the Fire Multiplier to achieve the final value that is used.