Burn

Active

Check this to enable simulation and caching of the burn channel.

Clip Below

If the Adaptive Container option is enabled, this value defines the minimum temperature that will cause the container will consider non-empty. If this value is zero, no voxels containing temperature will be lost. However, often shading settings are such that low values aren't visible anyway. In this case, increasing this threshold allows the sim to keep the container smaller.

Burn Diffusion

Specifies how fast Burn diffuses (see Temperature/Diffusion above for more details).

Decay Mode

Decaying a channel can be done by a constant amount or by a percentage of the current value. The Decay Mode specifies which will be used.

Linear

Decay the burn channel by subtracting a constant value in each frame. If you emit 1.0 into the burn channel, setting Decay to 0.1 in this mode will let the emission live for 10 frames before it drops to zero.

This mode works well for flames that have a sharp contour

Exponential

Decay the burn channel by subtracting a percentage of the current value (of a voxel) in each frame. As the current value becomes smaller and smaller, so does the value that is subtracted. Therefore the emission never drops back to zero - only very close. And the speed at which is drops will get slower and slower.

This is the type of decay that temperature and density have. It models the way dust slowly dissipates in air.

The decision of which mode to use is mostly driven by your shading settings. To get an idea of how the modes differ, compare the histograms in the shader's mapping curve editors. The exponentially decayed channel will have lots of low values and less high values, because the decay slows down the smaller the values get. The linear decay more uniform distribution of high to low values.

This makes shading sharp contours easier with the linear mode.

If you want the channel values to live long and build up a more smoke-like cloud, the exponential decay works better.

Decay

In linear mode, Decay specifies the constant value that will be subtracted from the current channel value in each frame.

In exponential mode, it's the percentage of the current value that will be subtracted.

Half-life

Instead of specifying the decay intensity as percentage per frame, you can specify the time it takes for the channel value to decay to half it's value. This may be more intuitive in many situations.

For example if Half-Life is one frame, the channel value drops to 50% within one frame. That corresponds to a Decay of 50%.