The Corona Volume Grid allows you to render objects which are truly volumetric and based on a 3D grid, as opposed to meshes with the Corona Mtl or Corona Volume Mtl which always require some kind of surface consisting of polygons to display the volume.

The typical uses of the Corona Volume Grid are fire, explosions, smoke, clouds, steam, and other visual effects based on particle or fluid simulations, objects which are not possible or hard to render when using classic polygonal mesh.

Only the OpenVDB (.vdb) format is supported. You can learn more about it at http://www.openvdb.org/, and you can download sample .vdb files at http://www.openvdb.org/download/

The Corona Volume Grid also supports animated OpenVDB files (.vdb sequences). 

Load from file – Loads sequence of OpenVdb from a disk. If one file is selected by the user, the sequence of files is auto-detected. If more files are selected by the user, only those files are loaded and they are placed into sequence by their alphabetical order.

Reload from disk – Reloads a previously loaded sequence of OpenVdb files from disk.

Online Help – Opens the knowledge article for Corona Volume Grid.

Model – Sets how the sequence of OpenVDB files are mapped onto the 3ds Max timeline.

Repeat – The whole sequence is periodically repeated (the first frame appears after the last frame).

Ping Pong – Sequence is repeated in a continuous way (e.g. frames 1 to 5 are repeated as follows: 1 2 3 4 5 4 3 2 1 2...).

Freeze out of range – Frames below the range of the loaded sequence are displayed as the first frame of the sequence and frames above the range as the last frame of the sequence.

Hide out of range – Out-of-range frames are not displayed.

Frame Offset – Determines how much the OpenVDB sequence is offset compered to the 3ds Max timeline. Setting offset to 1 displays frame 1 to the OpenVDB sequence when 3ds Max frame number is 2 (so the OpenVDB sequence is delayed by 1 frame).

Start – Determines the first frame to be displayed from the OpenVDB sequence. Setting this value to 2 for a sequence with frames from 0 to 9 results in skipping frames 0 and 1.

Length – Determines the number of consecutive frames used from the OpenVDB sequence. Setting this value to 5 for a sequence with frames from 0 to 9 results in skipping frames 5 to 9.

Speed multiplier – Determines how fast the OpenVDB sequence runs. Setting this value to 1 results in one OpenVDB frame per one 3ds Max frame, while setting this value to 2 results in two OpenVDB frames per one 3ds Max frame.

Load nearest if missing –When enabled, missing frames in Open VDB sequence are replaced by the closest frame.

Emission

Checkbox – Enables emissions in this volume.

Channel – Selects the channel that is used as a source for medium emission. Along with channels (grids) presented in the OpenVDB file, such as Temperature, Density, Velocity, etc., a special option can be selected:

Constant – Contains only the value 1, which fills the whole bounding box of the volume grid.

Tint – Enables modulation of the final color by text map. Only object XYZ and World XYZ mapping are supported.

Mode – Defines the way in which data in the selected emission channel is mapped to the final emission of the volume.

Raw data – Channel data are not mapped and are only influenced by scale and tint.
Channel mapping – Emission channel undergoes mapping to colors, which is accessible under the "Channel color mapping" button.
Blackbody – Emission channel is treated temperature of Kelvin and converted to color according to blackbody radiation laws.

Scale – Uniformly scales emission amount after mapping.

Temperature scale – If emission mode is set to Blackbody, the selected emission channel is treated as temperature in Kelvin and these parameters are used to linearly scale the temperature.

Channel color mapping – Opens the Channel color mapping window, where you can choose how values from the selected channel are mapped to colors.

Absorption

Channel – Еnables absorption in this volume. Selects the channel that is used as a source for absorption. Along with channels (grids) presented in the OpenVDB file, a special option can be selected:

Constant – Contains only the value 1, which fills the whole bounding box of the volume grid.

Tint – Enables modulation of the final color by text map. Only object XYZ and World XYZ mapping are supported.

Scale – Uniformly scales absorption amount after channel mapping.

Channel color mapping – Opens the Channel color mapping window, where you can choose how values from the selected channel are mapped to colors. Use the checkbox to enable mapping of channel value to colors. If mapping is switched off, float channel values are treated as luminance values, while color channel values are not changed. When channel values are colors and mapping is enabled, only the luminance of channel color is taken into account.

Example: Emission

Example: Emission Scale

Example: Absorbtion Scale

Example: Scattering Scale

Example: Emission Tint

Set to blue

Example of Scattering Tint

Set to blue

Interpolation – Specifies how emission/scattering/absorption values are computed from the channel.

None – No interpolation, the fastest method, but also prone to artifacts.
Linear – Linear interpolation, good quality, and reasonable speed.
Quadratic – Quadratic interpolation, the best results but the lowest speed.

Step size – Determines ray marching step size. Increasing the value leads to faster rendering with more noise and possibly some artifacts, while lowering the value leads to less noise but slower render times.

Alpha mode – Specifies how the object is visible in the alpha channel. Available modes are:

Default – This is the default mode. In this mode, the object's alpha value is derived from the data in the OpenVDB file.
Always black – In this mode, the object's alpha value is black (the object shows as transparent).
Always white – In this mode, the object's alpha value is white (the object shows as opaque).

Flip frame – Flips frame of the OpenVDB channels (more specifically the Y and Z axes).

Motion Blur – Defines how motion blur and frame interpolation are computed.

Disabled – Both frame interpolation and motion blur are disabled (in both cases we only use the frame closest to the current time).

Simple – Simple linear interpolation between two OpenVDB frames, may lead to artifacts.

Velocity-based – Enables smooth interpolation/motion blur using velocities stored in the OpenVDB file. For this mode to work properly, you must select a velocity channel.

Multiplier – Specifies the strength of the motion blur effect.

Viewport display – Determines how the current OpenVDB fame is displayed in the viewport.

Wire box – Only bounding box of selected OpenVDB channels is displayed.

Point cloud – The volume is displayed as a point cloud; each point position is determined by the volume density and its color by channel mapping/tint settings (requires loading of channels - slow for huge files).

Quality – Determines a total number of points displayed for the point cloud preview mode.

Example: Interpolation

Example: Step Size

Examples: Motion Blur

This example shows how to get dark, thick smoke, which does not emit or scatter light.

Emission and scattering are disabled (since we do not need them). Absorption is enabled and set to a high scale.

In this example we show how to achieve intensive fire with dark, thin smoke, which does scatter some light.

Emission is enabled since we want to render the emissive fire part. Emission scale is set to 5 to boost intensity. Blackbody mode is selected for the emission, since we are rendering realistic fire. 

Absorption is enabled to render the smoke, however its scale is reduced to 0,5 as we'd like the smoke to be thin. Scattering is enabled with the scale set to just 0,05 for a weak scattering effect.

This example shows how to create a bright steam.

We do not need emission for this example, so it is left disabled.  Absorption is enabled, however its scale is lowered to 0,05 to make the steam transparent.

Scattering scale is set to 1 to make the steam appear bright.

This example shows magic fire with colorful smoke.

We are using emission, absorption, and scattering. To achieve the purple hue of the smoke, we are using a custom scattering tint color.

For the colorful fire, we are using the channel color mapping option for emission (see below for further information and a mini-tutorial).

Channel color mapping can be used to colorize the properties of the volume grid (emission, absorption, scattering) based on a gradient. For example, a red-yellow gradient can be used to create realistic fire.

The channel mapping dialog allows for mapping floating point values stored in the channel to colors. The mapping is computed as follows:

Suggested workflow (based on emission)

1. Set emission channel to temperature.
2. Set mode to Channel mapping.
3. Open the Channel color mapping window. 
4. Press the Auto-scale button to automatically adjust the scaling value.

5. Set the desired gradient colors:

   The leftmost color represents the color of the darkest features of the volumetric effect (in case of using the temperature channel, this means the lowest temperature, so all areas outside of the visible fire) The right most color represents the color of the brightest features of the volumetric effect (in case of using the temperature channel, this means the highest temperature) All values in-between represent the transition between the extreme intensities (left and right most)

6. At this point, you can adjust the curve to your needs:

7. If you press OK and close the channel color mapping window, you can then return to the Shading rollout of the Corona Volume Grid object and adjust the Emission scale parameter. It works in a similar manner to the Input scaling inside the channel color mapping window, but adjusts the intensity after the color mapping is done. This means that if you have removed the visible bounding box from the volumetric effect using the controls provided in the channel color mapping window, then there is no way to show the bounding box again using the Emission scale parameter. This can be used to adjust the intensity of the effect after the overall color and visibility has been set up (e.g. making fire brighter).