Using Volumetrics with Octane Render for LightWave

The Open VDB tab contains a number of options if an Object is going to be used as an OpeVDB container.

• OpenVDB File: Points to a user chosen VDB file or sequence (a smoke simulation, for example)

• $FToken frame offset: This allows you to offset (or delay/advance) the VDB volume in an animation by X number of frames.

The next set of attributes are related to 'channels' embedded in the VDB such as density, temperature, velocity and so on. These used Signed Distance Fields (SDFs) which means they can be scaled to any size.

note

An important thing to remember with OpenVDB channels is that they each have a unique name assigned to them. For example, volume density information could be written to a channel named "bob" and still function exactly the same as the more common channel name "density." Thankfully, most applications follow fairly standard naming conventions but the key thing is to know what channels are contained in your VDB file - to know each channel's name and its purpose.

The reason this is so important is because volume channels tell Octanbe Render for LightWave exactly how your volume should be shaded, and you are likely to get vastly different results from each channel because they tend to have very different uses.

For example, temperature should be used to drive volume emission for realistic flames and explosions by making use of blackbody emission rather than using the density channel which is tailored for describing thick plumes of smoke or clouds.

Your VDB file should have had these channels baked during simulation. If they are present, the dropdown menu for each item will allow you to select the channel in use.

Absorption, Density, Emission.....IDs - Determines which imported VDB data is used for each of these attributes.

Example:

In this example (a Smoke simulation baked with Storm Solver), several channels are available in the drop-down.

i.e. None, Default, Density, Fuel, Pressure, Temperature and Velocity. These are all signed XYZ distance fields.

Similar to the way the clarity of an image is defined by its resolution a voxel grid has resolution as well. While a low resolution image will look jagged or pixelated, a low resolution volume will look blocky.

A volume that is simulated at a higher resolution will look smoother, it is important to create a volume with the appropriate resolution for your scene and its distance from the camera. The advanced remapping parameters of the volume shader can be used to help alleviate some of this blockiness but it is a tricky balancing act that may require sacrificing too much of the desired look of your volume, so ultimately what is most important is the actual resolution of your volume simulation.

The attributes controllable via SDFs include

• Absorption/Scattering: The Absorption and Scattering SDF grids normally would be derived from the density SDF embedded in the VDB object, but you could choose something else, of course.

• Emission: The Emission grid SDF normally would be your temperature SDF allowing you to drive it via Blackbody emission node.

• Velocity: Velocity channels are expected in either two forms:

  A single velocity grid in the float vector type which contains all the XYZ information in one grid.

  Example: velocity

  Three separate float scalar velocity grids that pertain to X, Y, and Z individually.

  Example: velocity.x , velocity.y , velocity.z

Absorption, Emission, Scatter, Velocity Scales - provides additional control for adjusting individual scales. These are typically left at the defaults and adjustments are made in an attached medium node.

Lastly, the remaining two (2) attributes available include:

• Scale: This is the unit scale of the VDB volume/simualtion you have imported if the source of the VDB object has no real world units embedded.

• Isovalue: The isovalue is the offset of the SDF from the original VDB object. Typically you want to keep this as close to zero as you can. Typical values are 0.004 in the event you get an 'Invalid Mesh' issue.

When "TFD" is selected as the Volumetrics mode, the TFD panel becomes active. For more information, please refer to the 'Using TurbulenceFD with Octane' page.