This is a special gradient node for Volumes like Smoke and Fire. It allows users to control the colour of a volume geometry (e.g., Flame) more precisely.
A Volume Ramp is connected to an Octane Volume MediumA shading system designed to render volumes such as smoke and fog. which ports the OpenVDBDreamworks’ open-source C++ library housing the data structures and tools implementation for storing and manipulating volume data, like smoke and other amorphous materials. The purpose of OpenVDB is mostly to have an efficient way to store volumetric data in memory and on disk. It has evolved into a more general toolkit that also lets you accomplish other things, such as fracturing volumes, converting meshes to volumes and vice versa. However, it does not include a computational fluid dynamics solver, and therefore it cannot procedurally generate smoke or fire. OpenVDB is fully integrated as a library in OctaneRender. For more information about OpenVDB, check at http://www.openvdb.org/. volume as an Octane Node. Without the Octane Volume Medium, the volume will not show in the render. The Octane Volume Medium is then applied to the smoke’s Domain geometry.
The absorption ramp takes the grid value as input. In the colour gradient, the colors near "0" on the left are used for mapping low grid values to some custom colour (as in this case, the lowest values are mapped to white). Higher grid values are mapped to colours on the right of the color gradient. Bear in mind that less saturated colors will cause there to be less pronounced absorption. Emission and scattering ramps operate in the same way.
Please note that volume ramps are restricted to static colors for performance reasons (ie. it is not possible to attach a series of other texture mappings/generators to colours in the ramps).
There is an important consequence of volume animations specifically related to volume ramps: There is a "Max value" on the ramps, which you must set to a reasonable value. This value is used to scale grid values to between 0 and 1, so that the ramp can map these back to colors in the color gradient. This is needed because maximum values in the grids sometimes differ greatly throughout VDBDreamworks’ open-source C++ library housing the data structures and tools implementation for storing and manipulating volume data, like smoke and other amorphous materials. The purpose of OpenVDB is mostly to have an efficient way to store volumetric data in memory and on disk. It has evolved into a more general toolkit that also lets you accomplish other things, such as fracturing volumes, converting meshes to volumes and vice versa. However, it does not include a computational fluid dynamics solver, and therefore it cannot procedurally generate smoke or fire. OpenVDB is fully integrated as a library in OctaneRender. For more information about OpenVDB, please see http://www.openvdb.org/. sequences. If you set a Max value too high or too low, this will still work, but you will only see a subset of the colors in the gradient that you specify. The maximum values for grids in the current VDB selected are now shown in the volume node's inspector pane. A good rule of thumb is to choose a value near to these, but you are free to customise as you like.