This how-to will try to explain a work-flow for rendering OpenVDB cache files using the VRayVolumeGrid. We'll be using the fire.vdb cache available on the OpenVDB site (Fire OpenVDB).
At this step we have the cache file in Katana. Now we'll inspect the attributes at the vrayvolume location. To do so select the location in the Scenegraph tab and then go to the Attributes tab. You'll see something like the image below.
The info attribute is the important one. The information in it give us how big the grid is, what are the channels in the cache file and how they've been mapped to Phoenix FD channels. In this case we have two channels: density and temperature and they've been mapped to the Liquid/Temperature and Smoke channels in VRayVolumeGrid. This is important when we set up the shading.
Now you're ready to render and you'll get an image like the one below:
This result looks fine if we want to see only the smoke, but if we want to see the fire (we know we have fire in the cache, because there is a temperature channel) we need to make some changes to the settings.
Set the fire_scale parameter to 50. This value is chosen arbitrary until it looks good.
At this point you'll get an image that looks like the one below:
To tweak the shading of the fluid we'll use the Shader Widgets dialog. To access it use the Shelf -> VRay -> Shader Widgets. At the top of the window, there is drop down you can use to select the VrayShadingNode that has the VRayVolumeGrid shader. If you open the window it will look like this:
Tweak the Fire intensity remap control and Fire color ramp control similarly to the image below:
To get an image that looks like this:
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