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Simply create a Geometry SOP. Dive inside the node and set the File SOP's Geometry File parameter to point to  your .vdb file.

That's it!

RenderMan for Houdini will automatically detect the bounding box and all the VDB grid names.

Assign PxrVolume or your volume Bxdf to the Geometry node. Don't forget to set the density primvar name (which matches the density grid name in your .vdb file) appropriately in PxrVolume.

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To render a Houdini volume network or a non-VDB file, connect it to a Convert VDB SOP.  Change "Convert To" to VDB. When you render, RenderMan for Houdini will automatically write out the .vdb file to the Houdini temporary directory. This way, we are utilizing the efficient VDB format and not emitting RiVolume data that could get very large.