I have a Pyro sim which I shaded, but wherever the density field is 0, the flame/temperature/fuel etc fields have these ugly holes in them whenever I render (in Karma, althought Mantra has the same issue).

What confuses me is that if I hook up a volume visualization node, it displays every field fully. For example, here is the fuel field from before.

I use a sparse pyro solver. Is this related?
Happy to provide additional info. Thanks!

Not related to sparse

The fact you are seeing those artifacts is that Karma and Mantra both use a single volume field to optimize acceleration structure for rendering therefore any fields outside of it will be culled

So if you have a field that encompasses all fields necessary you can use that one instead

https://www.sidefx.com/docs/houdini/props/karma.html#object
karma:object:volumesamplingchannel
Specifies the volume field by name that will be used for empty space culling. By default karma will use the 'density' field if it exists. If you are rendering an emissive volume in which some parts of the volume have a 0 density but still need to be rendered, you should specify a different field using this parameter.

However maybe easier is to keep it as density and just make sure your density is non-zero wherever your flame is which you can easily do in Pyro Post Process SOP [www.sidefx.com]
And make sure Flame Density is set to non-zero value like 0.001
This will make sure that density field contains at least this value wherever the flame field is and hence will fully contain the flame and not clip in the render with default sampling channel set to density

tamte
Not related to sparse

The fact you are seeing those artifacts is that Karma and Mantra both use a single volume field to optimize acceleration structure for rendering therefore any fields outside of it will be culled

So if you have a field that encompasses all fields necessary you can use that one instead

https://www.sidefx.com/docs/houdini/props/karma.html#object
karma:object:volumesamplingchannel
Specifies the volume field by name that will be used for empty space culling. By default karma will use the 'density' field if it exists. If you are rendering an emissive volume in which some parts of the volume have a 0 density but still need to be rendered, you should specify a different field using this parameter.

However maybe easier is to keep it as density and just make sure your density is non-zero wherever your flame is which you can easily do in Pyro Post Process SOP [www.sidefx.com]
And make sure Flame Density is set to non-zero value like 0.001
This will make sure that density field contains at least this value wherever the flame field is and hence will fully contain the flame and not clip in the render with default sampling channel set to density

Thanks Tomas. This fixed the issue for me. I used the Render Geometry Settings LOP to set the Sampling Channel to fuel, which was perfect for me. If I was planning to shade/render smoke as well, the Bake Pyro Volume trick would do well instead. Cheers