Meshing a grain sim via VDB gets inflated at rendertime

[Strob]

Hi everyone,

I am trying to use a grain simulation to create a sticky gum effect.

 (like this video) but I have a problem with the meshing.

To create a skin out of my grain I first convert my grain to vdb (with vdb from particlesfluid) and then I use convertvdb with the polygon polygon option to convert this to a polygon “skin”. Everything looks perfect in the viewport but when I try to render, it is inflated at rendertime, ie:  it’s not a perfect match with what I see in the viewport. If instead of grains, I use a sphere that I convert to vdb volume and then back to poly with the same convertvdb node with the polygon option I don’t get this volume increase at rendertime.

See yourself:

Is this a bug? Do you know any workaround I could get mantra to render the same result as I see in the viewport when using grain as a source?

I saw this thread where someone talk about creating properties in a shader to get to render vdb isosurface directly without converting to poly: https://www.sidefx.com/forum/topic/29678/ But I still have to investigate that one, looks a bit complicated.

As another workaround I could render my sim correctly by writing the vdb to disk and loading it in max to render with vray and it rendered correctly. But still I would like to know why mantra can't do it.

thanks

 

 

 

[Strob]

I found a workaround.

 

I just reloaded the vdb file in houdini and converted to poly and then it rendered correctly.

So the same vdb converted to poly directly does not work but if I write it to disk first and then load it back and convert the same way, it does work. So I think it just confirmed that it's a bug.

I submited it to SESI.

Edited June 12, 2016 by Strob

[Strob]

Finally it turns out it was not really a bug... but rather an automated feature of the shelf tool that turns on  the "Render as Points: Render Only Points" option in the object's render::geometry tab. We can see this on /obj/grain_particles. When I set this to any other choice the vdb to poly will render correctly.

So if you use grain to render anything else than points you have to know that you have to change that option. 

Edited June 15, 2016 by Strob