VDB multiplication by (-1) problem

[catchyid]

Hi,

I have FLIP simulation that I convert to VDB (i.e. in the Particle Fluid Surface node, I set output to  "Surface VDB"). I am rendering the VDB in an external renderer that requires negative VDB densities (or to be more clear, it requires positive densities inside the volume), so in a Volume VOP  I multiply incoming "density" by (-1) and wire the result to output "density" (plz, see attached image). However, when I do the multiplication, I get "new" voxels inside the VDB??? Meaning, I get positive densities in voxels that were zero before the multiplication??? The problem is gone when I converted the VDB into a native Houdini Volume then converted the result into VDB again (but this is inefficient). My question: why VDB multiplication creates new voxels?Is there a way to avoid this problem without converting back and forth between native and VDB volumes?

 

Thanks,