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bbentley81

Polygon to VDB issues

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What are the major gatchas with using the VDB from Polygon sop. I attempted to turn a stone arch into a vdb so I could dent it up a cut the surface, and then return it to a high poly  node. When I viualize the sop I just see a bunch of grayscale dots. I am think there is something up with my geom b/c if I just use a box it works just fine.

 

I used to do this with the iso surface node but I heard the VDB has better performance. Any one have any tips they can give me for working with this.

 

Thanks in advance!

 

Cheers,

B

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Do you have any holes in your polygon geometry ? VDB usually works better with watertight objects.  Maybe try adding a polycap.

You can also try and  adjust your settings like lowering the VDB resolution to try and fill the holes.

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If you are using a Volume VOP to sculpt the incoming VDB sdf volume, you can push the result beyond the active voxels. It is mandatory to manage the VDB active voxels when using Volume VOP sops.

 

The main difference between sculpting regular volume sdf's and VDB sdf's is needing to manage the active vdb voxels versus the inactive vdb voxels.

 

When you use the VDB from Polygons SOP, it sets active voxels +3 and -3 deep around the limit surface. This is a narrow band of active voxels around the 0 limit surface.

 

 If you push the surface in and out either +3 or -3 voxels in distance, you will blow in to non-active voxels and no surface will be generated then you get only the bits of the surface that are still in the boundary.

 

You need to use the VDB Activate SDF SOP to activate a wider band of active voxels for SDF type grids. Use the MMB info on the node to see what type of VDB grid you have as it will have (surface) to indicate an SDF type grid. Use the VDB Activate SOP to expand other density and velocity type grids.

 

You will also want to use the VDB Visualize Tree SOP to see which voxels are active in the VDB grid. Pretty much a must to debug VDB grids before and after operations.

 

post-371-0-47041800-1456842046_thumb.jpg

 

After you have finished your sculpting operations, append another VDB Activate SDF SOP and shrink the thin band back to 3 voxels either side of the new limit 0 surface on the sdf grid(s) to claw back memory and make the sdf grid as efficient as you can.

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i tend to put down a convert vdb to see the actual polys rather than relying explicitly on the viewport's openvdb rendering.  gray dots sounds like it's interpreting your source geo as a thin surface, which implies it might be open somewhere.

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Wow,

 

Thanks for all the info. Very very useful. I didn't know about the holes limitation. That did the trick. As soon as I added a polycap everything fell into place.

 

Thanks again,

B

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