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This PR exposes the OpenVDB dilate & erode operations as a single node. This node importantly _only_ affects the active state of voxels, and does not change their values at all. Dilating does expand the number of active voxels. It is of particular use in combination with the _Advect Grid_ node. A common source of frustration from users is that the velocity grid has to be slightly larger than the value grid for advection to work properly. Dilating the grid before advection is the approach taken by the crurrent "Volume Displace" modifier. ### Value of newly activated voxels When dilating, previously inactive voxels become active. There is some subtlety in what value these voxels have after the node. In openvdb grids, inactive voxels can have values stored which are different from the background value (although when sampling them, they will appear to have the background value). When a previously inactive voxel becomes active again, it will get the value which was stored for it previously. Often that is the background value, but not always. For example, when first eroding a grid with noise values and then dilating it again, these noise values will still be there (unless an entire tile was eroded in which case it is removed). If one wants to ensure that the newly active voxels have the background value, one needs to make sure that the inactive voxel values are properly set. This can be done with a slightly updated Set Grid Background (#153525) node that can force-override all inactive values in the grid. In the future, the node could support writing an explicit value to each newly activated voxel, or it could output a topology grid containing the voxels which have changed active status. This comes with a fair amount of overhead and is not necessary for many use-cases, so it's not implemented for now. Pull Request: https://projects.blender.org/blender/blender/pulls/147430 |
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