OpenVDB: Release Notes

OpenVDB 3.1.0

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Release Notes

  Version 3.1.0 - October 1, 2015

  Highlights:
  - New features: advection of arbitrary volumes, general-purpose preconditioned linear solver and Poisson solver, segmentation of topologically-enclosed regions of a volume, new and faster bitmask operators, concurrent paged array, volume diagnostics
  - Optimizations: threaded grid constructors and topology operations; faster mesh to volume conversion, SDF to fog volume conversion and grid pruning; faster, unbounded particle partitioning
  - New Houdini nodes: Advect, Diagnostics, Rasterize Points, Remap, Remove Divergence, Sort Points

  New features:
  - Added a volume advection tool for sparse advection of non-level-set volumes.
  - Added a preconditioned conjugate gradient solver.
  - Added a Poisson solver for functions sampled on grids.
  - Added extractEnclosedRegion, which detects topologically-enclosed (watertight) exterior regions (cavities) that can result from CSG union operations between level sets with concavities that are capped. (See the unit test `TestPoissonSolver::testSolveWithSegmentDomain` for an example in which this tool is used to identify regions of trapped fluid when solving for pressure in a volume of incompressible fluid.)
  - Added PagedArray, a concurrent, dynamic linear array data structure with fast O(1) value access (both random and sequential).
  - Added Sampler, which provides a unified API for both staggered and non-staggered interpolation of various orders.
  - Added equality and inequality operators to Metadata and MetaMap.
  - Added CheckLevelSet and CheckFogVolume tools that perform various tests on symmetric, narrow-band level sets and fog volumes, respectively, to diagnose potential issues.
  - Added support for value accessors that are not registered with their trees. (Bypassing accessor registration can improve performance in rare cases but should be used with caution, since the accessor will be left in an invalid state if the tree topology is modified.)
  - Added a stealNodes method that transfers ownership of all nodes in a tree of a certain type and inserts them into a linear array.
  - Added a tools::createLevelSetBox factory function for level-set grids.
  - Added Dense::offsetToCoord.
  - Added LeafNode::Buffer::data, which provides direct access to a leaf node’s voxel value array, avoiding out-of-core overhead. Use with caution.
  - Added a NodeMask::foreach method for efficient evaluation of complex bitwise operations.
  - Added a bitwise difference method to NodeMask.
  - Added a `-version` option to `vdb_print`, `vdb_render` and `vdb_view`.

  Improvements:
  - Deep, conversion and topology copy Grid constructors are now threaded and up to five times faster.
  - Grid::topologyUnion, Grid::topologyIntersection, and Grid::topologyDifference are now much faster due to threading.
  - Significantly improved the performance, parallel scaling and memory usage of the mesh to volume converter, and implemented a more robust inside/outside sign classification scheme.
  - Reimplemented the point partitioning tool for improved performance, concurrency and memory usage. The tool is now unbounded in the sense that points may be distributed anywhere in index space.
  - Significantly improved the performance of the SDF to fog volume converter.
  - Significantly improved the performance of the sdfInteriorMask tool and added support for both grid and tree inputs.
  - Made various optimizations and improvements to the level set morphing tool.
  - Aggregated DiscreteField and EnrightField (formerly in tools/LevelSetAdvect.h) and VelocitySampler and VelocityIntegrator (formerly in tools/PointAdvect.h) into a single header, tools/VelocityFields.h.
  - Modified the signed flood fill tool to accept grids of any signed scalar value type, not just floating-point grids.
  - The prune tool is now faster, and it employs an improved compression technique on trees with floating-point values.

  Bug fixes:
  - Fixed a build issue that could result in spurious “Blosc encoding is not supported” errors unless `OPENVDB_USE_BLOSC` was `#define`d when compiling client code.
  - Added NaN and inf checks to the point partitioning tool.
  - Fixed a `vdb_view` issue whereby the frame buffer size did not necessarily match the window size. [Contributed by Rafael Campos]
  - Fixed a roundoff issue in LevelSetTracker that could result in NaNs.
  - Changed CheckNormGrad to check the magnitude of the gradient rather than the square of the magnitude.
  - Fixed parameter type inconsistencies in math/Ray.h and tools/RayIntersector.h. [Contributed by Kévin Dietrich]
  - Fixed incorrect handling of signed values in the clip tool (and the Clip SOP).

  API changes:
  - Removed the `math::Hermite` class since it was no longer used and caused build issues for some.
  - Refactored the level set advection, level set filtering, level set measuring and level set tracking tools.
  - Extended the API of the Diagnose tool and disabled copy construction.
  - Extended and unified the API of various Samplers.
  - Added an optional template argument to the ValueAccessor class to allow for unregistered accessors.

  Houdini:
  - Added a Rasterize Points SOP that produces density volumes and transfers arbitrary point attributes using a weighted-average scheme. The node incorporates a VOP subnetwork for procedural modeling, and its accompanying creation script defines a default network with VEX procedures for cloud and velocity field modeling. (See the creation script file header for installation details.)
  - Merged the Advect Level Set SOP into a new Advect SOP that supports advection of arbitrary volumes, not just level sets.
  - Added a Remove Divergence SOP that eliminates divergence from a velocity field.
  - Added a Diagnostics SOP that can identify various problems with level sets, fog volumes and other grids.
  - Added a Sort Points SOP that spatially reorders a list of points so that points that are close together in space are also close together in the list. This can improve CPU cache coherency and performance for random-access operations.
  - Added a Remap SOP that maps voxel values in an input range to values in an output range through a user-defined transfer function.
  - Added an option to the Convert SOP to activate interior voxels. [Contributed by SESI]
  - The To Spheres SOP can now optionally output a `pscale` attribute.
  - Added `openvdb_houdini::SOP_NodeVDB::duplicateSourceStealable()`, which in conjunction with the Unload flag can help to minimize deep copying of grids between nodes. The Advect, Convert, Fill, Filter, Fracture, Noise, Offset Level Set, Prune, Remap, Remove Divergence, Renormalize Level Set, Resize Narrow Band, Smooth Level Set and Transform SOPs all have this optimization enabled, meaning that they can potentially steal, rather than copy, data from upstream nodes that have the Unload flag enabled. [Contributed by Double Negative]
  - Redesigned the UI of the Visualize SOP and added toggles to draw with or without color, to use the grid name as the attribute name for points with values, and to attach grid index coordinates to points.
  - Added toggles to the Filter, Rebuild Level Set, Resize Narrow Band, Smooth Level Set and To Spheres SOPs to specify units in either world space or index space.
  - Fixed an issue whereby grids generated by the Rebuild Level Set SOP did not always display as surfaces in the viewport.
  - The Metadata SOP now sets appropriate viewport visualization options when the grid class is changed.

  Version 3.0.0 - January 14, 2015
  - The File class now supports delayed loading of `.vdb` files, meaning that memory is not allocated for voxel values until the values are actually accessed. (This feature is enabled by default.) Until a grid has been fully loaded, its source `.vdb` file must not be modified or deleted, so for safety, File::open automatically makes private copies of source files that are smaller than a user-specified limit (see File::setCopyMaxBytes). The limit can be set to zero to disable copying, but if it cannot be guaranteed that a file will not be modified, then it is best not to enable delayed loading for that file.
  - `.vdb` files can now optionally be compressed with the Blosc LZ4 codec. Blosc compresses almost as well as ZLIB, but it is much faster.
  - Added PointPartitioner, a tool for fast spatial sorting of points stored in an external array, and PointIndexGrid, an acceleration structure for fast range and nearest-neighbor searches.
  - Added tree::NodeManager, which linearizes a tree to facilitate efficient multithreading across all tree levels.
  - Added tools::prune (and other variants), which replaces and outperforms `Tree::prune`.
  - Added tools::signedFloodFill, which replaces and outperforms `Tree::signedFloodFill`.
  - Added tools::changeBackground (and other variants), which replaces and outperforms `Tree::setBackground()`.
  - Added a fast but approximate narrow-band level set dilation method, a fast narrow-band level set erosion method, and a masked normalization method to LevelSetTracker.
  - Added Diagnose, which performs multithreaded diagnostics on grids to identify issues like values that are NaNs or out-of-range. It optionally generates a boolean grid of all values that fail user-defined tests.
  - Added optional alpha masks to LevelSetMorphing.
  - Fixed an intermittent crash in LevelSetMorphing.
  - Added `tools::topologyToLevelSet()`, which generates a level set from the implicit boundary between active and inactive voxels in an arbitrary input grid. [DWA internal]
  - Improved the performance of point scattering (by orders of magnitude) and added a DenseUniformPointScatter class as well as support for fractional numbers of particles per voxel.
  - Improved the performance and memory footprint of the ParticlesToLevelSet tool for large numbers (tens to hundreds of millions) of particles.
  - Added edge-adjacent (6+12=18 neighbors) and vertex-adjacent (6+12+8=26 neighbors) dilation algorithms to Morphology::dilateVoxels. The default dilation pattern is still face-adjacent (6 neighbors).
  - Added Tree::getNodes, which allows for fast construction of linear arrays of tree nodes for use in multithreaded code such as the LeafManager or tree::NodeManager.
  - Added math::Extrema and tools::extrema to efficiently compute minimum and maximum values in a grid.
  - Added support for material color grids to all level set shaders, and added an option to `vdb_render` that allows one to specify a reference grid to be used for material color lookups.
  - Added getLibraryVersionString and OPENVDB_LIBRARY_VERSION_STRING.
  - Modified the mesh to volume converter to always set the grid background value to the exterior narrow-band width, and added finite value checks to narrow band parameters.
  - tools::volumeToMesh now compiles for all grid types but throws an exception if the input grid does not have a scalar value type.
  - Added a File::readGrid overload and readBuffers overloads to the grid, tree and node classes that allow one to specify a bounding box against which to clip a grid while reading it. For large grids, clipping while reading can result in significantly lower memory usage than clipping after reading.
  - Added GridBase::clipGrid, which clips a grid against a world-space bounding box, and GridBase::clip and Tree::clip, which clip against an index-space bounding box.
  - Added tools::clip, which clips a grid either against a bounding box or against the active voxels of a mask grid.
  - File::readGridPartial allocates the nodes of a grid’s tree as before, but it now allocates leaf nodes without data buffers. (This feature is mainly for internal use. Partially-read grids should be used with care if at all, and they should be treated as read-only.)
  - Grid names retrieved using a File::NameIterator now always uniquely identify grids; they no longer generate ‘more than one grid named “x”’ warnings when there are multiple grids of the same name in a file (for files written starting with this version of the OpenVDB library).
  - Fixed a bug in Tree::ValueOffIter that could cause depth-bounded iterators to return incorrect values.
  - Eliminated a recursive call in TreeValueIteratorBase::next that could cause crashes on systems with a limited stack size.
  - Fixed memory leaks in RootNode::topologyDifference and RootNode::topologyIntersection.
  - Fixed a memory leak in io::Queue when the queue was full and a write task could not be added within the timeout interval.
  - Fixed a potential division by zero crash in tools::compDiv with integer-valued grids.
  - Fixed kernel normalization in the filter tool so that it is correct for integer-valued grids.
  - Fixed a bug in LeafNode::Buffer::getValue whereby Visual C++ would return a reference to a temporary. [Contributed by SESI]
  - Fixed a bug in tools::ParticlesToLevelSet related to attribute transfer when leaf nodes are produced without active values.
  - Added util::CpuTimer and removed the more simplistic `unittest_util::CpuTimer` from `unittest/util.h`.
  - Eliminated the use of `getopt` for command-line argument parsing in `vdb_test`.
  - openvdb::initialize now properly initializes log4cplus if it is enabled, eliminating “No appenders could be found” errors.
  - Fixed a bug in the QuantizedUnitVec::pack method that caused quantization artifacts.
  - Added convenience class AlphaMask
  - Added constructors and methods to both RandInt and Rand01 to set and reset the random seed value.
  - Added convenience methods for transforming bounding boxes to math::Transform.
  - `vdb_view` is now compatible with both GLFW 2 and GLFW 3.
  - Made many small changes to address type conversion and other warnings reported by newer compilers like GCC 4.8 and ICC 14.
  - Replaced the `HALF_INCL_DIR` and `HALF_LIB_DIR` Makefile variables with `ILMBASE_INCL_DIR` and `ILMBASE_LIB_DIR` and added `ILMBASE_LIB`, to match OpenEXR’s library organization. [Contributed by Double Negative]
  - Eliminated most local (function-scope) static variables, because Visual C++ doesn’t guarantee thread-safe initialization of local statics. [Contributed by SESI]
  - Fixed a bug in readString related to empty strings. [Contributed by Fabio Piparo]
  - Fixed a bug in the VolumeToMesh simplification scheme that was creating visual artifacts.

  API changes:
  - The addition of a GridBase::readBuffers virtual function overload and the GridBase::clip GridBase::readNonresidentBuffers and Tree::clipUnallocatedNodes virtual functions changes the grid ABI so that it is incompatible with earlier versions of the OpenVDB library (such as the ones in Houdini 12.5 and 13). Define the macro `OPENVDB_2_ABI_COMPATIBLE` when compiling OpenVDB to disable these changes and preserve ABI compatibility.
  - All shaders now have a template argument to specify the type of an optional material color grid, but the default type mimics the old, uniform color behavior.
  - Removed a deprecated io::Stream::write overload.
  - The point counts in UniformPointScatter and NonUniformPointScatter are now specified and returned as Index64.
  - RandInt has an extra template argument to specify the integer type. The RandomInt typedef is unchanged.
  - io::readData, io::HalfReader::read and io::HalfWriter::write now take a `uint32_t` argument indicating the type of compression instead of a `bool` indicating whether compression is enabled.
  - Removed `io::Archive::isCompressionEnabled()` and `io::Archive::setCompressionEnabled()` and renamed `io::Archive::compressionFlags()` and `io::Archive::setCompressionFlags()` to io::Archive::compression and io::Archive::setCompression.
  - Internal and leaf node classes are now required to provide "PartialCreate" constructors that optionally bypass the allocation of voxel buffers. Leaf node classes must now also provide allocate and isAllocated methods to manage the allocation of their buffers.
  - Removed `pruneInactive` and `pruneLevelSet` methods from the Tree and various node classes. These methods have been replaced by the much faster pruning functions found in tools/Prune.h.
  - Removed `signedFloodFill` methods from the Grid, Tree and various node classes. These methods have been replaced by the much faster functions found in tools/SignedFloodFill.h.
  - Removed `Grid::setBackground()` and `Tree::setBackground()` (use the faster changeBackground tool instead), and removed the default argument from RootNode::setBackground.

  Python:
  - Added grid methods `convertToPolygons()` and `convertToQuads()`, which convert volumes to meshes, and `createLevelSetFromPolygons()`, which converts meshes to volumes. NumPy is required.

  Maya:
  - Added an adaptive polygonal surface extraction node.

  Houdini:
  - Added a new Resize Narrow Band SOP that can efficiently adjust the width of a level set’s narrow band. This allows, for example, for a level set to be created quickly from points or polygons with a very narrow band that is then quickly resized to a desired width.
  - Fixed bugs in the Smooth Level Set and Reshape Level Set SOPs that caused them to ignore the selected discretization scheme.
  - Added a Morph Level Set SOP.
  - Added a From Points SOP to very quickly generate a level set from a point cloud, ignoring any radius attribute. [DWA internal]
  - Added a Voxel Scale mode to the Resample SOP.
  - Improved the performance and memory footprint of the From Particles SOP for large numbers (tens to hundreds of millions) of particles.
  - The Scatter SOP now accepts fractional numbers of particles per voxel.
  - Improved the performance of the Scatter SOP by more than an order of magnitude.
  - The Clip SOP now has a toggle to choose explicitly between a mask grid or a bounding box as the clipping region. As a consequence, the mask grid can now be unnamed.
  - Added the OpenVDB library version number to the Extended Operator Information for all SOPs.
  - SOPs are now linked with an rpath to the directory containing the OpenVDB library.
  - Like the native Houdini file SOP, the Read SOP now allows missing frames to be reported either as errors or as warnings.
  - The Read SOP now has an optional input for geometry, the bounding box of which can be used to clip grids as they are read. For large grids, clipping while reading can result in significantly lower memory usage than clipping after reading.
  - The From Polygons and Convert SOPs now default to using the polygon soup mesh representation, which uses less memory.

  Version 2.3.0 - April 23, 2014
  - Added extractSparseTree, which selectively extracts and transforms data from a dense grid to produce a sparse tree, and extractSparseTreeWithMask, which copies data from the index-space intersection of a sparse tree and a dense input grid.
  - Added copy constructors to the Grid, Tree, RootNode, InternalNode and LeafNode classes, and an assignment operator overload to RootNode, that allow the source and destination to have different value types.
  - Modified Tree::combine2 to permit combination of trees with different value types.
  - Added CanConvertType and RootNode::SameConfiguration metafunctions, which perform compile-time tests for value type and tree type compatibility, and a RootNode::hasCompatibleValueType method, which does runtime checking.
  - Added optional support for logging using log4cplus. See logging.h and the `INSTALL` file for details.
  - Added VolumeRayIntersector::hits, which returns all the hit segments along a ray. This is generally more efficient than repeated calls to VolumeRayIntersector::march.
  - Added member class Ray::TimeSpan and method Ray::valid, and deprecated method Ray::test.
  - Fixed a bug in VolumeHDDA that could cause rendering artifacts when a ray’s start time was zero. [Contributed by Mike Farnsworth]
  - Added a compositeToDense tool, which composites data from a sparse tree into a dense array, using a sparse alpha mask. Over, Add, Sub, Min, Max, Mult, and Set are supported operations.
  - Added a transformDense tool, which applies a functor to the value of each voxel of a dense grid within a given bounding box.
  - Improved the performance of node iterators.

  API changes:
  - Collected the digital differential analyzer code from math/Ray.h and tools/RayIntersector.h into a new header file, math/DDA.h.
  - Rewrote VolumeHDDA and made several changes to its API. (VolumeHDDA is used internally by VolumeRayIntersector, whose API is unchanged.)
  - Tree::combine2, RootNode::combine2, InternalNode::combine2, LeafNode::combine2 and CombineArgs all now require an additional template argument, which determines the type of the other tree.
  - Assignment operators for LeafManager::LeafRange::Iterator, BaseMaskIterator, NodeMask and RootNodeMask now return references to the respective objects.
  - Removed a number of methods that were deprecated in version 2.0.0 or earlier.

  Houdini:
  - Added a Clip SOP, which does volumetric clipping.
  - Added an Occlusion Mask SOP, which generates a mask of the voxels inside a camera frustum that are occluded by objects in an input grid.
  - The Combine SOP now applies the optional signed flood fill only to level set grids, since that operation isn’t meaningful for other grids.
  - The Filter SOP now processes all grid types, not just scalar grids.

  Version 2.2.0 - February 20, 2014
  - Added a simple, multithreaded volume renderer, and added volume rendering support to the `vdb_render` command-line renderer.
  - Added an option to the LevelSetRayIntersector and to `vdb_render` to specify the isovalue of the level set.
  - Added methods to the LevelSetRayIntersector to return the time of intersection along a world or index ray and to return the level set isovalue.
  - Improved the performance of the VolumeRayIntersector and added support for voxel dilation to account for interpolation kernels.
  - Added a section to the Cookbook on interpolation using BoxSampler, GridSampler, DualGridSampler, et al.
  - Added a section to the Overview on grids and grid metadata.
  - Modified DualGridSampler so it is more consistent with GridSampler.
  - The cpt, curl, laplacian, meanCurvature and normalize tools now output grids with appropriate vector types (covariant, contravariant, etc.).
  - Added a transformVectors tool, which applies an affine transformation to the voxel values of a vector-valued grid in accordance with the grid’s vector type and world space/local space setting.
  - Added a compDiv tool, which combines grids by dividing the values of corresponding voxels.
  - Fixed a bug in the mean curvature computation that could produce NaNs in regions with constant values.
  - Added a Grid::topologyDifference method.
  - Added exp and sum methods to Vec2, Vec3 and Vec4.
  - Improved the fillWithSpheres tool for small volumes that are just a few voxels across.
  - Improved the accuracy of the mesh to volume converter.
  - Fixed a bug in the mesh to volume converter that caused incorrect sign classifications for narrow-band level sets.
  - Fixed a bug in NonlinearFrustumMap::applyIJT that resulted in incorrect values when computing the gradient of a grid with a frustum transform.
  - Fixed a file I/O bug whereby some `.vdb` files could not be read correctly if they contained grids with more than two distinct inactive values.
  - Fixed an off-by-one bug in the numbering of unnamed grids in `.vdb` files. The first unnamed grid in a file is now retrieved using the name “`[0]`”, instead of “`[1]`”.
  - Fixed a build issue reported by Clang 3.2 in tools/GridOperators.h.
  - Fixed a memory leak in Film.
  - Added library and file format version number constants to the Python module.
  - Improved convergence in the volume renderer. [Contributed by Jerry Tessendorf and Mark Matthews]
  - Made various changes for compatibility with Houdini 13 and with C++11 compilers. [Contributed by SESI]

  API changes:
  - VolumeRayIntersector::march no longer returns an `int` to distinguish tile vs. voxel hits. Instead, it now returns `false` if no intersection is detected and `true` otherwise. Also, t0 and t1 might now correspond to the first and last hits of multiple adjacent leaf nodes and/or active tiles.
  - DualGridSampler is no longer templated on the target grid type, and the value accessor is now passed as an argument.
  - The `.vdb` file format has changed slightly. Tools built with older versions of OpenVDB should be recompiled to ensure that they can read files in the new format.

  Houdini:
  - Added topology union, intersection and difference operations to the Combine SOP. These operations combine the active voxel topologies of grids that may have different value types.
  - Added a Divide operation to the Combine SOP.
  - Added support for boolean grids to the Combine, Resample, Scatter, Prune and Visualize SOPs.
  - The Fill SOP now accepts a vector as the fill value, and it allows the fill region bounds to be specified either in index space (as before), in world space, or using the bounds of geometry connected to an optional new reference input.
  - Added a toggle to the Offset Level Set SOP to specify the offset in either world or voxel units.
  - Added a toggle to the Transform and Resample SOPs to apply the transform to the voxel values of vector-valued grids, in accordance with those grids’ vector types and world space/local space settings.
  - Added a Vector Type menu to the Vector Merge SOP.
  - Removed masking options from the Renormalize SOP (since masking is not supported yet).
  - Reimplemented the Vector Merge SOP for better performance and interruptibility and to fix a bug in the handling of tile values.

  Version 2.1.0 - December 12, 2013
  - Added a small number of Maya nodes, primarily for conversion of geometry to and from OpenVDB volumes and for visualization of volumes.
  - Added an initial implementation of level set morphing (with improvements to follow soon).
  - Added tools::LevelSetMeasure, which efficiently computes the surface area, volume and average mean-curvature of narrow-band level sets, in both world and voxel units. Those quantities are now exposed as intrinsic attributes on the Houdini VDB primitive and can be queried using the native Measure SOP.
  - tools::Dense now supports the XYZ memory layout used by Houdini and Maya in addition to the ZYX layout used in OpenVDB trees.
  - Improved the performance of masking in the level set filter tool and added inversion and scaling of the mask input, so that any scalar-valued volume can be used as a mask, not just volumes with a [0, 1] range.
  - Added optional masking to the non-level-set filters, to the grid operators (CPT, curl, divergence, gradient, Laplacian, mean curvature, magnitude, and normalize) and to the Analysis and Filter SOPs.
  - Added more narrow band controls to the Rebuild Level Set SOP.
  - Improved the accuracy of the level set rebuild tool.
  - Added tools::activate and tools::deactivate, which set the active states of tiles and voxels whose values are equal to or approximately equal to a given value, and added a Deactivate Background Voxels toggle to the Combine SOP.
  - Added BBox::applyMap and BBox::applyInverseMap, which allow for transformation of axis-aligned bounding boxes.
  - Added a position shader to the level set ray-tracer (primarily for debugging purposes).
  - Added an io::Queue class that manages a concurrent queue for asynchronous serialization of grids to files or streams.
  - Fixed a bug in io::Archive whereby writing unnamed, instanced grids (i.e., grids sharing a tree) to a file rendered the file unreadable.
  - Fixed a bug in the volume to mesh converter that caused it to generate invalid polygons when the zero crossing lay between active and inactive regions.
  - Fixed a bug in the point scatter tool (and the Scatter SOP) whereby the last voxel always remained empty.
  - Fixed a bug in the Read SOP that caused grids with the same name to be renamed with a numeric suffix (e.g., “grid[1]” “grid[2]”, etc.).
  - Fixed some unit test failures on 64-bit Itanium machines.

  API changes:
  - The Filter tool is now templated on a mask grid, and threading is controlled using a grain size, for consistency with most of the other level set tools.
  - The LevelSetFilter tool is now templated on a mask grid.
  - All shaders now take a ray direction instead of a ray.

  Version 2.0.0 - October 31, 2013
  - Added a Python module with functions for basic manipulation of grids (but no tools, yet).
  - Added ray intersector tools for efficient, hierarchical intersection of rays with level-set and generic volumes.
  - Added a Ray class and a hierarchical Digital Differential Analyzer for fast ray traversal.
  - Added a fully multi-threaded level set ray tracer and camera classes that mimic Houdini’s cameras.
  - Added a simple, command-line renderer (currently for level sets only).
  - Implemented a new meshing scheme that produces topologically robust two-manifold meshes and is twice as fast as the previous scheme.
  - Implemented a new, topologically robust (producing two-manifold meshes) level-set-based seamless fracture scheme. The new scheme eliminates visible scarring seen in the previous implementation by subdividing internal, nonplanar quads near fracture seams. In addition, fracture seam points are now tagged, allowing them to be used to drive pre-fracture dynamics such as local surface buckling.
  - Improved the performance of Tree::evalActiveVoxelBoundingBox and Tree::activeVoxelCount, and significantly improved the performance of Tree::evalLeafBoundingBox (by about 30x).
  - Added a tool (and a Houdini SOP) that fills a volume with adaptively-sized overlapping or non-overlapping spheres.
  - Added a Ray SOP that can be used to perform geometry projections using level-set ray intersections or closest-point queries.
  - Added a tool that performs accelerated closest surface point queries from arbitrary points in world space to narrow-band level sets.
  - Increased the speed of masked level set filtering by 20% for the most common cases.
  - Added math::BoxStencil, with support for trilinear interpolation and gradient computation.
  - Added Tree::topologyIntersection, which intersects a tree’s active values with those of another tree, and Tree::topologyDifference, which performs topological subtraction of one tree’s active values from another’s. In both cases, the `ValueType`s of the two trees need not be the same.
  - Added Tree::activeTileCount, which returns the number of active tiles in a tree.
  - Added math::MinIndex and math::MaxIndex, which find the minimum and maximum components of a vector without any branching.
  - Added BBox::minExtent, which returns a bounding box’s shortest axis.
  - The default BBox constructor now generates an invalid bounding box rather than an empty bounding box positioned at the origin. The new behavior is consistent with CoordBBox. [Thanks to Rick Hankins for suggesting this fix.]
  - Added CoordBBox::reset, which resets a bounding box to its initial, invalid state.
  - Fixed a bug in the default ScaleMap constructor that left some data used in the inverse uninitialized.
  - Added MapBase::applyJT, which applies the Jacobian transpose to a vector (the Jacobian transpose takes a range-space vector to a domain-space vector, e.g., world to index), and added MapBase::inverseMap, which returns a new map representing the inverse of the original map (except for NonlinearFrustumMap, which does not currently have a defined inverse map).
    Note: Houdini 12.5 uses an earlier version of OpenVDB, and maps created with that version lack virtual table entries for these new methods, so do not call these methods from Houdini 12.5.
  - Reimplemented math::RandomInt using Boost.Random instead of `rand()` (which is not thread-safe), and deprecated `math::randUniform()` and added math::Random01 to replace it.
  - Modified tools::copyFromDense and tools::copyToDense to allow for implicit type conversion (e.g., between a Dense<Int32> and a FloatTree) and fixed several bugs in tools::CopyFromDense.
  - Fixed bugs in math::Stats and math::Histogram that could produce `NaN`s or other incorrect behavior if certain methods were called on populations of size zero.
  - Renamed `struct tolerance` to math::Tolerance and `negative` to math::negative and removed `math::toleranceValue()`.
  - Implemented a closest point on line segment algorithm, math::closestPointOnSegmentToPoint.
  - Fixed meshing issues relating to masking and automatic partitioning.
  - Grid::merge and Tree::merge now accept an optional MergePolicy argument that specifies one of three new merging schemes. (The old merging scheme, which is no longer available, used logic for each tree level that was inconsistent with the other levels and that could result in active tiles being replaced with nodes having only inactive values.)
  - Renamed `LeafNode::coord2offset()`, `LeafNode::offset2coord()` and `LeafNode::offset2globalCoord()` to coordToOffset, offsetToLocalCoord, and offsetToGlobalCoord, respectively, and likewise for InternalNode. [Thanks to Rick Hankins for suggesting this change.]
  - Replaced Tree methods `setValueOnMin`, `setValueOnMax` and `setValueOnSum` with tools::setValueOnMin, tools::setValueOnMax and tools::setValueOnSum (and a new tools::setValueOnMult) and added Tree::modifyValue and Tree::modifyValueAndActiveState, which modify voxel values in-place via user-supplied functors. Similarly, replaced `ValueAccessor::setValueOnSum()` with ValueAccessor::modifyValue and ValueAccessor::modifyValueAndActiveState, and added a modifyValue method to all value iterators.
  - Removed `LeafNode::addValue` and `LeafNode::scaleValue`.
  - Added convenience classes tree::Tree3 and tree::Tree5 for custom tree configurations.
  - Added an option to the From Particles SOP to generate an alpha mask, which can be used to constrain level set filtering so as to preserve surface details.
  - The mesh to volume converter now handles point-degenerate polygons.
  - Fixed a bug in the Level Set Smooth, Level Set Renormalize and Level Set Offset SOPs that caused the group name to be ignored.
  - Fixed various OS X and Windows build issues. [Contributions from SESI and DD]

  Version 1.2.0 - June 28 2013
  - Level set filters now accept an optional alpha mask grid.
  - Implemented sharp feature extraction for level set surfacing. This enhances the quality of the output mesh and reduces aliasing artifacts.
  - Added masking options to the meshing tools, as well as a spatial multiplier for the adaptivity threshold, automatic partitioning, and the ability to preserve edges and corners when mesh adaptivity is applied.
  - The mesh to volume attribute transfer scheme now takes surface orientation into account, which improves accuracy in proximity to edges and corners.
  - Added a foreach method to tree::LeafManager that, like tools::foreach, applies a user-supplied functor to each leaf node in parallel.
  - Rewrote the particle to level set converter, simplifying the API, improving performance (especially when particles have a fixed radius), adding the capability to transfer arbitrary point attributes, and fixing a velocity trail bug.
  - Added utility methods Sign, SignChange, isApproxZero, Cbrt and ZeroCrossing to math/Math.h.
  - Added a probeNode method to the value accessor and to tree nodes that returns a pointer to the node that contains a given voxel.
  - Deprecated `LeafNode::addValue` and `LeafNode::scaleValue`.
  - Doubled the speed of the mesh to volume converter (which also improves the performance of the fracture and level set rebuild tools) and improved its inside/outside voxel classification near edges and corners.
  - GridSampler now accepts either a grid, a tree or a value accessor, and it offers faster index-based access methods and much better performance in cases where many instances are allocated.
  - Extended tools::Dense to make it more compatible with existing tools.
  - Fixed a crash in io::Archive whenever the library was unloaded from memory and then reloaded. [Contributed by Ollie Harding]
  - Fixed a bug in `GU_PrimVDB::buildFromPrimVolume()`, seen during the conversion from Houdini volumes to OpenVDB grids, that could cause signed flood fill to be applied to non-level set grids, resulting in active tiles with incorrect values.
  - Added a Prune SOP with several pruning schemes.

  Version 1.1.1 - May 10 2013
  - Added a simple dense grid class and tools to copy data from dense voxel arrays into OpenVDB grids and vice-versa.
  - Starting with Houdini 12.5.396, plugins built with this version of OpenVDB can coexist with native Houdini OpenVDB nodes.
  - The level set fracture tool now smooths seam line edges during mesh extraction, eliminating staircase artifacts.
  - Significantly improved the performance of the leafTopologyIntersection and leafTopologyDifference utilities and added a LeafNode::topologyDifference method.
  - Added convenience functions that provide simplified interfaces to the mesh to volume and volume to mesh converters.
  - Added a tools::accumulate function that is similar to tools::foreach but can be used to accumulate the results of computations over the values of a grid.
  - Added tools::statistics, tools::opStatistics and tools::histogram, which efficiently compute statistics (mean, variance, etc.) and histograms of grid values (using math::Stats and math::Histogram).
  - Modified CoordBBox to adhere to TBB’s splittable type requirements, so that, for example, a `CoordBBox` can be used as a blocked iteration range.
  - Added Tree::addTile, Tree::addLeaf and Tree::stealNode, for fine control over tree construction.
  - Addressed a numerical stability issue when performing Gaussian filtering of level set grids.
  - Changed the return type of CoordBBox::volume to reduce the risk of overflow.
  - When the input mesh is self-intersecting, the mesh to volume converter now produces a level set with a monotonic gradient field.
  - Fixed a threading bug in the mesh to volume converter that caused it to produce different results for the same input.
  - Fixed a bug in the particle to level set converter that prevented particles with zero velocity from being rasterized in Trail mode.
  - Added an optional input to the Create SOP into which to merge newly-created grids.
  - Fixed a bug in the Resample SOP that caused it to produce incorrect narrow-band widths when resampling level set grids.
  - Fixed a bug in the To Polygons SOP that caused intermittent crashes when the optional reference input was connected.
  - Fixed a bug in the Advect Level Set SOP that caused a crash when the velocity input was connected but empty.
  - The Scatter and Sample Point SOPs now warn instead of erroring when given empty grids.
  - Fixed a crash in `vdb_view` when stepping through multiple grids after changing render modes.
  - `vdb_view` can now render fog volumes and vector fields, and it now features interactively adjustable clipping planes that enable one to view the interior of a volume.

  Version 1.1.0 - April 4 2013
  - The resampleToMatch tool, the Resample SOP and the Combine SOP now use level set rebuild to correctly and safely resample level sets. Previously, scaling a level set would invalidate the signed distance field, leading to holes and other artifacts.
  - Added a mask-based topological erosion tool, and rewrote and simplified the dilation tool.
  - The LevelSetAdvection tool can now advect forward or backward in time.
  - Tree::pruneLevelSet() now replaces each pruned node with a tile having the inside or outside background value, instead of arbitrarily selecting one of the node’s tile or voxel values.
  - When a grid is saved to a file with saveFloatAsHalf set to `true`, the grid’s background value is now also quantized to 16 bits. (Not quantizing the background value caused a mismatch with the values of background tiles.)
  - As with tools::foreach, it is now possible to specify whether functors passed to tools::transformValues should be shared across threads.
  - tree::LeafManager can now be instantiated with a `const` tree, although buffer swapping with `const` trees is disabled.
  - Added a Grid::signedFloodFill overload that allows one to specify inside and outside values.
  - Fixed a bug in Grid::setBackground() so that now only the values of inactive voxels change.
  - Fixed Grid::topologyUnion so that it actually unions tree topology, instead of just the active states of tiles and voxels. The previous behavior broke multithreaded code that relied on input and output grids having compatible tree topology.
  - math::Transform now includes an isIdentity predicate and methods to pre- and postmultiply by a matrix.
  - Modified the node mask classes to permit octree-like tree configurations (i.e., with a branching factor of two) and to use 64-bit operations instead of 32-bit operations.
  - Implemented a new, more efficient closest point on triangle algorithm.
  - Implemented a new vertex normal scheme in the volume to mesh converter, and resolved some overlapping polygon issues.
  - The volume to mesh converter now meshes not just active voxels but also active tiles.
  - Fixed a bug in the mesh to volume converter that caused unsigned distance field conversion to produce empty grids.
  - Fixed a bug in the level set fracture tool whereby the cutter overlap toggle was ignored.
  - Fixed an infinite loop bug in `vdb_view`.
  - Updated `vdb_view` to use the faster and less memory-intensive OpenVDB volume to mesh converter instead of marching cubes, and rewrote the shader to be OpenGL 3.2 and GLSL 1.2 compatible.
  - Given multiple input files or a file containing multiple grids, `vdb_view` now displays one grid at a time. The left and right arrow keys cycle between grids.
  - The To Polygons SOP now has an option to associate the input grid’s name with each output polygon.

  Version 1.0.0 - March 14 2013
  - tools::levelSetRebuild now throws an exception when given a non-scalar or non-floating-point grid.
  - The tools in tools/GridOperators.h are now interruptible, as is the Analysis SOP.
  - Added a leaf node iterator and a TBB-compatible range class to the LeafManager.
  - Modified the VolumeToMesh tool to handle surface topology issues around fracture seam lines.
  - Modified the Makefile to allow `vdb_view` to compile on OS X systems (provided that GLFW is available).
  - Fixed a bug in the Create SOP that resulted in "invalid parameter name" warnings.
  - The Combine SOP now optionally resamples the A grid into the B grid’s index space (or vice-versa) if the A and B transforms differ.
  - The Vector Split and Vector Merge SOPs now skip inactive voxels by default, but they can optionally be made to include inactive voxels, as they did before.
  - The LevelSetFracture tool now supports custom rotations for each cutter instance, and the Fracture SOP now uses quaternions to generate uniformly-distributed random rotations.

  Version 0.104.0 - February 15 2013
  - Added a tool and a SOP to rebuild a level set from any scalar volume.
  - ``.vdb files are now saved using a mask-based compression scheme that is an order of magnitude faster than ZLIB and produces comparable file sizes for level set and fog volume grids. (ZLIB compression is still enabled by default for other classes of grids).
  - The Filter and LevelSetFilter tools now include a Gaussian filter, and mean (box) filtering is now 10-50x faster.
  - The isosurface meshing tool is now more robust (to level sets with one voxel wide narrow bands, for example).
  - Mesh to volume conversion is on average 1.5x faster and up to 5.5x faster for high-resolution meshes where the polygon/voxel size ratio is small.
  - Added createLevelSet and createLevelSetSphere factory functions for level set grids.
  - tree::ValueAccessor is now faster for trees of height 2, 3 and 4 (the latter is the default), and it now allows one to specify, via a template argument, the number of node levels to be cached, which can also improve performance in special cases.
  - Added a toggle to tools::foreach to specify whether or not the functor should be shared across threads.
  - Added Mat4s and Mat4d metadata types.
  - Added explicit pre- and postmultiplication methods to the `Transform`, `Map` and `Mat4` classes and deprecated the old accumulation methods.
  - Modified NonlinearFrustumMap to be more compatible with Houdini’s frustum transform.
  - Fixed a GridTransformer bug that caused it to translate the output grid incorrectly in some cases.
  - Fixed a bug in the tree-level LeafIterator that resulted in intermittent crashes in tools::dilateVoxels.
  - The `Hermite` data type and Hermite grids are no longer supported.
  - Added tools/GridOperators.h, which includes new, cleaner implementations of the closest point transform, curl, divergence, gradient, Laplacian, magnitude, mean curvature and normalize tools.
  - Interrupt support has been improved in several tools, including tools::ParticlesToLevelSet.
  - Simplified the API of the Stencil class and added an intersects method to test for intersection with a specified isovalue.
  - Renamed `voxelDimensions` to `voxelSize` in transform classes and elsewhere.
  - Deprecated `houdini_utils::ParmFactory::setChoiceList` in favor of `houdini_utils::ParmFactory::setChoiceListItems`, which requires a list of token, label string pairs.
  - Made various changes for Visual C++ compatibility. [Contributed by SESI]
  - Fixed a bug in `houdini_utils::getNodeChain()` that caused the Offset Level Set, Smooth Level Set and Renormalize Level Set SOPs to ignore frame changes. [Contributed by SESI]
  - The From Particles SOP now provides the option to write into an existing grid.
  - Added a SOP to edit grid metadata.
  - The Fracture SOP now supports multiple cutter objects.
  - Added a To Polygons SOP that complements the Fracture SOP and allows for elimination of seam lines, generation of correct vertex normals and grouping of polygons when surfacing fracture fragments, using the original level set or mesh as a reference.

  Version 0.103.1 - January 15 2013
  - tree::ValueAccessor read operations are now faster for four-level trees. (Preliminary benchmark tests suggest a 30-40% improvement.)
  - For vector-valued grids, tools::compMin and tools::compMax now compare vector magnitudes instead of individual components.
  - Migrated grid sampling code to a new file, Interpolation.h, and deprecated old files and classes.
  - Added a level-set fracture tool and a Fracture SOP.
  - Added tools::sdfInteriorMask, which creates a mask of the interior region of a level set grid.
  - Fixed a bug in the mesh to volume converter that produced unexpected nonzero values for voxels at the intersection of two polygons, and another bug that produced narrow-band widths that didn’t respect the background value when the half-band width was less than three voxels.
  - `houdini_utils::ParmFactory` can now correctly generate ramp multi-parms.
  - Made various changes for Visual C++ compatibility. [Contributed by SESI]
  - The Convert SOP can now convert between signed distance fields and fog volumes and from volumes to meshes. [Contributed by SESI]
  - For level sets, the From Mesh and From Particles SOPs now match the reference grid’s narrow-band width.
  - The Scatter SOP can now optionally scatter points in the interior of a level set.

  Version 0.103.0 - December 21 2012
  - The mesh to volume converter is now 60% faster at generating level sets with wide bands, and the From Mesh SOP is now interruptible.
  - Fixed a threading bug in the recently-added compReplace tool that caused it to produce incorrect output.
  - Added a probeConstLeaf method to the Tree, ValueAccessor and node classes.
  - The Houdini VDB primitive doesn’t create a `name` attribute unnecessarily (i.e., if its grid’s name is empty), but it now correctly allows the name to be changed to the empty string.
  - Fixed a crash in the Vector Merge SOP when fewer than three grids were merged.
  - The From Particles SOP now features a "maximum half-width" parameter to help avoid runaway computations.

  Version 0.102.0 - December 13 2012
  - Added tools::compReplace, which copies the active values of one grid into another, and added a "Replace A With Active B" mode to the Combine SOP.
  - Grid::signedFloodFill no longer enters an infinite loop when filling an empty grid.
  - Fixed a bug in the particle to level set converter that sometimes produced level sets with holes, and fixed a bug in the SOP that could result in random output.
  - Fixed an issue in the frustum preview feature of the Create SOP whereby rendering very large frustums could cause high CPU usage.
  - Added streamline support to the constrained advection scheme in the Advect Points SOP.
  - Added an Advect Level Set SOP.

  Version 0.101.1 - December 11 2012 (DWA internal release)
  - Partially reverted the Houdini VDB primitive’s grid accessor methods to their pre-0.98.0 behavior. A primitive’s grid can once again be accessed by shared pointer, but now also by reference. Accessor methods for grid metadata have also been added, and the primitive now ensures that metadata and transforms are never shared.
  - Fixed an intermittent crash in the From Particles SOP.

  Version 0.101.0 - December 6 2012 (DWA internal release)
  - Partially reverted the Grid’s tree and transform accessor methods to their pre-0.98.0 behavior, eliminating copy-on-write but preserving their return-by-reference semantics. These methods are now supplemented with a suite of shared pointer accessors.
  - Restructured the mesh to volume converter for a 40% speedup and to be more robust to non-manifold geometry, to better preserve sharp features, to support arbitrary tree configurations and to respect narrow-band limits.
  - Added a `getNodeBoundingBox` method to RootNode, InternalNode and LeafNode that returns the index space spanned by a node.
  - Made various changes for Visual C++ compatibility. [Contributed by SESI]
  - Renamed the Reshape Level Set SOP to Offset Level Set.
  - Fixed a crash in the Convert SOP and added support for conversion of empty grids.

  Version 0.100.0 - November 30 2012 (DWA internal release)
  - Greatly improved the performance of the level set to fog volume converter.
  - Improved the performance of the median filter and of level set CSG operations.
  - Reintroduced Tree::pruneLevelSet(), a specialized Tree::pruneInactive() for level-set grids.
  - Added utilities to the `houdini_utils` library to facilitate the collection of a chain of adjacent nodes of a particular type so that they can be cooked in a single step. (For example, adjacent `xform` SOPs could be collapsed by composing their transformation matrices into a single matrix.)
  - Added pruning and flood-filling options to the Convert SOP.
  - Reimplemented the Filter SOP, omitting level-set-specific filters and adding node chaining (to reduce memory usage when applying several filters in sequence).
  - Added a toggle to the Read SOP to read grid metadata and transforms only.
  - Changed the attribute transfer scheme on the From Mesh and From Particles SOPs to allow for custom grid names and vector type metadata.

  Version 0.99.0 - November 21 2012
  - Added Grid methods that return non-`const` tree and transform references without triggering deep copies, as well as `const` methods that return `const` shared pointers.
  - Added `Grid` methods to populate a grid’s metadata with statistics like the active voxel count, and to retrieve that metadata. By default, statistics are now computed and added to grids whenever they are written to `.vdb` files.
  - Added io::File::readGridMetadata and io::File::readAllGridMetadata methods to read just the grid metadata and transforms from a `.vdb` file.
  - Fixed numerical precision issues in the csgUnion, csgIntersection and csgDifference tools, and added toggles to optionally disable postprocess pruning.
  - Fixed an issue in `vdb_view` with the ordering of GL vertex buffer calls. [Contributed by Bill Katz]
  - Fixed an intermittent crash in the ParticlesToLevelSet tool, as well as a race condition that could cause data corruption.
  - The `ParticlesToLevelSet` tool and From Particles SOP can now transfer arbitrary point attribute values from the input particles to output voxels.
  - Fixed a bug in the Convert SOP whereby the names of primitives were lost during conversion, and another bug that resulted in arithmetic errors when converting empty grids.
  - Fixed a bug in the Combine SOP that caused the Operation selection to be lost.

  Version 0.98.0 - November 16 2012
  - Tree and Transform objects (and Grid objects in the context of Houdini SOPs) are now passed and accessed primarily by reference rather than by shared pointer. See Porting to OpenVDB 0.98.0 for details about this important API change. [Contributed by SESI]
  - Reimplemented CoordBBox to address several off-by-one bugs related to bounding box dimensions.
  - Fixed an off-by-one bug in evalActiveVoxelBoundingBox.
  - Introduced the LeafManager class, which will eventually replace the `LeafArray` class. `LeafManager` supports dynamic buffers stored as a structure of arrays (SOA), unlike `LeafArray`, which supports only static buffers stored as an array of structures (AOS).
  - Improved the performance of the LevelSetFilter and LevelSetTracker tools by rewriting them to use the new LeafManager class.
  - Added Tree::setValueOnly and ValueAccessor::setValueOnly methods, which change the value of a voxel without changing its active state, and Tree::probeLeaf and ValueAccessor::probeLeaf methods that return the leaf node that contains a given voxel (unless the voxel is represented by a tile).
  - Added a LevelSetAdvection tool that propagates and tracks narrow-band level sets.
  - Introduced a new GridSampler class that supports world-space (or index-space) sampling of grid values.
  - Changed the interpretation of the NonlinearFrustumMap’s taper parameter to be the ratio of the near and far plane depths.
  - Added a `ParmFactory::setChoiceList()` overload that accepts (token, label) string pairs, and a `setDefault()` overload that accepts an STL string.
  - Fixed a crash in the Combine SOP in Copy B mode.
  - Split the Level Set Filter SOP into three separate SOPs, Level Set Smooth, Level Set Reshape and Level Set Renormalize. When two or more of these nodes are connected in sequence, they interact to reduce memory usage: the last node in the sequence performs all of the operations in one step.
  - The Advect Points SOP can now output polyline streamlines that trace the paths of the points.
  - Added an option to the Analysis SOP to specify names for output grids.
  - Added camera-derived frustum transform support to the Create SOP.

  Version 0.97.0 - October 18 2012
  - Added a narrow-band level set interface tracking tool (up to fifth-order in space but currently only first-order in time, with higher temporal orders to be added soon).
  - Added a level set filter tool to perform unrestricted surface smoothing (e.g., Laplacian flow), filtering (e.g., mean value) and morphological operations (e.g., morphological opening).
  - Added adaptivity to the level set meshing tool for faster mesh extraction with fewer polygons, without postprocessing.
  - Added a ValueAccessor::touchLeaf method that creates (if necessary) and returns the leaf node containing a given voxel. It can be used to preallocate leaf nodes over which to run parallel algorithms.
  - Fixed a bug in Grid::merge whereby active tiles were sometimes lost.
  - Added LeafManager, which is similar to `LeafArray` but supports a dynamic buffer count and allocates buffers more efficiently. Useful for temporal integration (e.g., for level set propagation and interface tracking), `LeafManager` is meant to replace `LeafArray`, which will be deprecated in the next release.
  - Added a LeafNode::fill method to efficiently populate leaf nodes with constant values.
  - Added a Tree::visitActiveBBox method that applies a functor to the bounding boxes of all active tiles and leaf nodes and that can be used to improve the performance of ray intersection tests, rendering of bounding boxes, etc.
  - Added a Tree::voxelizeActiveTiles method to densify active tiles. While convenient and fast, this can produce large dense grids, so use it with caution.
  - Repackaged `Tree::pruneLevelSet()` as a Tree::pruneOp()-compatible functor. Tree::LevelSetPrune is a specialized Tree::pruneInactive for level-set grids and is used in interface tracking.
  - Added a GridBase::pruneGrid() method.
  - Added a Grid:hasUniformVoxels method.
  - Renamed `tools::dilate` to dilateVoxels and improved its performance. The new name reflects the fact that the current implementation ignores active tiles.
  - Added a tools::resampleToMatch function that resamples an input grid into an output grid with a different transform such that, after resampling, the input and output grids coincide, but the output grid’s transform is preserved.
  - Significantly improved the performance of depth-bounded value iterators (ValueOnIter, ValueAllIter, etc.) when the depth bound excludes leaf nodes.
  - Exposed the value buffers inside leaf nodes with LeafNode::buffer. This allows for very fast access (const and non-const) to voxel values using linear array offsets instead of coordinates.
  - In openvdb_houdini/UT_VDBTools.h, added operators for use with `processTypedGrid` that resample grids in several different ways.
  - Added a policy mechanism to `houdini_utils::OpFactory` that allows for customization of operator names, icons, and Help URLs.
  - Renamed many of the Houdini SOPs to make the names more consistent.
  - Added an Advect Points SOP.
  - Added a Level Set Filter SOP that allows for unrestricted surface deformations, unlike the older Filter SOP, which restricts surface motion to the initial narrow band.
  - Added staggered vector sampling to the Sample Points SOP.
  - Added a minimum radius threshold to the particle voxelization tool and SOP.
  - Merged the Composite and CSG SOPs into a single Combine SOP.
  - Added a tool and a SOP to efficiently generate narrow-band level set representations of spheres.
  - In the Visualize SOP, improved the performance of tree topology generation, which is now enabled by default.

  Version 0.96.0 - September 24 2012
  - Fixed a memory corruption bug in the mesh voxelizer tool.
  - Temporarily removed the optional clipping feature from the level set mesher.
  - Added "Staggered Vector Field" to the list of grid classes in the Create SOP.

  Version 0.95.0 - September 20 2012
  - Added a quad meshing tool for higher-quality level set meshing and updated the Visualizer SOP to use it.
  - Fixed a precision error in the mesh voxelizer and improved the quality of inside/outside voxel classification. Output grids are now also classified as either level sets or fog volumes.
  - Modified the GridResampler to use the signed flood fill optimization only on grids that are tagged as level sets.
  - Added a quaternion class to the math library and a method to return the trace of a `Mat3`.
  - Fixed a bug in the ValueAccessor copy constructor that caused the copy to reference the original.
  - Fixed a bug in RootNode::setActiveState that caused a crash when marking a (virtual) background voxel as inactive.
  - Added a `Tree::pruneLevelSet` method that is similar to but faster than Tree::pruneInactive() for level set grids.
  - Added fast leaf node voxel access methods that index by linear offset (as returned by ValueIter::pos) instead of by coordinates.
  - Added a Tree::touchLeaf method that can be used to preallocate a static tree topology over which to safely perform multithreaded processing.
  - Added a grain size argument to `LeafArray` for finer control of parallelism.
  - Modified the Makefile to make it easier to omit the `doc`, `vdb_test` and `vdb_view` targets.
  - Added utility functions (in `houdini/UT_VDBUtils.h`) to convert between Houdini and OpenVDB matrix and vector types. [Contributed by SESI]
  - Added accessors to `GEO_PrimVDB` that make it easier to directly access voxel data and that are used by the HScript volume expression functions in Houdini 12.5. [Contributed by SESI]
  - As of Houdini 12.1.77, the native transform SOP operates on OpenVDB primitives. [Contributed by SESI]
  - Added a Convert SOP that converts OpenVDB grids to Houdini volumes and vice-versa.

  Version 0.94.1 - September 7 2012
  - Fixed bugs in RootNode and InternalNode `setValue*`() and `fill()` methods that could cause neighboring voxels to become inactive.
  - Fixed a bug in Tree::hasSameTopology that caused false positives when only active states and not values differed.
  - Added a Tree::hasActiveTiles method.
  - For better cross-platform consistency, substituted bitwise AND operations for right shifts in the ValueAccessor hash key computation.
  - `vdb_view` no longer aborts when asked to surface a vector-valued grid—but it still doesn’t render the surface.
  - Made various changes for Visual C++ compatibility. [Contributed by SESI]
  - Added an option to the MeshVoxelizer SOP to convert both open and closed surfaces to unsigned distance fields.
  - The Filter SOP now allows multiple filters to be applied in user-specified order.

  Version 0.94.0 - August 30 2012
  - Added a method to union just the active states of voxels from one grid with those of another grid of a possibly different type.
  - Fixed an incorrect scale factor in the Laplacian diffusion filter.
  - Fixed a bug in Tree::merge that could leave a tree with invalid value accessors.
  - Added TreeValueIteratorBase::setActiveState and deprecated `setValueOn`.
  - Removed `tools/FastSweeping.h`. It will be replaced with a much more efficient implementation in the near future.
  - ZLIB compression of `.vdb` files is now optional, but enabled by default. [Contributed by SESI]
  - Made various changes for Clang and Visual C++ compatibility. [Contributed by SESI]
  - The MeshVoxelizer SOP can now transfer arbitrary point and primitive attribute values from the input mesh to output voxels.

  Version 0.93.0 - August 24 2012
  - Renamed symbols in math/Operators.h to avoid ambiguities that GCC 4.4 reports as errors.
  - Simplified the API for the stencil version of the closest-point transform operator.
  - Added logic to io::Archive::readGrid to set the grid name metadata from the descriptor if the metadata doesn’t already exist.
  - Added guards to prevent nesting of `openvdb_houdini::Interrupter::start()` and `end()` calls.

  Version 0.92.0 - August 23 2012
  - Added a Laplacian diffusion filter.
  - Fixed a bug in the initialization of the sparse contour tracer that caused mesh-to-volume conversion to fail in certain cases.
  - Fixed a bug in the curvature stencil that caused mean curvature filtering to produce wrong results.
  - Increased the speed of the GridTransformer by as much as 20% for fog volumes.
  - Added optional pruning to the Resample SOP.
  - Modified the PointSample SOP to allow it to work with ungrouped, anonymous grids.
  - Fixed a crash in the LevelSetNoise SOP.

  Version 0.91.0 - August 16 2012
  - tools::GridTransformer and tools::GridResampler now correctly (but not yet efficiently) process tiles in sparse grids.
  - Added an optional CopyPolicy argument to GridBase::copyGrid and to Grid::copy that specifies whether and how the grid’s tree should be copied.
  - Added a GridBase::newTree method that replaces a grid’s tree with a new, empty tree of the correct type.
  - Fixed a crash in Tree::setValueOff when the new value was equal to the background value.
  - Fixed bugs in Tree::prune() that could result in output tiles with incorrect active states.
  - Added `librt` to the link dependencies to address build failures on Ubuntu systems.
  - Made various small changes to the Makefile and the source code that should help with Mac OS X compatibility.
  - The Composite and Resample SOPs now correctly copy the input grid’s metadata to the output grid.

  Version 0.90.1 - August 7 2012
  - Fixed a bug in the BBox::getCenter() method.
  - Added missing header files to various files.
  - io::File::NameIterator::gridName() now returns a unique name of the form `"name[1]"`, `"name[2]"`, etc. if a file contains multiple grids with the same name.
  - Fixed a bug in the Writer SOP that caused grid names to be discarded.
  - The Resample SOP now correctly sets the background value of the output grid.

  Version 0.90.0 - August 3 2012 (initial public release)
  - Added a basic GL viewer for OpenVDB files.
  - Greatly improved the performance of two commonly-used `Tree` methods, evalActiveVoxelBoundingBox() and memUsage().
  - Eliminated the `GridMap` class. File I/O now uses STL containers of grid pointers instead.
  - Refactored stencil-based tools (Gradient, Laplacian, etc.) and rewrote some of them for generality and better performance. Most now behave correctly for grids with nonlinear index-to-world transforms.
  - Added a library of index-space finite difference operators.
  - Added a Hermite grid type that compactly stores each voxel’s upwind normals and can be used to convert volumes to and from polygonal meshes.
  - Added a tool (and a Houdini SOP) to scatter points randomly throughout a volume.