These are generic properties of grids (not stored in voxels) which are
useful to know in geometry nodes. The transform in particular defines
the voxel size. Background value is used outside of active voxels.
Pull Request: https://projects.blender.org/blender/blender/pulls/138592
This changes how the lazy-loading and unloading of volume grids works. With that
it should also fix#124164.
The cache is now moved to a deeper and more global level. This allows reloadable
volume grids to be unloaded automatically when a memory limit is reached. The
previous system for automatically unloading grids only worked in fairly specific
cases and also did not work all that well with caching (parts of) volume
sequences.
At its core, this patch adds a general cache system in `BLI_memory_cache.hh`. It
has a simple interface of the form `get(key, compute_if_not_cached_fn) ->
value`. To avoid growing the cache indefinitly, it uses the new
`BLI_memory_counter.hh` API to detect when the cache size limit is reached. In
this case it can automatically free some cached values. Currently, this uses an
LRU system, where the items that have not been used in a while are removed
first. Other heuristics can be implemented too, but especially for caches for
loading files from disk this works well already.
The new memory cache is internally used by `volume_grid_file_cache.cc` for
loading individual volume grids and their simplified variants. It could
potentially also be used to cache which grids are stored in a file.
Additionally, it can potentially also be used as caching layer in more places
like loading bakes or in import geometry nodes. It's not clear yet whether this
will need an extension to the API which currently is fairly minimal.
To allow different systems to use the same memory cache, it has to support
arbitrary identifiers for the cached data. Therefore, this patch also introduces
`GenericKey`, which is an abstract base class for any kind of key that is
comparable, hashable and copyable.
The implementation of the cache currently relies on a new `ConcurrentMap`
data-structure which is a thin wrapper around `tbb::concurrent_hash_map` with a
fallback implementation for when `tbb` is not available. This data structure
allows concurrent reads and writes to the cache. Note that adding data to the
cache is still serialized because of the memory counting.
The size of the cache depends on the `memory_cache_limit` property that's
already shown in the user preferences. While it has a generic name, it's
currently only used by the VSE which is currently using the `MEM_CacheLimiter`
API which has a similar purpose but seems to be less automatic, thread-safe and
also has no idea of implicit-sharing. It also seems to be designed in a way
where one is expected to create multiple "cache limiters" each of which has its
own limit. Longer term, we should probably strive towards unifying these
systems, which seems feasible but a bit out of scope right now. While it's not
ideal that these cache systems don't use a shared memory limit, it's essentially
what we already have for all cache systems in Blender, so it's nothing new.
Some tests for lazy-loading had to be removed because this behavior is more
implicit now and is not as easily observable from the outside.
Pull Request: https://projects.blender.org/blender/blender/pulls/126411
We often have the situation where it would be good if we could easily estimate
the memory usage of some value (e.g. a mesh, or volume). Examples of where we
ran into this in the past:
* Undo step size.
* Caching of volume grids.
* Caching of loaded geometries for import geometry nodes.
Generally, most caching systems would benefit from the ability to know how much
memory they currently use to make better decisions about which data to free and
when. The goal of this patch is to introduce a simple general API to count the
memory usage that is independent of any specific caching system. I'm doing this
to "fix" the chicken and egg problem that caches need to know the memory usage,
but we don't really need to count the memory usage without using it for caches.
Implementing caching and memory counting at the same time make both harder than
implementing them one after another.
The main difficulty with counting memory usage is that some memory may be shared
using implicit sharing. We want to avoid double counting such memory. How
exactly shared memory is treated depends a bit on the use case, so no specific
assumptions are made about that in the API. The gathered memory usage is not
expected to be exact. It's expected to be a decent approximation. It's neither a
lower nor an upper bound unless specified by some specific type. Cache systems
generally build on top of heuristics to decide when to free what anyway.
There are two sides to this API:
1. Get the amount of memory used by one or more values. This side is used by
caching systems and/or systems that want to present the used memory to the
user.
2. Tell the caller how much memory is used. This side is used by all kinds of
types that can report their memory usage such as meshes.
```cpp
/* Get how much memory is used by two meshes together. */
MemoryCounter memory;
mesh_a->count_memory(memory);
mesh_b->count_memory(memory);
int64_t bytes_used = memory.counted_bytes();
/* Tell the caller how much memory is used. */
void Mesh::count_memory(blender::MemoryCounter &memory) const
{
memory.add_shared(this->runtime->face_offsets_sharing_info,
this->face_offsets().size_in_bytes());
/* Forward memory counting to lower level types. This should be fairly common. */
CustomData_count_memory(this->vert_data, this->verts_num, memory);
}
void CustomData_count_memory(const CustomData &data,
const int totelem,
blender::MemoryCounter &memory)
{
for (const CustomDataLayer &layer : Span{data.layers, data.totlayer}) {
memory.add_shared(layer.sharing_info, [&](blender::MemoryCounter &shared_memory) {
/* Not quite correct for all types, but this is only a rough approximation anyway. */
const int64_t elem_size = CustomData_get_elem_size(&layer);
shared_memory.add(totelem * elem_size);
});
}
}
```
Pull Request: https://projects.blender.org/blender/blender/pulls/126295
The openvdb API was used incorrectly before. I didn't notice that this is
a static function and requires the type to be passed in.
Note that this does not solve the cache invalidation issue yet. Instead
it handles the case more gracefully when the stored .vdb file changes
when Blender doesn't expect it to change.
There was one functional issue with the previous API which was its
use in `VolumeGrid<T>::grid_for_write(tree_token)`. The issue was
that the tree token had to be received before the grid is accessed.
However, this `grid_for_write` method might create a copy of the
`VolumeGridData` internally and if it does, the passed in `tree_token`
corresponds to the wrong tree.
The solution is to output the token as part of the method. This has two
additional benefits:
* The API is more safe, because one can't pass an r-value into the methods
anymore. This generally shouldn't be done, because the token should
live at least as long as the OpenVDB tree is used and shouldn't be freed
immediatly.
* The API is a bit simpler, because it's not necessary to call the
`VolumeGrid.tree_access_token()` method anymore.
This refactors how volume grids are stored with the following new goals in mind:
* Get a **stand-alone volume grid** data structure that can be used by geometry nodes.
Previously, the `VolumeGrid` data structure was tightly coupled with the `Volume` data block.
* Support **implicit sharing of grids and trees**. Previously, it was possible to share data
when multiple `Volume` data blocks loaded grids from the same `.vdb` files but this was
not flexible enough.
* Get a safe API for **lazy-loading and unloading** of grids without requiring explicit calls
to some "load" function all the time.
* Get a safe API for **caching grids from files** that is not coupled to the `Volume` data block.
* Get a **tiered API** for different levels of `openvdb` involvement:
* No `OpenVDB`: Since `WITH_OPENVDB` is optional, it's helpful to have parts of the API that
still work in this case. This makes it possible to write high level code for volumes that does
not require `#ifdef WITH_OPENVDB` checks everywhere. This is in `BKE_volume_grid_fwd.hh`.
* Shallow `OpenVDB`: Code using this API requires `WITH_OPENVDB` checks. However, care
is taken to not include the expensive parts of `OpenVDB` and to use forward declarations as
much as possible. This is in `BKE_volume_grid.hh` and uses `openvdb_fwd.hh`.
* "Full" `OpenVDB`: This API requires more heavy `OpenVDB` includes. Fortunately, it turned
out to be not necessary for the common API. So this is only used for task specific APIs.
At the core of the new API is the `VolumeGridData` type. It's a wrapper around an
`openvdb::Grid` and adds some features on top like implicit sharing, lazy-loading and unloading.
Then there are `GVolumeGrid` and `VolumeGrid` which are containers for a volume grid.
Semantically, each `VolumeGrid` has its own independent grid, but this is cheap due to implicit
sharing. At highest level we currently have the `Volume` data-block which contains a list of
`VolumeGrid`.
```mermaid
flowchart LR
Volume --> VolumeGrid --> VolumeGridData --> openvdb::Grid
```
The loading of `.vdb` files is abstracted away behind the volume file cache API. This API makes
it easy to load and reuse entire files and individual grids from disk. It also supports caching
simplify levels for grids on disk.
An important new concept are the "tree access tokens". Whenever some code wants to work
with an openvdb tree, it has to retrieve an access token from the corresponding `VolumeGridData`.
This access token has to be kept alive for as long as the code works with the grid data. The same
token is valid for read and write access. The purpose of these access tokens is to make it possible
to detect when some code is currently working with the openvdb tree. This allows freeing it if it's
possible to reload it later on (e.g. from disk). It's possible to free a tree that is referenced by
multiple owners, but only no one is actively working with. In some sense, this is similar to the
existing `ImageUser` concept.
The most important new files to read are `BKE_volume_grid.hh` and `BKE_volume_grid_file_cache.hh`.
Most other changes are updates to existing code to use the new API.
Pull Request: https://projects.blender.org/blender/blender/pulls/116315
