Adds the option `merge_parent_xform` to the USD export operator and
panels so the transform and shapes are merged into a single USD Prim.
Without the option (existing default), a top-level mesh would be
exported as a top-level `Xform` that has a `Mesh` child:
```
def Xform "MyBlenderMeshObject"
{
matrix4d xformOp:transform = ...
def Mesh "MyBlenderMeshData"
{
}
}
```
This matches the Blender data model, where a transformable object
contains a geometric shape (like a mesh). This structure is also very
valid in USD, where we don't want to directly instantiate geometric
primitives[1]
However, "since number of prims on a stage is one of the primary factors
that governs how USD scale"[2], to reduce the number of prims in a
stage, geometric primitives *are transformable* themselves (see the
inheritence diagram[3]).
As such, the new export option allows to export geometric primitives
without the parent transform:
```
def Mesh "MyBlenderMeshObject"
{
matrix4d xformOp:transform = ...
}
```
This MR adds a the `is_object_data_context` flag to the
`HierarchyContext` context structure. The point of this change is to
make unambiguous in `USDHierarchyIterator::create_usd_export_context`
the fact that an `object` or a `data` is currently being exported: the
new `merge_parent_xform` option is meaningless for `object`. Only `data`
can be exported with a parent `Xform` or as a child of said `Xform`.
Ideally this flag would not be needed at all: the final USD prim path
*could* be computed in an override of the virtual
`AbstractHierarchyIterator::get_object_data_path` method. However, this
would mean that an `object` and a `data` would have the same export path.
This does not currently play well with
`AbstractHierarchyIterator::ensure_writer`, where `writers` are cached
*by their export path*: it would cache a transform writer, but will skip
the subsequent data writer.
Additionally, another new `is_parent` flag is added to handle the case
where merging the Xform is invalid: objects that are parents to other
objects should remain unmerged as otherwise that would yield invalid USD
files.
[1] https://openusd.org/release/glossary.html#usdglossary-gprim
[2] https://openusd.org/release/glossary.html#usdglossary-instancing
[3] https://openusd.org/release/api/class_usd_geom_xformable.html
Co-authored-by: Odréanne Breton <odreanne.breton@ubisoft.com>
Co-authored-by: Sttevan Carnali Joga <sttevan.carnali-joga@ubisoft.com>
Co-authored-by: Charles Flèche <charles.fleche@ubisoft.com>
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