Blender Git Loki

Support for CustomData in Alembic caches.

This is a skeleton feature that provides a general way of storing
CustomData types. Currently only ORIGINDEX layers are implemented, the
code is designed to make extension easy.

Storing CustomData layers in Alembic is a bit involved because the
complex structs often used as customdata need to be de-interleaved for
Alembic into a set of POD (plain-old-datatype) array properties.

In addition the property names should be unique, so that mapping abc
properties back to customdata layers is safe. This works by using 2
levels of compound properties: the first level stored per CD type
properties, with a number of properties for each layer of this type,
using either a name or index (for unnamed layers). The internal
properties can then in turn be compounds, if structs need to be
serialized into PODs.

Abc property readers/writers are created dynamically for the CustomData
compounds. This is necessary because we don't know in advance what kind
of data layers a DerivedMesh or other CD user will have, and this can
change each frame in theory. Alembic is easier with state data schemes,
but using it this way is possible (if somewhat cumbersome).