Reference/Release Notes/2.81/Library Overrides

= Blender 2.81: Library Overrides =

Library Overrides is the new system designed to replace and supersede Proxies. Most types of linked data-blocks can be overridden, and the properties of those overrides can then be edited. When the library data change, unmodified properties of the override one will be updated accordingly.

It is considered as experimental, and current proxy system will be kept alongside for a few releases. Automatic conversion from proxies to library overrides is not planned, there will be a manual operator to do that, but results on complex characters are not guaranteed.

Compared to proxies, library overrides support:
 * Multiple independent overrides of a same linked data (e.g. a whole character).
 * Adding new modifiers and constraints, anywhere in the stack.


 * Recursively chaining overrides (i.e. link and override overrides from another library file, etc.).

Please see the reference manual for more info about library overrides usage.

Technical Notes on Implementation
Internally, overrides are local data-blocks that are copied from and keep a reference to their linked source. They also keep a list of locally modified properties (using RNA paths).

When saving the .blend file, both data-blocks (local override and linked source) are diff'ed, and all allowed differences are stored in the .blend file.

When reading the .blend file, linked source are duplicated again into local overrides, and then all the diffing data is re-applied on the overrides.

This implies that even if you modify a "forbidden" property, on next file reload it will be restored to the linked data-block's value.

Known Limitations and Issues
Many more things could/will be made overridable. Focus has been on making overriding of characters for posing/animation working first, as an initial real-life test case, and direct proxy replacement.

Note that driven shape keys won't be working 'out of the box' currently (see for details).

Since overrides are basically local data-blocks for most of Blender, especially for tools and operators, there are still a lot of cases that can lead to editing non-overridable properties. While those cases are being hunted down, remember that any forbidden editing will be reset to the linked data status on next file reload anyway.

Performances, especially of the "diffing" process (where Blender compares override and linked source data, and generate the needed data to store the overrides), can be slow. By default this is ran on most of the undo steps, which can make working on heavy production scenes painful. The easiest way to work around that issue currently is to disable the 'auto override' feature, at least for the Armature objects (from python console, see [the API doc https://docs.blender.org/api/master/bpy.types.IDOverrideLibrary.html#bpy.types.IDOverrideLibrary.auto_generate]). This will prevent immediate automatic detection of edited (and hence overridden) properties for those data-blocks, which will only happen on file save.

As with proxies, in case a link to a library is broken, using the Relocate feature (in the Outliner) won't work really well, bone shapes e.g. are likely to be lost... Some data are currently lost in the "place-holder" step of missing data-blocks.

Overriding a geometry object (especially a mesh) copies locally all of the data, even though geometry itself is not (and will never be) overridable.

Future Ideas
There are several potential future extensions/improvements to library overrides, which are currently not exposed to the user (though some are partially implemented):
 * Differential overrides (for scaling e.g., store the difference as a factor of the original value, instead of an absolute value).
 * Protected/locked properties: forbid (by default) to edit some properties in the override data-blocks.
 * Templates: define in the library files how a given data-block should be overridden by default (which properties should be overridden, which should be protected from modifications, etc.).