Fix/Refactor NLA Track library override #105272

New Issue

When refactoring the NLA create track methods, some questions came up about library overrides.

Mainly:

What defines an NLA Track as a library override?
What behavior differs when the NLA tracks in the Animation Data are updated (new track added, removed, ordered, etc)

Currently, every newly created NLA track has the NLATRACK_OVERRIDELIBRARY_LOCAL flag set by default. It's not super clear why this flag is set, or how it indicates a library override.

The second open question, is how a library override impacts the NLA tracks. This comment suggests that the library override tracks need to be grouped together at the front of the list, and anything local should be after that. The codes doesn't seem to enforce this however, as tracks can be inserted before/after each other, regardless of library override status.

Next step

@dr.sybren and I decided that we should open this discussion to the larger community for feedback. We'll be discussing this issue in one of the upcoming Animation & Rigging module meetings.

When refactoring the [NLA create track methods](https://projects.blender.org/blender/blender/pulls/104929), some questions came up about library overrides. Mainly: 1. What defines an NLA Track as a library override? 2. What behavior differs when the NLA tracks in the Animation Data are updated (new track added, removed, ordered, etc) Currently, every newly created NLA track has the [NLATRACK_OVERRIDELIBRARY_LOCAL](https://projects.blender.org/blender/blender/src/branch/main/source/blender/blenkernel/intern/nla.c#L352) flag set by default. It's not super clear why this flag is set, or how it indicates a library override. The second open question, is how a library override impacts the NLA tracks. [This comment](https://projects.blender.org/blender/blender/src/branch/main/source/blender/blenkernel/intern/nla.c#L352) suggests that the library override tracks need to be grouped together at the front of the list, and anything local should be after that. The codes doesn't seem to enforce this however, as tracks can be inserted before/after each other, regardless of library override status. --- ### Next step @dr.sybren and I decided that we should open this discussion to the larger community for feedback. We'll be discussing this issue in one of the upcoming [Animation & Rigging module meetings](https://stuvel.eu/anim-meetings/).

Maybe #105260 could be tackled at the same time, under the umbrella of "properly working with linked data".

For context, systematically setting NLATRACK_OVERRIDELIBRARY_LOCAL is done to avoid as much Editor code as possible to need to be aware that it's working on liboverride data. The idea is that:

This flag is 100% ignored in purely local data;
This flag is 100% cleared in linked data;
That way, in liboverrides (which are local data), tracks that come from (are copied from) the linked ID do not have that flag, and tracks that have been added in the override have it.

This behavior is shared between all collections of items that support liboverride insertions (constraints, the various modifiers, and so on).

While it may look confusing, it has the key advantages of being both simple, and extremely robust (in the sense that it is almost impossible for a track to be wrongly tagged as linked or local, while only requiring a very few functions to have control over the flag). I would be most happy if a better system could be worked on though, but that would be a liboverride design topic covering all use-cases, not only NLA tracks.

(Added some precisions about this in the liboverride code doc).

Regarding ordering, all 'liboverride-insertable' collections share the same limitation, which is that linked data is not re-orderable. Local data however should be, so you can e.g. move a local-inserted modifier in-between two 'linked' modifiers, but you cannot change the order of these linked modifiers.

NLA tracks have additional limitations due to the fact that their names are not unique currently, which means that override diffing and applying code cannot use these names as reference. Since using indices for that purpose is extremely unreliable (insert local track 2 after track 5, good luck keeping this doing what it is supposed to do if anything changes in the linked data's NLA tracks).

To work around that, the additional limitation is that all linked tracks come first, and local-inserted ones are always kept after all linked ones.

For context, systematically setting `NLATRACK_OVERRIDELIBRARY_LOCAL` is done to avoid as much Editor code as possible to need to be aware that it's working on liboverride data. The idea is that: * This flag is 100% ignored in purely local data; * This flag is 100% cleared in linked data; That way, in liboverrides (which are local data), tracks that come from (are copied from) the linked ID do not have that flag, and tracks that have been added in the override have it. This behavior is shared between all collections of items that support liboverride insertions (constraints, the various modifiers, and so on). While it may look confusing, it has the key advantages of being both simple, and extremely robust (in the sense that it is almost impossible for a track to be wrongly tagged as linked or local, while only requiring a very few functions to have control over the flag). I would be most happy if a better system could be worked on though, but that would be a liboverride design topic covering all use-cases, not only NLA tracks. *(Added some precisions about this in the [liboverride code doc](https://wiki.blender.org/wiki/Source/Architecture/Overrides/Library)).* ------------------- Regarding ordering, all 'liboverride-insertable' collections share the same limitation, which is that linked data is not re-orderable. Local data however should be, so you can e.g. move a local-inserted modifier in-between two 'linked' modifiers, but you cannot change the order of these linked modifiers. NLA tracks have additional limitations due to the fact that their names are not unique currently, which means that override diffing and applying code cannot use these names as reference. Since using indices for that purpose is extremely unreliable (`insert local track 2 after track 5`, good luck keeping this doing what it is supposed to do if anything changes in the linked data's NLA tracks). To work around that, the additional limitation is that all linked tracks come first, and local-inserted ones are always kept after all linked ones.

Thanks for the clarification @mont29.

This was discussed in last week's module meeting, and decided to be put on hold until it blocks other development.

This was discussed in last week's [module meeting](https://devtalk.blender.org/t/2023-03-23-animation-rigging-module-meeting/28313), and decided to be put on hold until it blocks other development.

Download

What's New

Blender Studio

Manual

Developers Blog

Documentation

Benchmark

Blender Conference

Development Fund

One-time Donations

Fix/Refactor NLA Track library override #105272

Next step