[Bf-usd] Collections for USD

[Brecht Van Lommel]

Hi Michael,

On Fri, Aug 28, 2020 at 10:43 PM Michael A Kowalski <makowalski at nvidia.com>
wrote:

> Thank you very much for sharing the design document regarding using
> collections as part of the USD integration in Blender (
> https://developer.blender.org/T68933 ).  We have some follow-up questions
> to better understand the proposed design:
>
>
>
>    - What would the user interface for setting export properties look
>    like?  Would there still be an export dialog or panel for setting global
>    properties (like in the current exporter) and a UI for setting export
>    properties on a given collection as well?  It seems that the ability to
>    apply export settings per collection would be important.  For example, if a
>    collection represents a USD sublayer, one might want to specify a save path
>    for that layer.
>
> The export settings would be per collection. Most likely the properties
editor would get a new tab for settings of the active collection, and the
export settings would be there.


>
>    - The document states that “evolving Blender Collections to match USD
>    Layers more closely would help integrate well with USD”.  Can you elaborate
>    on what this might entail?
>
> There's many things we can do here, but some things that come to mind:

* USD Xform prims are closest to collection instances in Blender, in that
they have a transform and can instance objects. However Blender collection
instances were only designed to instance a collection that already exists
elsewhere, you can't directly edit their contents in the 3D viewport. So
the idea would be to make these collection instances work more like group
objects in other applications, where you can edit their contents in-place.
This would be a major change, but something that users want regardless of
USD.
* Blender has no concept of variants, this should become a native feature
of Blender collections or datablocks in general.
* USD has more flexible definitions of pivot points, transform order, ...
we may need to support that natively in Blender too.
* Blender collections and objects have enable/disable toggles for render
and viewport. Instead we could use something like USD purpose and
active/inactive, also for Blender itself it would be good to have an
explicit distinction between render/proxy/guide objects.


>    - Regarding the point that USD or Alembic files would be “non-editable
>    by default and then support overrides and make local just like linked
>    datablocks”, can you describe some ideas of how this data and UX might be
>    presented to the user?
>
> It would be the same as the existing UX for linked datablocks and
overrides (which is currently undergoing changes in 2.90 and 2.91).

Linked datablocks by default can't be edited, their properties are grayed
out and you can't go into edit mode. If you want to edit them, you
typically go into the outliner, right-click and make an independent local
copy, or make a local copy with overrides. And then from that point you can
edit them just like any other datablock.

Regards,
Brecht.
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