Use the Usd Asset Preview structure with the wg assets

[meshula]

Asset Previews in USD are described here:

https://openusd.org/release/wp_asset_previews.html

Fast, scalable browsing of numerous USD assets is a concern that grows in importance as the number of readily available assets grows. On many platforms it will be impractical to load and render all assets in a directory just-in-time to generate preview/thumbnail views, even using background threads and caching. We therefore propose an encoding in USD for specifying pre-generated, lightweight preview assets for USD scenes, that can be recovered from a USD asset with small expense.

We should undertake a few things, in an ordered set of PRs.

1. Create a "canonical" asset preview that shows how things work, and some best practices.
2. In the assets submission process, one of the check boxes should be that the asset has a preview structure set up, or an explanation of why a preview isn't useful.
3. Our automation can generate preview images; it could also set up the asset preview structure if necessary. See

   assets/scripts/thumbnail-generator/generate_thumbnail.py

   Line 271 in 907d5f1

   def link_image_to_subject(subject_stage, image_name):

   for a place already leveraging the preview system.
4. We should work through the existing assets, adding previews as necessary. Perhaps organized versus a spreadsheet.

[beersandrew]

@meshula for the canonical example, it seems we'd want both cards & thumbnails, right? Also for the canonical example, what's the preferred way to represent this in the repo, I can think of 3 ways:

1. a new usdz file with an updated usd file + all cards & thumbnail
2. a new usda file that references the existing repo asset + existing all cards & thumbnail
3. update the existing asset to reference the existing cards & thumbnails

seems like step 4 implies the third approach, does that make sense to you?

[meshula]

One thought: a nice aspect of a usda based sample is that it would easy to read and browse on the web here.