Problem with Arnold USD instancing in Bifrost.

I suspect like the other couple of bugs I posted here that this is more of a Bifrost bug than a USD bug, but this one is very important to me. I've used Arnold "standins" functionality to insert geometry into my Maya scenes for years, it's like a pre-USD functionality to handle large datasets in Maya. When USD was added to Maya, Arnold added the ability within Maya to "save selection as an Arnold USD" file, in addition to the .ASS file format of the previous Arnold instance reference archives.

These Arnold USD files follow the Maya scene descriptor hierarchy, including incorporating the Maya convention of making the meshes child objects of "shape nodes", and they include all the shader, gamma, and Arnold settings in the files.

The Arnold USD files can be used like the ASS files within Maya referenced by Arnold "Standins", which is very handy, and something I use all the time in my scenes. The Arnold USD files also work and render fine as USD files within Maya using the create stage from file option, and can then be used with all the new Maya USD functionality.

My problem is using them in Bifrost. These Arnold USD files render fine with all shaders in place in Bifrost when referenced as sublayers to a parent layer attached to a Stage, but they don't work with the instancing functionality currently implemented in Maya Bifrost.

In Bifrost USD instancing, the "define_usd_reference_from_file" node has to be used to add the path to these files - and that node doesn't handle the "extra" stuff in the Arnold USD file properly, and ends up completely failing to read the file, resulting in Arnold USD instances within Bifrost not being displayed or rendering.

To reproduce the problem follow these steps: 1. Create a Maya scene with a global light in Arnold, and an object with an Arnold Standard Surface shader with a texture. 2. Select the object, and export it using the Arnold USD file format (which is the format which works fine with Arnold "Standins" in Maya.) 3. Reference the Arnold USD file using an Arnold Standin in Maya and see how it renders. (It will/should render exactly the same as the original object) 4. Reference within a Bifrost graph the Arnold USD file using the standard method as a single file via a Stage, Layer container, and finally the file itself via a sublayer. 5. Render the image. It looks the same as the original object. 6. Instance the Arnold USD file using a Bifrost USD instancer node. 7. Render the image. (It will be blank, as not even previews work for Arnold USD files.)

I've found that using the Maya "export USD" of the selected file rather than the "Arnold USD" file results in USD archives that work in the Bifrost USD instancing, and render with shaders in place, but they do not react correctly to scene lighting, and do not of course follow all the extra Arnold rendering options like denoising settings. Looking at the USD files in the Maya proxy node in the Outliner shows the outstanding difference between a "regular USD" Maya export, and an "Arnold USD" export is that the base mesh in the regular USD is exposed directly in the scene hierarchy with no "shape node", whereas the Arnold USD has the mesh as a child of a "shape node".

Currently the big reason why this is so important to me is that whether I use the Bifrost Arnold USD instancing or not in my scene, I still continue to want to use the Arnold "Standin" functionality because it is so quick and convenient for dealing with single instances of heavy geometry within Maya. I guess I could switch completely over the USD and its proxies, perhaps that is what I should be aiming to do, but I am left with the dilemma of how to easily create these "new" USD files with included proxies within the Maya UI.

Currently making Arnold "standins" of selected heavy objects is just a single action (which I've assigned a key shortcut because I use it so often.) The obvious short term solution is for the Bifrost USD instancing to read these Arnold USD files correctly, but going forward, it seems that a logical step for Maya is for it to replicate the simplicity & efficiency of Arnold instances within the Maya USD UI functionality ie providing an easy way to export USD geometry that automatically builds a proxy object with all the Arnold shading and rendering options included for later use in instancing and referencing within the Maya USD functionality.

One extra observation is that by including a way to incorporate Arnold ":Standins" in the Bifrost graph would allow those who have built up large libraries of Arnold format archives to use them within the new Bifrost workflows.