USDZExporter: Store only unique geometry and reference it

[qeeqez]

Fixed #21748

Description

Now we are storing only unique geometries in different geometry.usd file.
model.usda have only references to geometries (meshes).

Also, geometry do not store material references. Xforms from model.usda now do it.
It could help us with storing of many identical objects but with different materials binded

[mrdoob]

Excellent! Thanks!

[mrdoob]

Now we are storing only unique geometries in different geometry.usd file.

Out of curiosity, why do we need to store them in a different file?

[qeeqez]

Now we are storing only unique geometries in different geometry.usd file.

Out of curiosity, why do we need to store them in a different file?

If we'll add geometry to the "output" - it will be instantiated in zero point. And we will see geometry itself in zero point + instantiated objects in correct places.

We have multiple solutions:

1. Make geometry invisible.
   But overriding this invisible state in the instance do not work thus we could not see instances ¯\_(ツ)_/¯
2. Store geometry in the first object's instance and then reference it.
   Good solution but we'll have some problems with overriding materials and etc.
3. Store geometry in a different file. This one is merged.
   Pixar's USD examples uses this approach in many places so I think it's a best solution.
   • Some complex example
   • Simplier example
     p.s. looks like they are storing every geometry in a separate file: we could do the same so we'll further improve memory usage during file creation. But final USDZ will be a little bit bigger as every .usd file should have a header on the top.

Note: not all of the features of USDZ format are implemented or work in AR QuickLook.
We could have some solutions working in the Xcode AR View but not in the AR Quick Look

[mrdoob]

p.s. looks like they are storing every geometry in a separate file: we could do the same so we'll further improve memory usage during file creation.

Yes, I was thinking if that would be better. I found it weird when I tested this and I saw the geometry.usd file with all geometries on top of each other. Makes it hard to debug.

But final USDZ will be a little bit bigger as every .usd file should have a header on the top.

I think it'd be worth it.

[qeeqez]

@mrdoob
Question about file structure we want to have:

Now	Proposed
model.usda def Xform "Object_ID" references = @./geometry.usd@</Geometry_ID>	model.usda def Xform "Object_ID" references = @./geometries/Geometry_ID.usd@
geometry.usd def "Geometry_ID" def Mesh "Geometry_ID"	geometry_ID.usd def "Geometry" def Mesh "Geometry"

Basically my proposal - store geometry ID in file names (similar to current textures storage). And inside the file we will only use "Geometry" word without ID.

[mrdoob]

The proposed approach looks great to me 👍