[hdEmbree] Initial UsdLux reference implementation (hdEmbree-UsdLux-PR06)

[pmolodo]

Description of Change(s)

This modifies the hdEmbree example plugin to do direct lighting so as to provide a reference implementation of the expected behaviour for UsdLux.

If no lights are present in the stage, the old ambient occlusion path will be used.

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Related PRs:

This PR is part of a chain of PRs that provide a reference implementation of UsdLux as part of hdEmbree.

◀️ Previous PR in chain:	🔽 This PR changes only:	▶️ Next PR in chain:
#3198	Diff vs Previous Due to a github limitation, this PR must include all previous PRs in the chain. To see JUST the changes in this PR vs the previous in the chain, click this link	#3196

⛓️ All PRs in this chain:

• [hdEmbree] Add UsdLux support to hdEmbree, to make it a reference UsdLux implementation #3199

📃 Documentation PR:
This chain of PRs (implementing UsdLux support in hdEmbree), was made separate from the PR documenting expected UsdLux behavior:

• [UsdLux] Add explanation of UsdLux quantities and behavior #3182

✳️ All UsdLux update related PRs:
To see ALL UsdLux update related PRs (documentation AND reference implementation) in one place, see:

• [UsdLux] [hdEmbree] Update documentation for UsdLux, and make hdEmbree provide a reference implementation #3200

⌛ Original PR:
These PRs are an update of this original PR:

• UsdLux update #2758

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But... why??

Why make these PRs in the first place?

The current specifications of the various UsdLux prims + attributes are imprecise or vague in many places, and as a result, actual implementations of them by various renderers have diverged, sometimes quite significantly. For instance, here is Intel's 4004 Moore Lane scene, with the same UsdLux lights defined, in 3 different renderers:

Karma:

Arnold:

Omniverse RTX:

For a full descpription of the problem, see here:

• Problem Statement

Why so many PRs?
This was my attempt to break up a rather large change into smaller, more easily reviewable changes, that can be merged in incrementally, to help ease the burden for code reviewers.

If you find this confusing, and would rather just one big PR (ie, just this one), or have them organized in some other way, please let me know!

Why are the documentation changes in their own PR?
In some ways, the documentation changes are the heart of this effort - we wish to specify more exactly what the various UsdLux prims and attributes represent. However, building consensus on this may take time - so we expect some dialogue on the exact language or formulas.

Because that may take time, the hdEmbree reference implementation changes are separate, and broken up, so we can hopefully start integrating portions of them even before final consensus has been reached on the final form of the specification.

Why are the documentation changes not broken up into smaller pieces, like the hdEmbree reference implementation changes?
Because I wasn't sure if that would be desirable or not! If people think that would be helpful, I can do so - perhaps breaking out by schema or individual attribute?

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• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9904

[jesschimein]

/AzurePipelines run

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[jesschimein]

/AzurePipelines run

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