Fix color artifacts in PointCloud projection due to CUDA race condition

[ManuCorrea]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

It solves the artifacts described in #6612 which happen to be because a race condition on the CUDA code.

The existing ProjectCUDA implementation suffered from a race condition in the color assignment pass. Multiple threads with similar depths in the color assignment pass (within the precision_bound) could simultaneously pass the depth test and write to the RGB color buffer, leading to the artifacts shown in the mentioned issue.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

To fix it I implemented a buffer to keep track in the pass 1 of which worker should write to the color tensor in the pass 2.

Pass 1 (Depth & Index Acquisition):

• Packs the depth (32-bit float bits) into the High Bits and the Point Index (32-bit workload_idx) into the Low Bits.
• Uses atomicMin on the 64-bit value. This ensures an unique worker for every pixel based on the minimum depth.

Pass 2 (Deterministic Color Assignment):

• Each thread reads the 64-bit winner from the index buffer.
• By applying a 32-bit mask (& 0xFFFFFFFF), we extract the unique ID.
• Only the thread whose workload_idx matches the ID writes to the color tensor.

Old graph from the issue:

My results:

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[ManuCorrea]

I will add testing and the changes in CHANGELOG.md in the following days.