Open3D Python cannot find DLLs in add_dll_directory-locations

[fegorsch]

Checklist

• I have searched for similar issues.
• For Python issues, I have tested with the latest development wheel.
• I have checked the release documentation and the latest documentation (for main branch).

Describe the issue

I try to import custom-build Open3D with CUDA on Windows. In __init__.py when open3d/cuda/pybind.cp310-win_amd64.pyd is loaded, open3d/tbb12.dll is not found, although its location has been added to the DLL-search-directories with os.add_dll_directory before.

Workarounds:

1. Copying tbb12.dll into Windows/system32 resolves the problem.
2. Loading tbb12.dll explicitly with CDLL(str(Path(__file__).parent / "tbb12.dll")) in __init__.py before loading cuda/pybind.cp310-win_amd64.pyd resolves the problem.
3. Adding tbb12.dll's location to PATH resolves the problem. I.e. doing os.environ['PATH'] = str(Path(__file__).parent) + ';' + os.environ.get('PATH', '') in __init__.py.

Main question: can anyone reproduce the problem or has any insights into why add_dll_directory seems not to be respected here? If this is a general problem, I'd open a PR that applies workaround 3.

Steps to reproduce the bug

Build Open3D with the software versions stated below and try to import it with `import open3d`.

Error message

I get the warning below. Note that the hint about the CUDA libraries is wrong in my case and only the "Reported error" is relevant.

"Open3D was built with CUDA support, but an error ocurred while loading the Open3D CUDA Python bindings. This is usually because the CUDA libraries could not be found. Check your CUDA installation. Falling back to the CPU pybind library. Reported error: Could not find module 'c:\workspace\visionkit\venv310\lib\site-packages\open3d\cuda\pybind.cp310-win_amd64.pyd' (or one of its dependencies). Try using the full path with constructor syntax."

Expected behavior

pybind.cp310-win_amd64.pyd is loaded without error. I.e. tbb12.dll can be located and loaded during the call to CDLL().

Open3D, Python and System information

- Operating system: Windows 11
- Python version: Python 3.10.11
- Open3D version: 0.18.0+8e434558a
- System architecture: x86
- Is this a remote workstation?: no
- How did you install Open3D?: build from source
- Compiler version (if built from source): msvc 14.3

Additional information

If I understand this correctly, the problem could perhaps also be that tbb12.dll is not finding its dependencies and not that tbb12.dll is not found. I did not follow this particular rabbit hole, further, though. If this is the problem, Open3D should perhaps implement a workaround or switch to a different tbb-version.

[edit: with some further research I found that this problem also exists for CUDA-DLLs, so I'm pretty sure the issue above has nothing to do with this issue.]

[fegorsch]

I reproduced the experiment from here with the open3d-package, i.e. tried all combinations of winmode=0/winmode=None and add_dll_directory / PATH. I did this for Python 3.10 and 3.13, all on Windows 11 with my custom build and came to the same conclusion as in the thread: only the following combinations work:

• winmode=0 and adding the DLL-search-directory to PATH,
• winmode=None and adding the DLL-search-directory via os.add_dll_directory.

Open3D currently uses winmode=0 + os.add_dll_directory, this seems to be the problem.

I'll open a PR with a change to winmode=None + os.add_dll_directory, as this is the documented (i.e. winmode=0 is undocumented) and recommended way of setting DLL-search-directories.

I guess winmode=0worked originally, because it causes PATH to be respected and the author of the PR had the CUDA-DLL-directory listed in there. At the same time he probably did not build with anything, that puts DLLs into the open3d-package directory (e.g. tbb). To also support this use-case, I propose to explicitly get the CUDA-DLL-directory from PATH and add it via add_dll_directory.