small problem in README.md in examples

[cwreynolds]

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).

My Question

In the Examples, I was following the recipe on Find Pre-Installed Open3D Package in CMake. It all worked (yay!) but there was a minor speed-bump. In Step 2 for the On Ubuntu/macOS case, it says to copy the sample draw app from the local Open3D repository by:

cp -ar Open3D/examples/cmake/open3d-cmake-find-package .

On my macOS (14.6.1) laptop, the command did not run, saying: cp: the -R and -r options may not be specified together

First of all I did not specify the -R and -r options together, but that must be a macOS cp problem. On the other hand it is not immediately clear why I needed any of -a, -r, or -R. There are no subdirectories, so -r recursion is not an issue, and the files are normal source code text files, so -a preservation of file attributes similarly seems a non-issue.

Perhaps this is meant to be a more general template that would work in more complicated situations? Perhaps it has to do with differences in the cp command on Linux and macOS? (See e.g. this versus that.) Or maybe it is just an error/typo? In any case it worked OK for me to run without -R/-r:

cp -a Open3D/examples/cmake/open3d-cmake-find-package . 

Although from the second doc above, it seems like the macOS command has no -a option. I would be happy to fix this README.md file as my first foray into pull requests in Open3D, except that I need some feedback on what it should be changed to. Unless I'm missing something, it would simplify the example to remove -ar.

[cwreynolds]

Similarly minor, but to avoid potential confusion to a first time user of Open3D, or cmake, or git, or whatever:

The Cloning Open3D step of Build from source should include a cd Open3D after git clone https://github.com/isl-org/Open3D and before subsequent steps.