compiling_the_schema

How To Build

This project is built the same way as every other project in this repository. With a couple minor changes:

cd build
USD_INSTALL_ROOT=/wherever/you/installed/USD/to cmake ..
make install

Note make is now make install and there's no ./run_it command. That's because we're building a library to share with other projects, not creating an executable program.

How To Test If It Worked

There are two companion projects which can be used to test if the schemas were built correctly. One tests C++ and the other tests the Python bindings.

The C++ Project

The Python Project

Project Explanation

Most of the files in this project were created by running this command:

cd src
usdGenSchema .

usdGenSchema will create a bunch of files, all of which are completely unaltered. Some files were created by-hand though. This is the full list:

module.cpp

moduleDeps.cpp

__init__.py

__packageinit__.py

The latter .py files are basically just to make the Python bindings importable for when we use the schema classes in Python. They're only "necessary" because of how this project is set up. Technically, they aren't really needed.

The former files though, module.cpp and moduleDeps.cpp, are absolutely necessary.

module.cpp and moduleDeps.cpp

To understand what these files do, open src/CMakeLists.txt, remove these lines, and re-build this project

add_library(${USDPLUGIN_PYTHON_NAME}
    SHARED
        wrapTokens.cpp
        wrapSimple.cpp
        wrapComplex.cpp
        wrapParamsAPI.cpp
)

Once you've done that, you'll probably get an error like this:

selecaoone@SelecaoOne:/home/selecaoone/projects/usd_experiments/examples/tricks/custom_schemas_with_python_bindings/compiling_the_schema/build$ PYTHONPATH=/usr/local/USD-19.07/lib/python:$PWD/instal
l/lib/python2.7/site-packages:$PYTHONPATH PXR_PLUGINPATH_NAME=$PWD/install/plugin/usd/testout/resources python ~/temp/schema_test_testout.py
Traceback (most recent call last):
  File "/home/selecaoone////temp/schema_test_testout.py", line 4, in <module>
    from testout import Testout
  File "/home/selecaoone/projects/usd_experiments/examples/tricks/custom_schemas_with_python_bindings/compiling_the_schema/build/install/lib/python2.7/site-packages/testout/Testout/__init__.py", lin
e 1, in <module>
    from . import _testout
ImportError: dynamic module does not define init function (init_testout)

This is also why every schema class that USD provides also has a module.cpp file

From what I can see searching online, it looks like when you add dynamic library to your PYTHONPATH and try to import them in a Python script, Python expects for that library to have an "entry" init function. USD does things a bit differently. Instead, "module.cpp" wraps the schema classes that you generated with usdGenSchema, which removes the need to define an init function yourself. Again, this is just a guess. Similarly, "moduleDeps.cpp" links USD libraries so that you don't get any linker errors when "module.cpp" is sourced.

TODO : Check if that's true

Important: From a quick test using Python, it looks like this schema example project doesn't need "moduleDeps.cpp". Only "module.cpp" is actually required. But it's still good to know about "moduleDeps.cpp", just in case.

Troubleshooting Build Issues

Every C++ project in this repository is compiled with GCC 8.3.0 except for this one. For whatever reason, when compiling with later versions of GCC, I got exceptions when importing the Python bindings library. The exact error isn't that important but, for reference, this was the error:

Traceback (most recent call last):
  File "/home/selecaoone////temp/schema_test_testout.py", line 4, in <module>
	from testout import Testout
  File "/home/selecaoone/projects/usd_experiments/cpp_test/schema_resolver_hybrid/build/install/lib/python2.7/site-packages/testout/Testout/__init__.py", line 1, in <module>
	from . import _testout
ImportError: /home/selecaoone/projects/usd_experiments/cpp_test/schema_resolver_hybrid/build/install/lib/python2.7/site-packages/testout/Testout/../../../../../plugin/usd/testout.so: undefined symbo
l: _ZN32pxrInternal_v0_19__pxrReserved__6TfType7DeclareERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE

You may also see similar errors where a USD library fails to link some STL library, like this if you use a later version of GCC. For this project, it is highly recommended to use whatever version of GCC USD recommends. At time time of writing, this was GCC 4.8.

References

Explanation of the init function error

Building and exporting CMake projects