CMake Compatibility with FetchContent

[mprat]

To allow for projects using CMake's FetchContent framework, instances of CMAKE_SOURCE_DIR should be replaced with PROJECT_SOURCE_DIR. FetchContent uses add_subdirectory() under the cmake hood, and using CMAKE_SOURCE_DIR cannot be set manually; it is set as the parent project source directory. For correct add_subdirectory() cmake builds using librealsense as a dependency, these should be changed to PROJECT_SOURCE_DIR.

[maloel]

Hi @mprat
Thanks for the PR!

Question, just based on a really quick look:

PROJECT_SOURCE_DIR is the source directory of the most recent project() command.

So, for example, in a file like src/gl/CMakeLists.txt, we have:

project(realsense2-gl)
...
include(${CMAKE_SOURCE_DIR}/CMake/opengl_config.cmake)

This evaluated to the CMake directory in the root librealsense structure.
But now, if we replace by PROJECT_SOURCE_DIR, it will evaluate to src/gl/CMake/opengl_config.cmake?

[mprat]

Hi @mprat Thanks for the PR!

Question, just based on a really quick look:

PROJECT_SOURCE_DIR is the source directory of the most recent project() command.

So, for example, in a file like src/gl/CMakeLists.txt, we have:

project(realsense2-gl)
...
include(${CMAKE_SOURCE_DIR}/CMake/opengl_config.cmake)

This evaluated to the CMake directory in the root librealsense structure. But now, if we replace by PROJECT_SOURCE_DIR, it will evaluate to src/gl/CMake/opengl_config.cmake?

Not quite - the CMakeLists.txt file in src/gl/ is called by the call from add_subdirectory(src/gl) in the CMakeLists.txt in librealsense/CMakeLists.txt. According to the CMake documentation, PROJECT_SOURCE_DIR is not affected by calls to add_subdirectory() (https://cmake.org/cmake/help/latest/variable/PROJECT_SOURCE_DIR.html). This means that the previous call to project() (which appears in the top-level librealsense/CMakeLists.txt sets the PROJECT_SOURCE_DIR to librealsense/. The line in src/gl/CMakeLists.txt then will evaluate to librealsense/CMake/opengl_config.cmake, as it expected.

If you want to get rid of this ambiguity, the opengl_config.cmake can be moved into src/gl/CMake and the use of CMAKE_SOURCE_DIR/PROJECT_SOURCE_DIR can be removed completely.