Cannot link g2o to ROS project, after building successfully from source and installing to default system paths

[JeffR1992]

I built and installed g2o from source, and calling
which g2o

shows me that the g2o binary is installed to:
/usr/local/bin/g2o

Furthermore, I can find the headers by going to:
/usr/local/include/g2o

which gives the following directories:
EXTERNAL apps autodiff config.h core examples solvers stuff types

However, in my ROS Noetic project's CMakeLists.txt file, when I add
find_package(G2O REQUIRED)

I get the error:

CMake Error at /home/jeff/projects/catkin_ws/src/mapper/CMakeLists.txt:34 (find_package):
  By not providing "FindG2O.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "G2O", but
  CMake did not find one.

  Could not find a package configuration file provided by "G2O" with any of
  the following names:

    G2OConfig.cmake
    g2o-config.cmake

  Add the installation prefix of "G2O" to CMAKE_PREFIX_PATH or set "G2O_DIR"
  to a directory containing one of the above files.  If "G2O" provides a
  separate development package or SDK, be sure it has been installed.

I also tried copying over the cmake_modules directory from the g2o source directory into my project root directory and added the cmake_modules directory to the CMAKE_MODULE_PATH with:
list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake_modules)

In this way, I don't get the above error anymore, but if I print out the g2o environment variables using:
message(STATUS "G2O_INCLUDE_DIR: ${G2O_INCLUDE_DIR}")
message(STATUS "G2O_LIBRARIES: ${G2O_LIBRARIES}")

I see the following for the headers:
G2O_INCLUDE_DIR: /usr/local/include

but for the libraries I see a blank path:
G2O_LIBRARIES:

Which causes linking errors if I try and actually use something like g2o::SparseOptimizer in my project.

So my question is, how in the world do I get CMake to see g2o so I can link my project to it correctly? I feel like what I've tried should have solved the issue, yet I've had no luck. Any help would be appreciated.

[RainerKuemmerle]

There is a ROS package available in ROS1. Did you try using that instead of building from src? See for example, https://github.com/rst-tu-dortmund/teb_local_planner for a package using that.

[geoeo]

This still seems to be an issue on master and is not related to ROS. I cant link use this library when built work source.

Edit: It seems to me there is an inconsistency between CMake and the expectation to find "g2o" and "G2O". For example the generated configs are g2oConfig.cmake which is not compatible with find_package(G2O..)

Edit #2. You need to set your cmake module path to point the supplied FindG2O.cmake file:

set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} <your_path>/g2o/cmake_modules)

It would be nice to clarify this on the README.md. Ideally it should be copied into the cmake/share folder

[apmilko]

@RainerKuemmerle can you please provide an answer to this issue?