apriltag-cmake-example

A MWE of using the apriltag library with cmake to generate makefiles.

I had a very hard time getting the apriltag library to work with cmake. I use cmake all the time, so decided to put together a little minimal working example. I'm being sort of brief, but if you have questions, just open an issue.

Contents:

• a working CMakeLists.txt.
• example opencv_demo.cc from the apriltag example directory.
• example apriltag_demo.c from the apriltag example directory.
• these instructions.
• I also added opencv-demo-read.**, so you can read from a directory, or use a camera.

Install apriltag

Install the apriltag lib. Set the install path. I do this by:

cmake .
cmake-gui . 

the first cmake gets everything set up with the defaults, then in the cmake-gui step, you can easily alter the install path without remembering flags. I'll use /usr/local/apriltag are the install path. Then push the configure and generate buttons. Done!

Then ...

make
sudo make install

clone this repo

git clone https://github.com/amy-tabb/apriltag-cmake-example.git

then

cd apriltag-cmake-example
mkdir build
cd build

Make sure pkg-config can find apriltag

Here, you need to do some searching we're going to use pkg-config to do all of the work of setting the compiler and linker options. Trouble is, sometimes the apriltag pkg-config file is not on the PKG_CONFIG_PATH.

First, what's your path?

 echo $PKG_CONFIG_PATH

See anything with apriltag in there? If not, you will need to add the folder with apriltag.pc:

export PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/usr/local/apriltag/lib/pkgconfig

to make this stick across bash sessions, put an export PKG_CONFIG_PATH= in ~/.profile (more commentary on setting environment variables in Ubuntu here), but use the output of echo $PKG_CONFIG_PATH. Otherwise, the PKG_CONFIG_PATH=$PKG_CONFIG_PATH will result in a longer and longer string. (Note, I have never had to do this will other libraries.)

cross your fingers and try cmake

Now you can try to generate a Makefile with cmake. I am totally skipping OpenCV, but I am assuming that you have OpenCV installed. If you don't, no worries. Comment out all of the OpenCV stuff in the CMakeLists.txt file using a #:

• find_package(OpenCV REQUIRED)
• (all 3 lines) add_executable(opencv_demo opencv_demo.cc )
• target_link_libraries(opencv_demo ${apriltag_LIBRARIES} ${OpenCV_LIBS})

The OpenCV demo will not be built, but the apriltag one will.

Then, there are a few different options for cmake,

• cmake ../src (basic)
• cmake -DCMAKE_BUILD_TYPE=Release ../src (Release configuration)
• cmake -DCMAKE_BUILD_TYPE=Debug ../src (Debug configuration)
• cmake -G"Eclipse CDT4 - Unix Makefiles" -DCMAKE_ECLIPSE_GENERATE_SOURCE_PROJECT=TRUE ../src/ (Create an Eclipse project to import -- it will be in the build folder)

In case you have installed OpenCV and cmake can't find it, you need to specify the location of OpenCVConfig.cmake. Don't know where it is? Find out with locate OpenCVConfig.cmake. Then append

-DCMAKE_PREFIX_PATH=dir

to the command line. Alternatively, you could use the cmake-gui option I used earlier.

Make and test

Then make and test,

make

you should have two files (if you used the OpenCV option too), opencv_demo and apriltag_demo.

run as usual, ./apriltag_demo.

The new demo is opencv-demo-read. Run that one with the help by ./opencv-demo-read --help.

Other resources

My source for this working CMakeLists.txt was this other CMakeLists.txt from the apriltag_ros project. FYI if you have a more complex project.