Use Tesseract in CMAKE project

I am trying to compile a cmake project that requires Tesseract and OpenCV. Everything is being done in Ubuntu 20.04. But I have been struggling to get it compiled. I compiled Tesseract from source and have been going through several SO posts and whatever article I find online, but still no luck. I noticed that Tesseract doesn't export a .cmake file under /usr/local/lib/cmake/, so you cannot do find_package(Tesseract x.x.x REQUIRED).

Looking around, I see that people recommend doing it this way:

find_package(PkgConfig REQUIRED)
pkg_search_module(TESSERACT REQUIRED tesseract)
pkg_search_module(LEPTONICA REQUIRED lept)
include_directories(${TESSERACT_INCLUDE_DIRS})
include_directories(${LEPTONICA_INCLUDE_DIRS})
link_directories( ${TESSERACT_LIBRARY_DIRS} )
link_directories( ${LEPTONICA_LIBRARY_DIRS} )

Which seems to find something. After doing pkg_search_module(TESSERACT REQUIRED tesseract), I can see that TESSERACT_INCLUDE_DIRS is populated with this list: /usr/local/include;/usr/include/leptonica. This is despite the fact that Tesseract header files are located under /usr/local/include/tesseract/. Anyways, running cmake goes through without an error.

However, running make afterwards simply adds strange errors of

error: 'l_int32' does not name a type; did you mean 'u_int32_t'?
   54 |     l_int32        num;       /*!< number of unknowns                     */
      |     ^~~~~~~

to my project. Even though I have not even included any header files of Tesseract or Leptonica.

This post is the only one that seems to be close to a solution.

I have also tried the following without success:

Add the path to Tesseract and Leptonica header files directly using

include_directories(/usr/local/include/tesseract/)
include_directories(/usr/local/include/leptonica/)

Also tried to make my own .cmake file so that I can use find_package(Tesseract 4.1.1 REQUIRED) directly.
I even tried to compile Leptonica library from source, but that doesn't help.
Reading through the cmake file of the Tesseract project itself also didn't shed a light on this.

Solution

So Ubuntu's libtesseract-dev package unfortunately does not bundle its CMake config files. But that doesn't mean you shouldn't look for them first.

find_package(Tesseract 4)
if (NOT Tesseract_FOUND)
    find_package(PkgConfig REQUIRED)

    pkg_check_modules(Tesseract REQUIRED IMPORTED_TARGET tesseract lept)
    add_library(Tesseract::libtesseract ALIAS PkgConfig::Tesseract)
endif ()

# ... later ...
target_link_libraries(my_app PRIVATE Tesseract::libtesseract)

Always use imported targets. Never ever use include_directories or link_directories. Forget those commands exist at all.

This code will try to find the Tesseract::libtesseract target in the Tesseract CMake package. If it can't, it will look for a pkg-config module for Tesseract. If it can't find that, the build fails (REQUIRED) but if it can, the pkg_check_modules command will create the PkgConfig::tesseract imported target. The add_library(.. ALIAS ..) command adds an alias to make the target names match no matter which method succeeded.

You then use target_link_libraries to link your application to Tesseract. CMake will handle the usage requirements (ie. link and include directories, compiler flags, etc.) for you.