CMake find_package can't find libpng (16)

I'm not the first to run into libpng issues, especially when I want to link to self-built sources on Windows. I'm using the libpng1638 sources from https://github.com/glennrp/libpng. Semi official - the reason for this version is it has a CMake build/install script.

zlib is built/found and the library installs in c:\Program Files\libpng (I have set CMAKE_INSTALL_PREFIX for this)

The problem occurs when doing a find_package(PNG 16). With debug flag on, a number of search directories is listed, but ultimately it fails to find the PNG-config.cmake script that can link up to the installed paths and files.

It doesn't seem to help when I set CMAKE_PREFIX_PATH to the correct folder, and the reason is explained below.

This the command I used in the script (zlib is already found):

set(CMAKE_FIND_DEBUG_MODE TRUE)
find_package(PNG 16         PATHS "C:\\Program Files\\libpng\\lib\\libpng")
set(CMAKE_FIND_DEBUG_MODE FALSE)

This is the output of the debug trace:

...
find_package considered the following locations for the Config module:
  ...
  C:/Program Files/CMake/PNGConfig.cmake
  C:/Program Files/CMake/png-config.cmake
  C:/Program Files (x86)/PNGConfig.cmake
  C:/Program Files (x86)/png-config.cmake
  C:/Program Files/libpng/lib/libpng/PNGConfig.cmake
  C:/Program Files/libpng/lib/libpng/png-config.cmake

Indeed, no png-config.cmake file can be found anywhere on my system. In the source CMakeLists.txt of libpng, the section to create these XXX-config.cmake scripts has been deliberately disabled for Win_32 systems:

  # Install the pkg-config files.
  if(NOT CMAKE_HOST_WIN32 OR CYGWIN OR MINGW)
    install(FILES ${CMAKE_CURRENT_BINARY_DIR}/libpng.pc
            DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
    install(PROGRAMS ${CMAKE_CURRENT_BINARY_DIR}/libpng-config
            DESTINATION ${CMAKE_INSTALL_BINDIR})
    install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${PNGLIB_NAME}.pc
            DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
    install(PROGRAMS ${CMAKE_CURRENT_BINARY_DIR}/${PNGLIB_NAME}-config
            DESTINATION ${CMAKE_INSTALL_BINDIR})
  endif()

Since I don't want to dabble in the source files (which ultimately are pulled from the web directly), I want to understand. Why would this be disabled for native Win32 builds? (most of the online info uses the Linux subsystem or a package manager.. )

Other packages like zlib have their XXX-config.cmake files properly copied to the install folders where FindXXX.cmake scripts can pick them up properly.

For completeness, here is the output of the libpng build (msvc)

  -- Install configuration: "Debug"
  -- Installing: C:/Program Files/libpng/lib/libpng16d.lib
  -- Installing: C:/Program Files/libpng/bin/libpng16d.dll
  -- Installing: C:/Program Files/libpng/lib/libpng16_staticd.lib
  -- Installing: C:/Program Files/libpng/include/png.h
  -- Installing: C:/Program Files/libpng/include/pngconf.h
  -- Installing: C:/Program Files/libpng/include/pnglibconf.h
  -- Installing: C:/Program Files/libpng/include/libpng16/png.h
  -- Installing: C:/Program Files/libpng/include/libpng16/pngconf.h
  -- Installing: C:/Program Files/libpng/include/libpng16/pnglibconf.h
  -- Installing: C:/Program Files/libpng/bin/pngfix.exe
  -- Installing: C:/Program Files/libpng/bin/png-fix-itxt.exe
  -- Installing: C:/Program Files/libpng/share/man/man3/libpng.3
  -- Installing: C:/Program Files/libpng/share/man/man3/libpngpf.3
  -- Installing: C:/Program Files/libpng/share/man/man5/png.5
  -- Installing: C:/Program Files/libpng/lib/libpng/libpng16.cmake
  -- Installing: C:/Program Files/libpng/lib/libpng/libpng16-debug.cmake

All insight greatly appreciated!

Update 1

After fiddling with the install commands, I have to conclude that libpng-libpng16.zip from https://github.com/glennrp/libpng is not maintained with Windows as build target in mind.

lpng1637.zip from https://sourceforge.net/projects/libpng/ has the same problem.

Update2 :

So far I have tried setting PNG_DIR, setting the CMAKE_PREFIX_PATH and a number of other things that failed. Just one seems to work, which is setting the environment PNG_ROOT variable to the correct installation folder. This is dead ugly, but it seems to be the only option that works.

Solution

So, for people who, like me, are trying to use freetype2 in a Windows project, here's how to build the dependencies:

make sure your zlib is built and installed also in the C:\Program Files\Zlib folder
zlib can usually be found without issues. To be sure, remove all other occurrences especially those under C:\Program Files (x86) if you have them. This will make sure find_package(..) cannot confuse between the different targets. Use -DCMAKE_INSTALL_PREFIX to force building/installing into that directory.
set the environment PNG_ROOT environment variable to C:\Program Files\libpng
set the environment BZIP2_ROOT environment variable to C:\Program Files\bzip2
set the environment ZLIB_ROOT environment variable to C:\Program Files\zlib

I did the same for dependencies Brotli and Harfbuzz, but no luck so far. Harfbuzz seems to have some strange chicken/egg relationship to freetype2.

For setting up the build for freetype2 with CMake, after building the dependencies, these flags work:

-DFT_DISABLE_HARFBUZZ=True 
-DFT_REQUIRE_PNG=True 
-DFT_DISABLE_BROTLI=True 
-DFT_REQUIRE_BZIP2=True 
-DCMAKE_PREFIX_PATH="C:\Program Files\libpng;C:\Program Files\bzip2;C:\Program Files\brotli;C:\Program Files\zlib;C:\Program Files\harfbuzz"

Note that libpng pulls in zlib as dependency, so there's no real need to specify that flag imho, so I'm not touching it.