Merge pull request opencv#24718 from asmorkalov/as/system_spng

Add support for external libspng. opencv#24718

Fixes opencv#24683
Related patch to libspng: randy408/libspng#264

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

CMakeLists.txt

@@ -1399,12 +1399,14 @@ if(WITH_AVIF OR HAVE_AVIF)
   endif()
 endif()
 
-if(WITH_PNG OR HAVE_PNG OR WITH_SPNG)
-  if(WITH_SPNG)
+if(WITH_SPNG)
+  if(BUILD_SPNG)
     status("    PNG:" "build-${SPNG_LIBRARY} (ver ${SPNG_VERSION})")
-  else()
-    status("    PNG:"  PNG_FOUND  THEN "${PNG_LIBRARY} (ver ${PNG_VERSION})" ELSE "build (ver ${PNG_VERSION})")
+  elseif(HAVE_SPNG)
+    status("    PNG:" "${SPNG_LIBRARY} (ver ${SPNG_VERSION})")
   endif()
+elseif(WITH_PNG OR HAVE_PNG)
+  status("    PNG:"  PNG_FOUND  THEN "${PNG_LIBRARY} (ver ${PNG_VERSION})" ELSE "build (ver ${PNG_VERSION})")
 endif()
 
 if(WITH_TIFF OR HAVE_TIFF)

cmake/OpenCVFindLibsGrfmt.cmake

@@ -232,16 +232,38 @@ if(WITH_JASPER AND NOT HAVE_OPENJPEG)
 endif()
 
 if(WITH_SPNG)
-  set(SPNG_LIBRARY libspng CACHE INTERNAL "")
-  set(SPNG_LIBRARIES ${SPNG_LIBRARY})
-  add_subdirectory("${OpenCV_SOURCE_DIR}/3rdparty/libspng")
-  set(SPNG_INCLUDE_DIR "${${SPNG_LIBRARY}_SOURCE_DIR}" CACHE INTERNAL "")
-  set(SPNG_DEFINITIONS "")
-  ocv_parse_header("${SPNG_INCLUDE_DIR}/spng.h" SPNG_VERSION_LINES SPNG_VERSION_MAJOR SPNG_VERSION_MINOR SPNG_VERSION_PATCH)
-
-  set(HAVE_SPNG YES)
-  set(SPNG_VERSION "${SPNG_VERSION_MAJOR}.${SPNG_VERSION_MINOR}.${SPNG_VERSION_PATCH}")
-  message(STATUS "imgcodecs: PNG codec will use SPNG, version: ${SPNG_VERSION} ")
+  if(BUILD_SPNG)
+    ocv_clear_vars(PNG_FOUND)
+  else()
+    # CMakeConfig bug in SPNG, include is missing there in version 0.7.4 and older
+    # See https://github.com/randy408/libspng/pull/264
+    include(CMakeFindDependencyMacro)
+    find_package(SPNG QUIET)
+    if(SPNG_FOUND)
+      set(SPNG_LIBRARY "spng::spng" CACHE INTERNAL "")
+      set(SPNG_LIBRARIES ${SPNG_LIBRARY})
+    else()
+      if(PkgConfig_FOUND)
+        pkg_check_modules(SPNG QUIET spng)
+      endif()
+    endif()
+    if(SPNG_FOUND)
+      set(HAVE_SPNG YES)
+      message(STATUS "imgcodecs: PNG codec will use SPNG, version: ${SPNG_VERSION}")
+    endif()
+  endif()
+  if(NOT SPNG_FOUND)
+    set(SPNG_LIBRARY libspng CACHE INTERNAL "")
+    set(SPNG_LIBRARIES ${SPNG_LIBRARY})
+    add_subdirectory("${OpenCV_SOURCE_DIR}/3rdparty/libspng")
+    set(SPNG_INCLUDE_DIR "${${SPNG_LIBRARY}_SOURCE_DIR}" CACHE INTERNAL "")
+    set(SPNG_DEFINITIONS "")
+    ocv_parse_header("${SPNG_INCLUDE_DIR}/spng.h" SPNG_VERSION_LINES SPNG_VERSION_MAJOR SPNG_VERSION_MINOR SPNG_VERSION_PATCH)
+
+    set(HAVE_SPNG YES)
+    set(SPNG_VERSION "${SPNG_VERSION_MAJOR}.${SPNG_VERSION_MINOR}.${SPNG_VERSION_PATCH}")
+    message(STATUS "imgcodecs: PNG codec will use SPNG, version: ${SPNG_VERSION} ")
+  endif()
 endif()
 
 # --- libpng (optional, should be searched after zlib) ---