Refine logic of parsing PNG version

Currently, if `PNG_FOUND`, cmake scripts will check include and parse
header while we can use `PNG_VERSION_STRING` conveniently. If
`BUILD_PNG`, parse version from `PNG_LIBPNG_VER_STRING` directly is more
convenient than parsing major, minor and patch and concatenate them.

The comment of png.h also supports this.
```
/* These should match the first 3 components of PNG_LIBPNG_VER_STRING: */
```
https://github.com/glennrp/libpng/blob/libpng16/png.h#L287

This patch also modifies `ocv_parse_header_version` macro to receive
another parameter to make it more general.

The reason why changing `PNG_VERSION` to `PNG_VERSION_STRING` is to be
consistent with cmake's FindPNG.

This patch removes `HAVE_LIBPNG_PNG_H` variable because `PNG_INCLUDE_DIR`
is where to find png.h, etc according to
https://cmake.org/cmake/help/latest/module/FindPNG.html.

This patch also removes `PNG_PNG_INCLUDE_DIR` variable which is an
advanced variable used in cmake's FindPNG and is not used in opencv.

CMakeLists.txt

@@ -1406,7 +1406,7 @@ if(WITH_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 ${PNG_VERSION})")
+  status("    PNG:"  PNG_FOUND  THEN "${PNG_LIBRARY} (ver ${PNG_VERSION_STRING})" ELSE "build (ver ${PNG_VERSION_STRING})")
 endif()
 
 if(WITH_TIFF OR HAVE_TIFF)

cmake/OpenCVFindLibsGrfmt.cmake

@@ -34,7 +34,7 @@ if(NOT ZLIB_FOUND)
   set(ZLIB_INCLUDE_DIRS ${ZLIB_INCLUDE_DIR})
   set(ZLIB_LIBRARIES ${ZLIB_LIBRARY})
 
-  ocv_parse_header_version(ZLIB "${${ZLIB_LIBRARY}_SOURCE_DIR}/zlib.h")
+  ocv_parse_header_version(ZLIB "${${ZLIB_LIBRARY}_SOURCE_DIR}/zlib.h" ZLIB_VERSION)
 endif()
 
 # --- libavif (optional) ---
@@ -273,31 +273,21 @@ if(NOT HAVE_SPNG AND WITH_PNG)
     ocv_clear_vars(PNG_FOUND)
   else()
     ocv_clear_internal_cache_vars(PNG_LIBRARY PNG_INCLUDE_DIR)
-    include(FindPNG)
-    if(PNG_FOUND)
-      include(CheckIncludeFile)
-      check_include_file("${PNG_PNG_INCLUDE_DIR}/libpng/png.h" HAVE_LIBPNG_PNG_H)
-      if(HAVE_LIBPNG_PNG_H)
-        ocv_parse_header("${PNG_PNG_INCLUDE_DIR}/libpng/png.h" PNG_VERSION_LINES PNG_LIBPNG_VER_MAJOR PNG_LIBPNG_VER_MINOR PNG_LIBPNG_VER_RELEASE)
-      else()
-        ocv_parse_header("${PNG_PNG_INCLUDE_DIR}/png.h" PNG_VERSION_LINES PNG_LIBPNG_VER_MAJOR PNG_LIBPNG_VER_MINOR PNG_LIBPNG_VER_RELEASE)
-      endif()
-    endif()
+    find_package(PNG QUIET)
   endif()
 
   if(NOT PNG_FOUND)
-    ocv_clear_vars(PNG_LIBRARY PNG_LIBRARIES PNG_INCLUDE_DIR PNG_PNG_INCLUDE_DIR HAVE_LIBPNG_PNG_H PNG_DEFINITIONS)
+    ocv_clear_vars(PNG_LIBRARY PNG_LIBRARIES PNG_INCLUDE_DIR PNG_DEFINITIONS)
 
     set(PNG_LIBRARY libpng CACHE INTERNAL "")
     set(PNG_LIBRARIES ${PNG_LIBRARY})
     add_subdirectory("${OpenCV_SOURCE_DIR}/3rdparty/libpng")
     set(PNG_INCLUDE_DIR "${${PNG_LIBRARY}_SOURCE_DIR}" CACHE INTERNAL "")
     set(PNG_DEFINITIONS "")
-    ocv_parse_header("${PNG_INCLUDE_DIR}/png.h" PNG_VERSION_LINES PNG_LIBPNG_VER_MAJOR PNG_LIBPNG_VER_MINOR PNG_LIBPNG_VER_RELEASE)
+    ocv_parse_header_version(PNG "${PNG_INCLUDE_DIR}/png.h" PNG_LIBPNG_VER_STRING)
   endif()
 
   set(HAVE_PNG YES)
-  set(PNG_VERSION "${PNG_LIBPNG_VER_MAJOR}.${PNG_LIBPNG_VER_MINOR}.${PNG_LIBPNG_VER_RELEASE}")
 endif()
 
 

cmake/OpenCVUtils.cmake

@@ -1430,15 +1430,15 @@ macro(ocv_parse_header2 LIBNAME HDR_PATH VARNAME)
   endif()
 endmacro()
 
-# set ${LIBNAME}_VERSION_STRING to ${LIBNAME}_VERSION without quotes
-macro(ocv_parse_header_version LIBNAME HDR_PATH)
+# set ${LIBNAME}_VERSION_STRING to ${LIBVER} without quotes
+macro(ocv_parse_header_version LIBNAME HDR_PATH LIBVER)
   ocv_clear_vars(${LIBNAME}_VERSION_STRING)
   set(${LIBNAME}_H "")
   if(EXISTS "${HDR_PATH}")
-    file(STRINGS "${HDR_PATH}" ${LIBNAME}_H REGEX "^#define[ \t]+${LIBNAME}_VERSION[ \t]+\"[^\"]*\".*$" LIMIT_COUNT 1)
+    file(STRINGS "${HDR_PATH}" ${LIBNAME}_H REGEX "^#define[ \t]+${LIBVER}[ \t]+\"[^\"]*\".*$" LIMIT_COUNT 1)
   endif()
   if(${LIBNAME}_H)
-    string(REGEX REPLACE "^.*[ \t]${LIBNAME}_VERSION[ \t]+\"([0-9\.]+)\"$" "\\1" ${LIBNAME}_VERSION_STRING "${${LIBNAME}_H}")
+    string(REGEX REPLACE "^.*[ \t]${LIBVER}[ \t]+\"([0-9\.]+)\"$" "\\1" ${LIBNAME}_VERSION_STRING "${${LIBNAME}_H}")
   endif()
 endmacro()
 

cmake/templates/cvconfig.h.in

@@ -78,9 +78,6 @@
 /* IJG JPEG codec */
 #cmakedefine HAVE_JPEG
 
-/* libpng/png.h needs to be included */
-#cmakedefine HAVE_LIBPNG_PNG_H
-
 /* GDCM DICOM codec */
 #cmakedefine HAVE_GDCM
 

modules/imgcodecs/src/grfmt_png.cpp

@@ -58,11 +58,7 @@
 #  define _FILE_OFFSET_BITS 0
 #endif
 
-#ifdef HAVE_LIBPNG_PNG_H
-#include <libpng/png.h>
-#else
 #include <png.h>
-#endif
 #include <zlib.h>
 
 #include "grfmt_png.hpp"