OpenCV+FFmpeg on Windows (4.3.2 from www.gyan.dev)

ffmpeg-config.cmake

# Location: <ffmpeg>/cmake/

if(NOT DEFINED FFMPEG_ROOT)
  get_filename_component(FFMPEG_ROOT "${CMAKE_CURRENT_LIST_DIR}/.." ABSOLUTE)
endif()

function (_ffmpeg_find component headername)
  find_path("FFMPEG_${component}_INCLUDE_DIR"
    NAMES
      "lib${component}/${headername}"
    PATHS
      "${FFMPEG_ROOT}/include"
      ~/Library/Frameworks
      /Library/Frameworks
      /usr/local/include
      /usr/include
      /sw/include # Fink
      /opt/local/include # DarwinPorts
      /opt/csw/include # Blastwave
      /opt/include
      /usr/freeware/include
    PATH_SUFFIXES
      ffmpeg
    DOC "FFMPEG's ${component} include directory")
  mark_as_advanced("FFMPEG_${component}_INCLUDE_DIR")

  # On Windows, static FFMPEG is sometimes built as `lib<name>.a`.
  if (WIN32)
    list(APPEND CMAKE_FIND_LIBRARY_SUFFIXES ".a" ".lib")
    list(APPEND CMAKE_FIND_LIBRARY_PREFIXES "" "lib")
  endif ()

  find_library("FFMPEG_${component}_LIBRARY"
    NAMES
      "${component}"
    PATHS
      "${FFMPEG_ROOT}/lib"
      ~/Library/Frameworks
      /Library/Frameworks
      /usr/local/lib
      /usr/local/lib64
      /usr/lib
      /usr/lib64
      /sw/lib
      /opt/local/lib
      /opt/csw/lib
      /opt/lib
      /usr/freeware/lib64
      "${FFMPEG_ROOT}/bin"
    DOC "FFMPEG's ${component} library")
  mark_as_advanced("FFMPEG_${component}_LIBRARY")

  if (FFMPEG_${component}_LIBRARY AND FFMPEG_${component}_INCLUDE_DIR)
    set(_deps_found TRUE)
    set(_deps_link)
    foreach (_ffmpeg_dep IN LISTS ARGN)
      if (TARGET "FFMPEG::${_ffmpeg_dep}")
        list(APPEND _deps_link "FFMPEG::${_ffmpeg_dep}")
      else ()
        set(_deps_found FALSE)
      endif ()
    endforeach ()
    if (_deps_found)
      if (NOT TARGET "FFMPEG::${component}")
        add_library("FFMPEG::${component}" UNKNOWN IMPORTED)
        set_target_properties("FFMPEG::${component}" PROPERTIES
          IMPORTED_LOCATION "${FFMPEG_${component}_LIBRARY}"
          INTERFACE_INCLUDE_DIRECTORIES "${FFMPEG_${component}_INCLUDE_DIR}"
          IMPORTED_LINK_INTERFACE_LIBRARIES "${_deps_link}")
      endif ()
      set("FFMPEG_${component}_FOUND" 1
        PARENT_SCOPE)

      set(version_header_path "${FFMPEG_${component}_INCLUDE_DIR}/lib${component}/version.h")
      if (EXISTS "${version_header_path}")
        string(TOUPPER "${component}" component_upper)
        file(STRINGS "${version_header_path}" version
          REGEX "#define  *LIB${component_upper}_VERSION_(MAJOR|MINOR|MICRO) ")
        string(REGEX REPLACE ".*_MAJOR *\([0-9]*\).*" "\\1" major "${version}")
        string(REGEX REPLACE ".*_MINOR *\([0-9]*\).*" "\\1" minor "${version}")
        string(REGEX REPLACE ".*_MICRO *\([0-9]*\).*" "\\1" micro "${version}")
        if (NOT major STREQUAL "" AND
            NOT minor STREQUAL "" AND
            NOT micro STREQUAL "")
          set("FFMPEG_${component}_VERSION" "${major}.${minor}.${micro}"
            PARENT_SCOPE)
          set("FFMPEG_lib${component}_VERSION" "${major}.${minor}.${micro}" CACHE INTERNAL "")
        endif ()
      endif ()
    else ()
      set("FFMPEG_${component}_FOUND" 0
        PARENT_SCOPE)
      set(what)
      if (NOT FFMPEG_${component}_LIBRARY)
        set(what "library")
      endif ()
      if (NOT FFMPEG_${component}_INCLUDE_DIR)
        if (what)
          string(APPEND what " or headers")
        else ()
          set(what "headers")
        endif ()
      endif ()
      set("FFMPEG_${component}_NOT_FOUND_MESSAGE"
        "Could not find the ${what} for ${component}."
        PARENT_SCOPE)
    endif ()
  endif ()
endfunction ()

_ffmpeg_find(avutil     avutil.h)
_ffmpeg_find(avresample avresample.h
  avutil)
_ffmpeg_find(swresample swresample.h
  avutil)
_ffmpeg_find(swscale    swscale.h
  avutil)
_ffmpeg_find(avcodec    avcodec.h
  avutil)
_ffmpeg_find(avformat   avformat.h
  avcodec avutil)
_ffmpeg_find(avfilter   avfilter.h
  avutil)
_ffmpeg_find(avdevice   avdevice.h
  avformat avutil)

if(NOT FFMPEG_FIND_COMPONENTS)
  set(FFMPEG_FIND_COMPONENTS avutil swresample swscale avcodec avformat avfilter avdevice)  # optional avresample
endif()

if (TARGET FFMPEG::avutil)
  set(_ffmpeg_version_header_path "${FFMPEG_avutil_INCLUDE_DIR}/libavutil/ffversion.h")
  if (EXISTS "${_ffmpeg_version_header_path}")
    file(STRINGS "${_ffmpeg_version_header_path}" _ffmpeg_version
      REGEX "FFMPEG_VERSION")
    string(REGEX REPLACE ".*\"n?\(.*\)\"" "\\1" FFMPEG_VERSION "${_ffmpeg_version}")
    unset(_ffmpeg_version)
  else ()
    set(FFMPEG_VERSION FFMPEG_VERSION-NOTFOUND)
  endif ()
  unset(_ffmpeg_version_header_path)
endif ()

set(FFMPEG_INCLUDE_DIRS)
set(FFMPEG_LIBRARIES)
set(__ffmpeg_version_vars)
set(_ffmpeg_required_vars)
foreach (_ffmpeg_component IN LISTS FFMPEG_FIND_COMPONENTS)
  if (TARGET "FFMPEG::${_ffmpeg_component}")
    set(FFMPEG_${_ffmpeg_component}_INCLUDE_DIRS
      "${FFMPEG_${_ffmpeg_component}_INCLUDE_DIR}")
    set(FFMPEG_${_ffmpeg_component}_LIBRARIES
      "${FFMPEG_${_ffmpeg_component}_LIBRARY}")
    list(APPEND FFMPEG_INCLUDE_DIRS
      "${FFMPEG_${_ffmpeg_component}_INCLUDE_DIRS}")
    list(APPEND FFMPEG_LIBRARIES
      "${FFMPEG_${_ffmpeg_component}_LIBRARIES}")
    if (FFMEG_FIND_REQUIRED_${_ffmpeg_component})
      list(APPEND _ffmpeg_required_vars
        "FFMPEG_${_ffmpeg_required_vars}_INCLUDE_DIRS"
        "FFMPEG_${_ffmpeg_required_vars}_LIBRARIES")
    endif ()
  endif ()
endforeach ()
unset(_ffmpeg_component)

if(FFMPEG_INCLUDE_DIRS)
  list(REMOVE_DUPLICATES FFMPEG_INCLUDE_DIRS)
endif ()

include(FindPackageHandleStandardArgs)
find_package_handle_standard_args(FFMPEG
  REQUIRED_VARS FFMPEG_INCLUDE_DIRS FFMPEG_LIBRARIES ${_ffmpeg_required_vars}
  VERSION_VAR FFMPEG_VERSION
  HANDLE_COMPONENTS)
unset(_ffmpeg_required_vars)

instructions.txt

# 1. Download FFmpeg from https://www.gyan.dev/ffmpeg/builds/packages/
#    2021.02: verified with *full_build-shared* archive

# 2. Create 'cmake' subdirectory and copy 'ffmpeg-config' there.

# 3.
SET PATH=<ffmpeg>\bin;%PATH%
cmake ... -DOPENCV_FFMPEG_USE_FIND_PACKAGE=FFMPEG
cmake --build . ...

# 4.
# Python 3.8+ ignores system PATH and requires to use os.add_dll_directory() instead