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• ffplay does not exit in forked child

  6 septembre 2019, par user12030145

  ffplay  -autoexit does not exit in a forked child

  I need to pipe my application (stdout) to ffplay (stdin). I do this by forking ffplay as a child and using -i pipe:0 as argument.

  #include 
  
  #include 
  
  #include <sys></sys>types.h>
  
  #include <sys></sys>wait.h>
  
  
  
  int main(int argc, const char** argv)
  
  {
  
  int tube[2];
  
  int c;
  
  FILE* f = fopen(argv[1], "rb");
  
  pid_t pid;
  
  if (argc &lt; 2) return -1;
  
  if (pipe(tube))  {
  
      perror("Pipe");
  
      return -1;
  
    }
  
  
  
  // main process cats a .mlp file to stdout, sent to a child ffplay stdin through a pipe
  
  char* const arg[] = {"-i", "pipe:0", "-f", "mlp", "-nodisp", "-autoexit", NULL};
  
  switch (pid = fork())    {
  
              case -1:
  
                  fprintf(stderr,"%s\n", "Could not launch ffplay");
  
                  break;
  
  
  
              case 0:
  
                  close(tube[1]);
  
                  dup2(tube[0], STDIN_FILENO);
  
                  execv("/usr/bin/ffplay", arg);
  
                  fprintf(stderr, "%s\n", "Runtime failure in ffplay child process");
  
                  return -2;
  
  
  
              default:
  
                  close(tube[0]);
  
                  dup2(tube[1], STDOUT_FILENO);
  
          }
  
  
  
  // Here the main process code sending the .mlp file to stdout...
  
  
  
  while ((c = fgetc(f)) != EOF) putchar(c);
  
  
  
  waitpid(pid, NULL, 0);
  
  fclose(f);
  
  
  
  // main never returns
  
  return 0;
  
  }

  The issue is that in this context, ffplay -autoexit never exits (GNU-Linux platform). In a main process, ffplay -autoexit always exits at the end of a media file.
  Is there a pure C workaround without using system, popen or scripting ?
  Is this a feature or a bug of ffplay (I cannot tell) ?

• MSVC fatal error LNK1120 : 1 unresolved externals with FFMPEG libs

  16 août 2019, par Jared

  I am trying to utilize the ffmpeg libraries in a program of my own and am having trouble linking them. Specifically, In my a basic program I am receiving fatal error LNK1120: 1 unresolved externals errors. The program is :

  #include <iostream>
  
  #include <libswresample></libswresample>swresample.h>
  
  
  
  int main()
  
  {
  
      std::cout &lt;&lt; "Hello World!\n";
  
      struct SwrContext* swr_ctx = swr_alloc();
  
      if (!swr_ctx) {
  
          std::cout &lt;&lt; "Could not allocate resampler context";
  
      }
  
  }
  
  </iostream>

  I downloaded prebuild libraries from https://ffmpeg.zeranoe.com/builds/, specifically the Windows x64 dev package which includes the .def/.lib as well as .dll files.

  I originally tried (and intend to ultimately use) cmake to generate the MSVC sln files. The cmake file is :

  cmake_minimum_required(VERSION 3.5)
  
  
  
  project(ffmpeg_jni)
  
  
  
  # Find the JNI bits
  
  find_package(JNI)
  
  
  
  # Search for the ffmpeg libraries
  
  set(ffmpeg_include_hint "ffmpeg-dev/include")
  
  set(ffmpeg_lib_hint "ffmpeg-dev/lib")
  
  
  
  find_path(SWRESAMPLE_INCLUDE_DIR libswresample/swresample.h PATHS ${ffmpeg_include_hint})
  
  find_library(SWRESAMPLE_LIBRARY swresample PATHS ${ffmpeg_lib_hint})
  
  add_library(swresample SHARED IMPORTED)
  
  set_target_properties(swresample PROPERTIES
  
      IMPORTED_LOCATION "${SWRESAMPLE_LIBRARY}"
  
      IMPORTED_IMPLIB "${SWRESAMPLE_LIBRARY}"
  
      INTERFACE_INCLUDE_DIRECTORIES "${SWRESAMPLE_INCLUDE_DIR}"
  
  )
  
  
  
  # Setup basic include dirs
  
  set(includeDIRS
  
          src/main/cpp
  
          ${JAVA_INCLUDE_PATH})
  
  
  
  # Setup windows specific includes
  
  set(includeDIRS
  
          ${includeDIRS}
  
          ${JAVA_INCLUDE_PATH}/Win32)
  
  
  
  include_directories(${includeDIRS})
  
  
  
  set(WRAPPER_SRC
  
          src/main/cpp/logging.c
  
          src/main/cpp/logging.h
  
          src/main/cpp/main.cpp)
  
  
  
  add_library(ffmpeg_jni SHARED ${WRAPPER_SRC})
  
  target_link_libraries(ffmpeg_jni PRIVATE swresample)

  The generated solution compiles and has proper access to the include files (Visual Studio can even take me to the declarations). The issue comes in the linking phase of the build where I receive :

  error LNK2019 : unresolved external symbol "struct SwrContext * __cdecl
  swr_alloc(void)" (?swr_alloc@@YAPEAUSwrContext@@XZ) referenced in
  function main

  Thinking that I perhaps had something wrong in cmake since I am still pretty new with it I tried making a simple demo as a pure visual studio project following what I have found in countless online demos for adding an external library to a project. Specifically this included :

  • Adding the directory containing the header files to Properties->C/C++->General->Additional Include Directories
  • Adding the directory containing the .lib files to Properties->Linker->General->Additional Library Directories (Note that the cmake path did not do this but instead added the lib file via a relative path)
  • Adding the .lib file to Properties->Linker->Input->Additional Dependencies

  At this point any searching efforts I undertake show me different people doing the same things which tells me I’ve been looking at this too long to find the answer myself and its something trivial that I’m missing/not understanding.

• How to accurately match an image to a video frame and exit with ffmpeg

  10 avril 2020, par Hans J

  In Bash,
I am trying to match an image to a frame in ffmpeg. I also want to exit the ffmpeg process when the match is found. Here is a (simplified version) of the code currently :

  



  ffmpeg --hide_banner -ss 0 -to 60 \
-i "video.mp4" -i "image.jpg" -filter_complex \
"blend=difference, blackframe" -f null - null 2>log.txt &
pid=$!
trap "kill $pid 2>/dev/null" EXIT
while kill -0 $pid 2>/dev/null; do
     # (grep command to monitor log file)
     # if grep finds blackframe match, return blackframe time
done


  



  To my understanding, if the video actually contains a blackframe I will get a false-positive. How can I effectively mitigate this ?

  



  While this is unnecessary to answer the question, I would like to exit the ffmpeg process without having to use grep to constantly monitor the log file, instead using pure ffmpeg

  



  Edit : I say this because while I understand the blend filter is computing the difference, I am getting a false positive on a blackframe in my video and I don't know why.

  



  Edit : A possible solution to this issue is to not use blackframe at all, but psnr (Peak Signal to Noise Ratio) but normal usage is by comparing two videos frame by frame, and I don't know how to effectively use it with an image as input.