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• FFmpeg - avcodec_receive_frame returns 0 but frames are invalid

  9 juillet 2019, par Jinx

  I’ve been trying to extract images from videos, but not those with PNG codec. My code works fine with those with JPEG. avcodec_receive_frame worked successfully but the data of the frames was like trash ? Do I have to do something special to demuxing when dealing with PNG ?

  Exception thrown at 0x00007FFF7DF34B9A (msvcrt.dll) in Program.exe : 0xC0000005 : Access violation reading location 0x00000000000003F0 when calling avcodec_send_frame in my saveImage function, which means I was accessing invalid or unalloacted memory I guess. How this happened ?

  

  Just suppose all the function calls returned 0 until exception thrown.

  Decoding :

  bool ImageExtractor::decode() {
  
      // some other code here
  
      ret = avcodec_send_packet(codec_ctx, packet); // returned 0
  
      ret = avcodec_receive_frame(codec_ctx, frame); // returned 0
  
      if (ret == 0) {
  
          if (count >= target_frame) {
  
              snprintf(buf, sizeof(buf), "%s/%d.png", destination.toLocal8Bit().data(), count);
  
              saveImage(frame, buf); // a function that writes images on disk 
  
         }
  
      // some other code here
  
  }
  
  
  
  
  
  bool ImageExtractor::saveImage(AVFrame *frame, char *destination) {
  
       AVFormatContext *imgFmtCtx = avformat_alloc_context();
  
       pFormatCtx->oformat = av_guess_format("mjpeg", NULL, NULL);
  
  
  
       // some other code here
  
  
  
      if (!frame)
  
          std::cout << "invalid frame \n";
  
  
  
      if (!imgCodecCtx) // AV_CODEC_ID_MJPEG(7)
  
          std::cout << "invalid codec ctx \n";
  
  
  
      ret = avcodec_send_frame(imgCodecCtx, frame); // everything stopped here
  
  }

  Demuxing :

  avformat_open_input(&format_ctx, source.toLocal8Bit(), nullptr, nullptr);
  
  vsi = av_find_best_stream(format_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, nullptr, 0);
  
  codec_par = avcodec_parameters_alloc();
  
  avcodec_parameters_copy(codec_par, format_ctx->streams[vsi]->codecpar);
  
  
  
  AVCodec* codec = avcodec_find_decoder(codec_par->codec_id); // AV_CODEC_ID_PNG(61)
  
  codec_ctx = avcodec_alloc_context3(codec);
  
  avcodec_parameters_to_context(codec_ctx, codec_par);
  
  avcodec_parameters_free(&codec_par);
  
  avcodec_open2(codec_ctx, codec, 0);

• Android : BitmapFactory.decodeStream() returns null after first success

  27 avril 2014, par giraffe

  My code is intended to update an ImageView with an image from a server when a UI button is pressed.

  The client side code shown below is a Runnable that runs when the button is pressed. The server is a desktop Java application with ffmpeg running in the background, continuously updating image.png with an image from the webcam. When the button is pressed on the Android app, the Android app attempts to receive image.png from the server, and because ffmpeg is constantly updating this image, it should be the most recent image taken with the server’s webcam.

  My problem is that the first button press shows the correct image, but every subsequent button press will just clear out my ImageView. BitmapFactory.decodeStream() is returning null every time I call it after the first time.

  Client side (runs when button is pressed) :

  InputStream inputStream = s.getInputStream();
  
  img = BitmapFactory.decodeStream(inputStream);          
  
  jpgView.setImageBitmap(img);        
  
  jpgView.invalidate();

  Server side :

  ServerSocket sock = new ServerSocket(PORT_NUMBER);
  
  Socket clientSocket = sock.accept();
  
  for (;;) {
  
      File f = new File("C:/folder/image.png");
  
      FileInputStream fileInput = new FileInputStream(f);
  
      BufferedInputStream bufferedInput = new BufferedInputStream(fileInput);
  
      OutputStream outStream = clientSocket.getOutputStream();
  
      try {
  
          byte[] outBuffer = new byte[fSize];
  
          int bRead = bufferedInput.read(outBuffer, 0, outBuffer.length);
  
          outStream.write(outBuffer, 0, bRead);
  
          outStream.flush();
  
          } catch (Exception e) {
  
              e.printStackTrace();
  
          } finally {
  
              bufferedInput.close();
  
          }
  
      }

• Why process output reader returns null data ? When same code in other process works correct

  26 août 2019, par Владимир Водов

  I try to get output from ffmpeg process but cant get output.
  In another processes and commands it works correctly but output returns immideately when start !

          using (var process = new Process())
  
          {
  
              process.StartInfo = new ProcessStartInfo()
  
              {
  
                  FileName = LinkHelper.IPFS_PATH,
  
                  Arguments = cmd,
  
                  UseShellExecute = false,
  
                  CreateNoWindow = true,
  
                  RedirectStandardOutput = true
  
              };
  
  
  
              process.ErrorDataReceived += FfmpegErrorRecieved;
  
              process.Start();
  
  
  
              using (StreamReader reader = process.StandardOutput)
  
              {
  
                  string output = await reader.ReadToEndAsync();
  
                  Console.WriteLine(output);
  
              }               
  
              process.WaitForExit();
  
          }

  Before output handle !

  Update :
  As szatmary said ffmpeg use output error instead standard output, so when you initialize process.StandartInfo don’t forget initialize property "RedirectStandardError" to TRUE !

  Here is correct code :

  private async Task DetectFFmpegCamerasAsync()
  
      {
  
          var cmd = "-list_devices true -f dshow -i dummy";
  
          using (var process = new Process())
  
          {
  
              process.StartInfo = new ProcessStartInfo()
  
              {
  
                  FileName = LinkHelper.FFMPEG_PATH,
  
                  Arguments = cmd,
  
                  UseShellExecute = false,
  
                  CreateNoWindow = true,
  
                  RedirectStandardError = true
  
              };
  
  
  
              process.Start();
  
  
  
              using (StreamReader reader = process.StandardError)
  
              {
  
                  string output = await reader.ReadToEndAsync();
  
                  Console.WriteLine($"Camera detection output: \n {output}");
  
              }               
  
              process.WaitForExit();
  
          }
  
      }