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• ffmpeg screen blending mode with transparency

  17 novembre 2013, par gilad s

  I have a sequence of PNG files with transparency and a video- video1.mp4

  1. I'd like to convert the PNG files into a video with an alpha channel- video2.mp4

  2. I'd like to merge video2.mp4 with video1.mp4 while the transparent parts in video2 are

  not seen and the other parts are blended using screen blending mode with video1

  when I do :

  ffmpeg -i "%03d.png" -vf scale=640:480 -vcodec libx264 video2.mp4

  a video is created with a black color where the transperent parts are supposed to be (I'm not sure if it's really transparent - I tryied other encoders - all gave the same result).

  afterwards when I do :

  ffmpeg -i video1.mp4 -i video2.mp4 -filter_complex "blend=all_mode='screen'" output.mp4

  the videos are indeed merged but the video has a pink overlay ?!

• Showing a video using C (not C++ or C#)

  1er octobre 2013, par user2833591

  I learned programming in C using the Tscope-library (which is incompatible with C++ and C#), so I'm completely stuck with C.

  The Tscope-library is used to program small psychological experiments, it allows for functions that generate random numbers or produce images on the screen. Not sure if it might be a problem, but Tscope does generate it's own 'window'.

  So I wanted my experiment to show videos (currently in .wmv-format, but that can be changed, no problem), but I don't know how to do so (neither in code nor concept).

  I have come across FFmpeg, but the longer I see its code, the more I worry it's not meant for C (as in, parts of the code appear completely unknown to me). Could someone please help me ? If FFmpeg is indeed the answer, could someone give a quick run-down of the idea behind how it works (I've seen something about frames being put together) ?

• JNI crash when I split code in two functions

  13 septembre 2013, par Lescott

  I have a properly working native C function which I call from my java code. But when I split this code in two functions and sequentially call them both I got fatal error.

  //global variables
  
  AVFormatContext *pFormatCtx;
  
  AVFrame         *pFrame;
  
  AVFrame         *pFrameRGB;
  
  AVCodecContext  *pCodecCtx;
  
  AVCodec         *pCodec;
  
  uint8_t         *buffer;
  
  int             videoStream;
  
  struct SwsContext      *sws_ctx = NULL;
  
  int outWidth, outHeight;

  Working unsplitted function

  JNIEXPORT void JNICALL Java_foo(JNIEnv * env, jclass class) {
  
      av_register_all();
  
      const char* videoPath = "11.mp4";
  
  
  
      int             numBytes;
  
      AVDictionary    *optionsDict = NULL;
  
  
  
      pFrame = NULL; 
  
      pFrameRGB = NULL;
  
      buffer = NULL;
  
      pCodec = NULL;
  
      pFormatCtx = NULL;
  
  
  
      // Open video file
  
      if(avformat_open_input(&pFormatCtx, videoPath, NULL, NULL)!=0)
  
              exit(1); // Couldn't open file
  
  
  
  
  
      // Retrieve stream information
  
      if(avformat_find_stream_info(pFormatCtx, NULL)<0)
  
              exit(1); // Couldn't find stream information
  
  
  
       av_dump_format(pFormatCtx, 0,videoPath, 0);
  
  
  
  
  
      // Find the first video stream
  
      videoStream=-1;
  
      int i;
  
      for(i=0; inb_streams; i++) {
  
              if(pFormatCtx->streams[i]->codec->codec_type==AVMEDIA_TYPE_VIDEO) {
  
                      videoStream=i;
  
                      break;
  
              }
  
      }
  
  
  
      if(videoStream==-1)
  
              exit(1); // Didn't find a video stream
  
  
  
      // Get a pointer to the codec context for the video stream
  
      pCodecCtx=pFormatCtx->streams[videoStream]->codec;
  
  
  
  
  
      // Find the decoder for the video stream
  
      pCodec=avcodec_find_decoder(pCodecCtx->codec_id);
  
      if(pCodec==NULL) {
  
              fprintf(stderr, "Unsupported codec!\n");
  
              exit(1); // Codec not found
  
      }
  
  
  
      // Open codec
  
      if(avcodec_open2(pCodecCtx, pCodec, &optionsDict)<0)
  
              exit(1); // Could not open codec
  
  
  
      // Allocate video frame
  
      pFrame=avcodec_alloc_frame();
  
  
  
      // Allocate an AVFrame structure
  
      pFrameRGB=avcodec_alloc_frame();
  
      if(pFrameRGB==NULL)
  
              exit(1);
  
  
  
      outWidth = 128;
  
      outHeight = 128;
  
  
  
      // Determine required buffer size and allocate buffer
  
      numBytes=avpicture_get_size(PIX_FMT_RGB24, outWidth, outHeight);
  
      buffer=(uint8_t *)av_malloc(numBytes*sizeof(uint8_t));
  
  
  
      sws_ctx = sws_getContext(
  
                              pCodecCtx->width,
  
                              pCodecCtx->height,
  
                              pCodecCtx->pix_fmt,
  
                              outWidth,
  
                              outHeight,
  
                              PIX_FMT_RGB24,
  
                              SWS_BILINEAR,
  
                              NULL,
  
                              NULL,
  
                              NULL
  
                      );
  
  
  
      // Assign appropriate parts of buffer to image planes in pFrameRGB
  
      // Note that pFrameRGB is an AVFrame, but AVFrame is a superset
  
      // of AVPicture
  
      avpicture_fill((AVPicture *)pFrameRGB, buffer, PIX_FMT_RGB24, outWidth, outHeight);
  
  }

  Failing splitted functions

  JNIEXPORT void JNICALL Java_foo1(JNIEnv * env, jclass class) {
  
      av_register_all();
  
  }
  
  
  
  JNIEXPORT void JNICALL Java_foo2(JNIEnv * env, jclass class) {
  
      //all lines of code from Java_foo exept the first
  
  }

  Java code

  System.loadLibrary("mylib");
  
  Mylib.foo1();
  
  Mylib.foo2(); //fatal error
  
  
  
  
  
  #
  
  # A fatal error has been detected by the Java Runtime Environment:
  
  #
  
  #  SIGSEGV (0xb) at pc=0x00007faab5012dc0, pid=15571, tid=140371352766208

  Any ideas ?