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• Understanding FFMPEG Video Encoding

  20 juin 2014, par SetSlapShot

  Got this from the encoding example in ffmpeg. I can somewhat follow the authors example for audio encoding, but I find myself befuddled looking at the C code (I commented in block numbers to help me reference what I’m talking about)...

  static void video_encode_example(const char *filename)
  
  {
  
  AVCodec *codec;
  
  AVCodecContext *c= NULL;
  
  int i, out_size, size, x, y, outbuf_size;
  
  FILE *f;
  
  AVFrame *picture;
  
  uint8_t *outbuf, *picture_buf;              //BLOCK ONE
  
  printf("Video encoding\n");
  
  
  
  /* find the mpeg1 video encoder */
  
  codec = avcodec_find_encoder(CODEC_ID_MPEG1VIDEO);
  
  if (!codec) {
  
      fprintf(stderr, "codec not found\n");
  
      exit(1);                                //BLOCK TWO
  
  }
  
  
  
  c= avcodec_alloc_context();
  
  picture= avcodec_alloc_frame();
  
  /* put sample parameters */
  
  c->bit_rate = 400000;
  
  /* resolution must be a multiple of two */
  
  c->width = 352;
  
  c->height = 288;
  
  /* frames per second */
  
  c->time_base= (AVRational){1,25};
  
  c->gop_size = 10; /* emit one intra frame every ten frames */
  
  c->max_b_frames=1;
  
  c->pix_fmt = PIX_FMT_YUV420P;                   //BLOCK THREE
  
  
  
  /* open it */
  
  if (avcodec_open(c, codec) < 0) {
  
      fprintf(stderr, "could not open codec\n");
  
      exit(1);
  
  }
  
  f = fopen(filename, "wb");
  
  if (!f) {
  
      fprintf(stderr, "could not open %s\n", filename);
  
      exit(1);
  
  }                                               //BLOCK FOUR
  
  
  
  /* alloc image and output buffer */
  
  outbuf_size = 100000;
  
  outbuf = malloc(outbuf_size);
  
  size = c->width * c->height;
  
  picture_buf = malloc((size * 3) / 2); /* size for YUV 420 */
  
  picture->data[0] = picture_buf;
  
  picture->data[1] = picture->data[0] + size;
  
  picture->data[2] = picture->data[1] + size / 4;
  
  picture->linesize[0] = c->width;
  
  picture->linesize[1] = c->width / 2;
  
  picture->linesize[2] = c->width / 2;              //BLOCK FIVE
  
  
  
  /* encode 1 second of video */
  
  for(i=0;i<25;i++) {
  
      fflush(stdout);
  
      /* prepare a dummy image */
  
      /* Y */
  
      for(y=0;yheight;y++) {
  
          for(x=0;xwidth;x++) {
  
              picture->data[0][y * picture->linesize[0] + x] = x + y + i * 3;
  
          }
  
      }                                            //BLOCK SIX
  
  
  
      /* Cb and Cr */
  
      for(y=0;yheight/2;y++) {
  
          for(x=0;xwidth/2;x++) {
  
              picture->data[1][y * picture->linesize[1] + x] = 128 + y + i * 2;
  
              picture->data[2][y * picture->linesize[2] + x] = 64 + x + i * 5;
  
          }
  
      }                                           //BLOCK SEVEN
  
  
  
      /* encode the image */
  
      out_size = avcodec_encode_video(c, outbuf, outbuf_size, picture);
  
      printf("encoding frame %3d (size=%5d)\n", i, out_size);
  
      fwrite(outbuf, 1, out_size, f);
  
  }                                              //BLOCK EIGHT
  
  
  
  /* get the delayed frames */
  
  for(; out_size; i++) {
  
      fflush(stdout);
  
      out_size = avcodec_encode_video(c, outbuf, outbuf_size, NULL);
  
      printf("write frame %3d (size=%5d)\n", i, out_size);
  
      fwrite(outbuf, 1, out_size, f);
  
  }                                             //BLOCK NINE
  
  
  
  /* add sequence end code to have a real mpeg file */
  
  outbuf[0] = 0x00;
  
  outbuf[1] = 0x00;
  
  outbuf[2] = 0x01;
  
  outbuf[3] = 0xb7;
  
  fwrite(outbuf, 1, 4, f);
  
  fclose(f);
  
  free(picture_buf);
  
  free(outbuf);
  
  avcodec_close(c);
  
  av_free(c);
  
  av_free(picture);
  
  }                                            //BLOCK TEN

  Here’s what I can get from the authors code block by block...

  BLOCK ONE : Initializing Variables and pointers. I couldn’t find the AVFrame struct yet in the ffmpeg source code so I don’t know what its referencing

  BLOCK TWO : Uses a codec from the file, if not found close.

  BLOCK THREE : Sets sample video parameters. Only thing I don’t really get is gop size. I read about intra frames and I still don’t get what they are.

  BLOCK FOUR : Open the file for writing...

  BLOCK FIVE : Here’s where they really start losing me. Part is probably because I don’t know exactly what AVFrame is, but why do they only use 3/2 of the image size ?

  BLOCK SIX & SEVEN : I don’t understand what they are trying to accomplish with this math.

  BLOCK EIGHT : It looks like the avcodec function does all the work here, not concerned with that for the time being..

  BLOCK NINE : Since it’s outside the 25 frame for loop I assume it gets the leftover frames ?

  BLOCK TEN : Close, free mem, etc...

  I know this is a large block of code to be confused with, any input would be helpful. I got put in over my head at work. Thanks in advance SO.

• What is the meaning of the hexadecimal value that follows the avc1 in an m3u8 variant playlist

  16 février 2013, par Anand

  From this apple's sample variant playlist

  https://devimages.apple.com.edgekey.net/resources/http-streaming/examples/bipbop_4x3/bipbop_4x3_variant.m3u8

  i see hexadecimal numbers follow the av1 like avc1.4d4015,avc1.4d401e,avc1.4d401e,avc1.4d401f can anybody know what is the meaning of them

• ffmpeg with a set of image files not formatted as %d.jpg

  12 juin 2012, par Rob Lourens

  I'm using ffmpeg to compile a set of jpgs into a video. There is plenty written about this, but it seems that the only way to do it is to have images named as consecutive padded numbers, e.g. 0001.jpg, 0002.jpg...

  The ffmpeg documentation states that it is possible to use other types of patterns, such as %*.jpg to capture all *.jpg files, but the only pattern that I have gotten to work on my own is the %0Nd-type pattern. The man page only mentions that type.

  I really want to have ffmpeg use a set of images with arbitrary names. It would simplify my app quite a lot, make it easier to keep thumbnails and metadata in sync as images are inserted and deleted, etc. Creating links is not an option since I'm working on Android. Is there any way to do this ?

  I'm also willing to modify the ffmpeg source or work with the C api to get it to do what I want, but I can't find the right spot in the code to do it, or appropriate docs for the C api. Any advice ? Thanks.