FFmpeg SDK开发模型之一：解码器

简介
本例讲解了如何使用ffmpeg SDK解码媒体文件；
参考源码是ffmpeg 自带的apiexample.c

一、源代码
#include
#include
#include
#include

#ifdef HAVE_AV_CONFIG_H
#undef HAVE_AV_CONFIG_H
#endif

#include "libavcodec/avcodec.h"
#define INBUF_SIZE 4096

void pgm_save(unsigned char *buf,int wrap, int xsize,int ysize,char *filename)
{
  FILE *f;
  int i;

  f=fopen(filename,"w");
  fprintf(f,"P5\n%d %d\n%d\n",xsize,ysize,255);
  for(i=0;i   fwrite(buf + i * wrap,1,xsize,f);
  fclose(f);
}

/*
 * Video decoding
 */
void video_decode_example(const char *outfilename, const char *filename)
{
  AVCodec *codec;
  AVCodecContext *c= NULL;
  int frame, size, got_picture, len;
  FILE *f;
  AVFrame *picture;
  uint8_t inbuf[INBUF_SIZE + FF_INPUT_BUFFER_PADDING_SIZE], *inbuf_ptr;
  char buf[1024];

  uint8_t *pkt_ptr, pkt_len;

  /* set end of buffer to 0 (this ensures that no overreading happens for damaged mpeg streams) */
  memset(inbuf + INBUF_SIZE, 0, FF_INPUT_BUFFER_PADDING_SIZE);

  printf("Video decoding\n");

  /* find the mpeg1 video decoder */
  //codec = avcodec_find_decoder(CODEC_ID_H264);
  codec = avcodec_find_decoder(CODEC_ID_MPEG2VIDEO);
  if (!codec) {
    fprintf(stderr, "codec not found\n");
    exit(1);
  }

  c= avcodec_alloc_context();
  picture= avcodec_alloc_frame();

  if(codec->capabilities&CODEC_CAP_TRUNCATED)
  c->flags|= CODEC_FLAG_TRUNCATED; /* we dont send complete frames */

  /* for some codecs, such as msmpeg4 and mpeg4, width and height
     MUST be initialized there because these info are not available
     in the bitstream */

  /* open it */
  if (avcodec_open(c, codec) < 0) {
    fprintf(stderr, "could not open codec\n");
    exit(1);
  }

  /* the codec gives us the frame size, in samples */
  f = fopen(filename, "rb");
  if (!f) {
    fprintf(stderr, "could not open %s\n", filename);
    exit(1);
  }
  frame = 0;
  for(;;) {
    AVPacket pkt;

    if (av_read_frame(c, &pkt) < 0) {
      continue;
    }

    //        size = fread(inbuf, 1, INBUF_SIZE, f);
    //        if (size == 0)
    //            break;

    /* NOTE1: some codecs are stream based (mpegvideo, mpegaudio)
       and this is the only method to use them because you cannot
       know the compressed data size before analysing it.

       BUT some other codecs (msmpeg4, mpeg4) are inherently frame
       based, so you must call them with all the data for one
       frame exactly. You must also initialize 'width' and
       'height' before initializing them. */

    /* NOTE2: some codecs allow the raw parameters (frame size,
       sample rate) to be changed at any frame. We handle this, so
       you should also take care of it */

    /* here, we use a stream based decoder (mpeg1video), so we
       feed decoder and see if it could decode a frame */
    pkt_len = pkt.size;
    pkt_ptr = pkt.data;

    //  inbuf_ptr = inbuf;
    while (pkt_len > 0) {
      len = avcodec_decode_video2(c, picture, &got_picture,
            &pkt);
      if (len < 0) {
        fprintf(stderr, "Error while decoding frame %d\n", frame);
        exit(1);
      }
      if (got_picture) {
        printf("saving frame %3d\n", frame);
        fflush(stdout);

        /* the picture is allocated by the decoder. no need to
           free it */
        snprintf(buf, sizeof(buf), outfilename, frame);
        pgm_save(picture->data[0], picture->linesize[0],
              c->width, c->height, buf);
        frame++;
      }
      size -= len;
      inbuf_ptr += len;
    }
    av_free_packet(&pkt);
  }

  /* some codecs, such as MPEG, transmit the I and P frame with a
     latency of one frame. You must do the following to have a
     chance to get the last frame of the video */
  len = avcodec_decode_video2(c, picture, &got_picture,
        NULL);
  if (got_picture) {
    printf("saving last frame %3d\n", frame);
    fflush(stdout);

    /* the picture is allocated by the decoder. no need to
       free it */
    snprintf(buf, sizeof(buf), outfilename, frame);
    pgm_save(picture->data[0], picture->linesize[0],
          c->width, c->height, buf);
    frame++;
  }

  fclose(f);

  avcodec_close(c);
  av_free(c);
  av_free(picture);
  printf("\n");
}

int main(int argc, char **argv)
{
  const char *filename;

  /* must be called before using avcodec lib */
  avcodec_init();

  /* register all the codecs (you can also register only the codec
     you wish to have smaller code */
  avcodec_register_all();

  if (argc <= 1) {
    //        audio_encode_example("/tmp/test.mp2");
    //        audio_decode_example("/tmp/test.sw", "/tmp/test.mp2");

    video_encode_example("/tmp/test.mpg");
    filename = "/tmp/test.mpg";
  } else {
    filename = argv[1];
  }

  //    audio_decode_example("/tmp/test.sw", filename);
  video_decode_example("/tmp/test%d.pgm", filename);

  return 0;
}

二、代码分析
01  avcodec_init(), 初始化libavcodec
02  avcodec_register_all(), 注册所有编解码器和格式

03  codec = avcodec_find_decoder(CODEC_ID_MPEG2VIDEO), 查找mpeg2 video解码器
04  c     = avcodec_alloc_context(), 分配AVCodecContext空间并初始化
05  picture = avcodec_alloc_frame(), 分配AVFrame的空间
06  avcodec_open(c, codec),  使用codec来初始化c

07  FOR循环 {
      07.1 av_read_frame(c, &pkt), 从输入文件中读取一个包
      07.2 pkt_len = pkt.size;
           pkt_ptr = pkt.data;
      07.3 while(pkt_len > 0) {
             07.3.1 avcodec_decode_video2(c, picture, &got_picture, &pkt), 解码当前包
             07.3.2 if (got_picture) {
                       pgm_save(picture->data[0], ...), 对解码后的YUV数据进行处理；
                       frame++;
                    }
             07.3.3 size -= len;
           }
      07.4 av_free_packet(&pkt), 释放当前包
    }
08  len = avcodec_decode_video2(c, picture, &got_picture, NULL); 解码最后的帧；
09  if (got_picture) { ...}, 处理最后帧的YUV数据

10  avcodec_close(c);
11  av_free(c)
12  av_free(picture);