使用FFmpeg开发2-比特流过滤器

本文介绍: 在mp4（H264编码）文件中，一般使用的是AVVC格式，因为封装了同步信息，就去掉了上述的起始码，如果见数据直接丢到解码器，解码器是没法找到数据头进行解码的，因此需要过滤器来为数据加上这个起始码。这里给出一个实现参考，从i mx的h ant ro中抠出来的，从中也可以一窥用法，支持的FFmpeg 版本比较旧，有时间再整理下新版的代码。在wrap的使用博文中介绍了裸流解码，这时候如果搭配了FFmpeg的过滤器，即可以实现读取 mp4文件，通过过滤器分离出裸H264数据，然后送到解码器解码，实现对封装数据的解码。

在使用FFmpeg 处理视频文件时，会遇到需要提取视频流的情况。例如有一个H264编码的mp4文件，需要提取帧数据送到解码器解码，这时候过滤器就派上用场了。

为什么要这么干，因为我用的是自有的硬件解码器，还未集成到FFmpeg中，没法在FFm pe g中直接调用。

对于H264裸流文件，一般使用的Ann e xB格式，每一帧由NAL单元加上一个起始码组成（三字节00 00 01或者四字节00 00 00 01）。解码器就是根据这个头来分割数据的。

在mp4（H264编码）文件中，一般使用的是AVVC格式，因为封装了同步信息，就去掉了上述的起始码，如果见数据直接丢到解码器，解码器是没法找到数据头进行解码的，因此需要过滤器来为数据加上这个起始码。h264_mp4t oa nn e xb 码流过滤器就解决了这个问题。它识别文件头，并提取里面的ext r a data，ext r a data这个数据就是封装信息。然后给每个NAL单元添加上起始码，再把数据送到硬件解码器。

对于mp4（H265编码）文件，则使用he vc _mp4t oa nn e x b。

在wrap的使用博文中介绍了裸流解码，这时候如果搭配了FFmp e g的过滤器，即可以实现读取mp4文件，通过过滤器分离出裸H264数据，然后送到解码器解码，实现对封装数据的解码。

imx VPU解码分析3-wrap的示例-CSDN博客

这里给出一个实现参考，从i mx的hant ro中抠出来的，从中也可以一窥用法，支持的FFmp e g版本比较旧，有时间再整理下新版的代码。貌似新版的FFmp e g将过滤器集成到了读入视频的过程，不需要额外处理了。

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#include "libav-wrapper.h"

#include "libavcodec/avcodec.h"
#include "libavformat/avformat.h"
#include "libavcodec/bsf.h"
#include "libavutil/mem.h"

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static AVFormatContext *p_format_ctx = NULL;
static AVCodecContext *p_codec_ctx = NULL;
static AVCodecParameters *origin_par = NULL;
//gan 20230907 AVBitStreamFilterContext is instand by AVBSFContext
//static AVBitStreamFilterContext *bsfc = NULL;
static AVBSFContext *bsfc = NULL;

static int video_stream = -1;
static int raw_h264 = 0;

uint8_t *byte_buffer = NULL;
int byte_buffer_size = 0;
static int num;
void libav_init() {
  //av_register_all();

  //av_log(NULL, AV_LOG_INFO, "Using %snConfig: %snn", LIBAVCODEC_IDENT,
  //       avcodec_configuration());
}

void libav_release() {
  if (bsfc)
    //av_bitstream_filter_close(bsfc);
    av_bsf_free(bsfc);
  if (p_format_ctx)
    avformat_close_input(&amp;p_format_ctx);
}

int libav_open(const char *fname) {
  printf("libav_openn");
  num = 1;
  /* Open video file */
  if (avformat_open_input(&amp;p_format_ctx, fname, NULL, NULL) != 0) {
    av_log(NULL, AV_LOG_ERROR, "Couldn't open file!n");
    return -1;
  }

  /* Retrieve stream information */
  if (avformat_find_stream_info(p_format_ctx, NULL) < 0) {
    /* this is not fatal error, yet */
    av_log(NULL, AV_LOG_ERROR, "Couldn't find stream information!n");
  }

  /* Dump information about file onto standard error */
  av_dump_format(p_format_ctx, 0, fname, 0);

  /* Find the video stream */
  video_stream = av_find_best_stream(p_format_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);

  if (video_stream < 0) {
    av_log(NULL, AV_LOG_ERROR, "Didn't find a video stream!n");
    return -1;
  }
  origin_par = p_format_ctx->streams[video_stream]->codecpar;
  /* Get a pointer to the codec context for the video stream */
  //p_codec_ctx = p_format_ctx->streams[video_stream]->codec;
  p_codec_ctx = avcodec_alloc_context3(avcodec_find_decoder(origin_par->codec_id));  
  avcodec_parameters_to_context(p_codec_ctx, origin_par); 
  printf("p_format_ctx->iformat->long_name,%sn", p_format_ctx->iformat->long_name);
  if (!av_strcasecmp(p_format_ctx->iformat->long_name, "raw H.264 video")) {
    raw_h264 = 1;
    printf("raw_h264n");
  } else {
    /* have to use a filter to get the byte stream out */
    //bsfc = av_bitstream_filter_init("h264_mp4toannexb");
    printf("not h264,have to filtern");
    const AVBitStreamFilter *pfilter = av_bsf_get_by_name("h264_mp4toannexb");
    av_bsf_alloc(pfilter, &amp;bsfc);
    if (!bsfc) {
      av_log(p_codec_ctx, AV_LOG_ERROR,
             "Couldn't open the h264_mp4toannexb BSF!n");
      return -1;
    }
  }

  //byte_buffer_size = av_image_get_buffer_size(p_codec_ctx->pix_fmt, p_codec_ctx->width, p_codec_ctx->height, 16);
  //byte_buffer = av_malloc(byte_buffer_size);
  //printf("**byte_buffer_size=%dn", byte_buffer_size);
  return 0;
}

//return -1 ,over;0, not video;other, ok 
int libav_read_frame(char *buffer) {
  //printf("libav_read_framen");

  int ret;
  uint8_t *frame_data = NULL;  // Initialize to NULL

  AVPacket *pkt = av_packet_alloc();
  pkt->data = NULL;
  pkt->size = 0;
  ret = av_read_frame(p_format_ctx, pkt);
  //read over
  if (ret != 0) {
    return 0;
  }
  for(int i=0;i<pkt->size;i++)
    printf("%2d ", *(pkt->data+i));

  if(ret == 0 &amp;&amp; pkt->stream_index == video_stream){
    
    printf("av_read_frame, num=%d, pkt->size=%dn", num++, pkt->size);

    if (!raw_h264) {
      uint8_t *orig_extradata = NULL;
      int orig_extradata_size = 0;

      orig_extradata_size = p_codec_ctx->extradata_size;
      orig_extradata = av_mallocz(orig_extradata_size + AV_INPUT_BUFFER_PADDING_SIZE);
      memcpy(orig_extradata, p_codec_ctx->extradata, orig_extradata_size);

      //ret = av_bitstream_filter_filter(bsfc, p_codec_ctx, NULL, &frame_data, &frame_size, pkt->data, pkt->size, 0);
      av_bsf_send_packet(bsfc, pkt);
      printf("av_read_frame, 1, pkt->size=%dn", pkt->size);
      ret = av_bsf_receive_packet(bsfc, pkt);
      printf("av_read_frame, 2, pkt->size=%dn", pkt->size);
      for(int i=0;i<pkt->size;i++)
        printf("%2d ", *(pkt->data+i));

      if (p_codec_ctx->extradata == NULL) {
        p_codec_ctx->extradata = orig_extradata;
        p_codec_ctx->extradata_size = orig_extradata_size;
      } else {
        av_free(orig_extradata);
      }
      av_bsf_free(&bsfc);
    }

    memcpy(buffer, pkt->data, pkt->size);

    return pkt->size;
  }
  else{
    printf("av_read_frame, no videon");
    return 0;
  }
}