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FFmpeg中直接调用。
对于H264裸流文件，一般使用的AnnexB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_mp4toannexb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extradata，extradata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hevc_mp4toannexb。
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。
这里给出一个实现参考，从imx的hantro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。貌似新版的FFmpeg将过滤器集成到了读入视频的过程，不需要额外处理了。
cpp
/*------------------------------------------------------------------------------
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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 %s\nConfig: %s\n\n", 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(&p_format_ctx);
}
int libav_open(const char *fname) {
printf("libav_open\n");
num = 1;
/* Open video file */
if (avformat_open_input(&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,%s\n", p_format_ctx->iformat->long_name);
if (!av_strcasecmp(p_format_ctx->iformat->long_name, "raw H.264 video")) {
raw_h264 = 1;
printf("raw_h264\n");
} else {
/* have to use a filter to get the byte stream out */
//bsfc = av_bitstream_filter_init("h264_mp4toannexb");
printf("not h264,have to filter\n");
const AVBitStreamFilter *pfilter = av_bsf_get_by_name("h264_mp4toannexb");
av_bsf_alloc(pfilter, &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=%d\n", byte_buffer_size);
return 0;
}
//return -1 ,over;0, not video;other, ok
int libav_read_frame(char *buffer) {
//printf("libav_read_frame\n");
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 && pkt->stream_index == video_stream){
printf("av_read_frame, num=%d, pkt->size=%d\n", 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=%d\n", pkt->size);
ret = av_bsf_receive_packet(bsfc, pkt);
printf("av_read_frame, 2, pkt->size=%d\n", 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 video\n");
return 0;
}
}从中可以看出主要是 这二个函数：
av_bsf_send_packet(bsfc, pkt);
av_bsf_receive_packet(bsfc, pkt);
av_bitstream_filter_filter函数已经被废弃。
在4.2版本上改改测试了下，是可行的。后期再试试新版。
如果不想使用FFmepg来读取H264裸流文件，可以参考这篇博文，使用纯C简单实现。