在有一些嵌入式平台中，H264数据流一般来自芯片内部的硬编码器， AAC音频数据则是通过采集PCM进行软编码，但是如何对它实时进行封装多媒体文件 ，参考ffmpeg example，花了一些时间终于实现了该功能。
流程图如下：
本文只展示DEMO
一.视频输入流 创建
//内存数据回调部分
static int read_packet(void *opaque, uint8_t *buf, int buf_size)
{
char * input_filename = (char *)opaque;
static FILE *fl = NULL;
if(fl == NULL){
fl = fopen(input_filename,"r");
}
static unsigned long long read_len=0;
static unsigned long long fps_count=0;
int len=0;
int i =0;
if(!feof(fl))
len = fread(buf,1,buf_size,fl);
else
return AVERROR_EOF;
read_len+= len;
printf("%s len:%d read_len:%d\n",__FUNCTION__, len ,read_len);
for(i=0;i<4091;i++){
if(buf[i+0] == 0
&&buf[i+1] == 0
&&buf[i+2] == 0
&&buf[i+3] == 1)
{
// int data = buf[i+4] &=31;
printf("0 0 0 1 %x %d\n",buf[i+4],fps_count);
fps_count++;
}
}
return len;
}
static AVFormatContext * getInputVideoCtx(const char *fileName) {
uint8_t *avio_ctx_buffer = NULL;
AVIOContext *avio_ctx = NULL;
//缓存buffersize
size_t buffer_size, avio_ctx_buffer_size = 4096;
AVFormatContext * video_fmt_ctx = NULL;
int ret = 0;
if (!(video_fmt_ctx = avformat_alloc_context())) {
ret = AVERROR(ENOMEM);
return NULL;
}
//创建数据缓存Buffer
avio_ctx_buffer = av_malloc(avio_ctx_buffer_size);
if (!avio_ctx_buffer) {
ret = AVERROR(ENOMEM);
return NULL;
}
avio_ctx = avio_alloc_context(avio_ctx_buffer, avio_ctx_buffer_size,
0, fileName, &read_packet, NULL, NULL);
if (!avio_ctx) {
ret = AVERROR(ENOMEM);
return NULL;
}
video_fmt_ctx->pb = avio_ctx;
//打开数据
ret = avformat_open_input(&video_fmt_ctx, NULL, NULL, NULL);
if (ret < 0) {
fprintf(stderr, "Could not open input\n");
return NULL;
}
//获取数据格式
ret = avformat_find_stream_info(video_fmt_ctx, NULL);
if (ret < 0) {
fprintf(stderr, "Could not find stream information\n");
return NULL;
}
//打印数据参数
av_dump_format(video_fmt_ctx, 0, fileName, 0);
return video_fmt_ctx;
}
1.注册内存回调read_packet，avformat_find_stream_info会从回调里读取大概2S的h264视频数据并解析。首先会读取SPS PPS，然后是帧数据，读取2S的数据结束，如果给的数据不对，解析不正常会一直读，所以要确保刚开始给的数据是否正常。av_dump_format打印出数据格式
执行如下：
二.创建多媒体输出，添加视频输出流音频输出流
avformat_alloc_output_context2(&oc, NULL, NULL, filename);
...
//
fmt = oc->oformat;
if (fmt->video_codec != AV_CODEC_ID_NONE) {
add_video_stream(&video_st, oc, video_fmt_ctx, fmt->video_codec);
...
}
/* Add the audio and video streams using the default format codecs
* and initialize the codecs. */
if (fmt->audio_codec != AV_CODEC_ID_NONE) {
add_audio_stream(&audio_st, oc, &audio_codec, fmt->audio_codec);
...
}
1.添加视频流和初始化
/* media file output */
static void add_video_stream(OutputStream *ost, AVFormatContext *oc,
const AVFormatContext *video_fmt_ctx,
enum AVCodecID codec_id)
{
...
//创建一个输出流
ost->st = avformat_new_stream(oc, NULL);
...
ost->st->id = oc->nb_streams-1;
c = avcodec_alloc_context3(NULL);
...
//流的time_base初始化
for (i = 0; i < video_fmt_ctx->nb_streams; i++) {
if(video_fmt_ctx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO){
avcodec_parameters_to_context(c, video_fmt_ctx->streams[i]->codecpar);
video_fmt_ctx->streams[i]->time_base.den = video_fmt_ctx->streams[i]->avg_frame_rate.num;
}
}
//初始化av_packet
ost->tmp_pkt = av_packet_alloc();
...
ost->enc = c;
}
2.添加音频流 初始化编解码器
/* Add an output stream. */
static void add_audio_stream(OutputStream *ost, AVFormatContext *oc,
const AVCodec **codec,
enum AVCodecID codec_id)
{
*codec = avcodec_find_encoder(codec_id);
...
//初始化有音频packet
ost->tmp_pkt = av_packet_alloc();
...
//初始化流
ost->st = avformat_new_stream(oc, NULL);
...
switch ((*codec)->type) {
case AVMEDIA_TYPE_AUDIO:
c->sample_fmt = (*codec)->sample_fmts ?
(*codec)->sample_fmts[0] : AV_SAMPLE_FMT_FLTP;
c->bit_rate = 64000;
c->sample_rate = 44100;//采样率
if ((*codec)->supported_samplerates) {
c->sample_rate = (*codec)->supported_samplerates[0];
for (i = 0; (*codec)->supported_samplerates[i]; i++) {
if ((*codec)->supported_samplerates[i] == 44100)
c->sample_rate = 44100;
}
}
av_channel_layout_copy(&c->ch_layout, &(AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO);
//输出audio流的time_base初始化
ost->st->time_base = (AVRational){ 1, c->sample_rate };
break;
default:
break;
}
}
3.初始化输出流音频和视频codecpar
static int open_video(AVFormatContext *oc, const AVCodec *codec,AVFormatContext *vedio_fmt_ctx,
OutputStream *ost)
{
...
ret = avcodec_parameters_copy(ost->st->codecpar, vedio_fmt_ctx->streams[index]->codecpar);
...
}
static void open_audio(AVFormatContext *oc, const AVCodec *codec,
OutputStream *ost, AVDictionary *opt_arg)
{
...
/* copy the stream parameters to the muxer */
ret = avcodec_parameters_from_context(ost->st->codecpar, c);
if (ret < 0) {
fprintf(stderr, "Could not copy the stream parameters\n");
exit(1);
}
...
}
三.开始写入多媒体文件
1.比较写入音视频的时间戳，判断下一次要写入音频还是视频
while (encode_video) {
/* select the stream to encode */
if (encode_video &&
( !encode_audio || av_compare_ts(video_st.next_pts, video_fmt_ctx->streams[v_ctx_index]->time_base,
audio_st.next_pts, audio_st.enc->time_base) <= 0)) {
encode_video = !write_video_frame(oc, video_fmt_ctx, &video_st, video_st.tmp_pkt);
} else {
encode_audio = !write_audio_frame(oc, &audio_st);
}
}
av_compare_ts 通过对比当前Audio Video帧的写入量判断当前要写入Audio 还是Video
（例如： Video= 写入10帧* 1/25 > Audio 写入 10240*1/44100 则写入audio）
2.写入一帧Video
static int write_video_frame(AVFormatContext *oc,AVFormatContext *vic, OutputStream *ost, AVPacket *pkt)
{
int ret,i;
static int frame_index = 0;
AVStream *in_stream, *out_stream;
int stream_index;
stream_index = av_find_best_stream(vic, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
//读一帧H264
ret = av_read_frame(vic, pkt);
if(ret == AVERROR_EOF)
return ret == AVERROR_EOF ? 1 : 0;
av_packet_rescale_ts(pkt, ost->enc->time_base, ost->st->time_base);
if(pkt->pts==AV_NOPTS_VALUE){
in_stream = vic->streams[stream_index];
out_stream = ost->st;
//Write PTS
AVRational time_base1=in_stream->time_base;
int64_t calc_duration=(double)AV_TIME_BASE/av_q2d(in_stream->avg_frame_rate);
//计算出包的解码时间
pkt->pts=(double)(frame_index*calc_duration)/(double)(av_q2d(time_base1)*AV_TIME_BASE);
pkt->dts=pkt->pts;
pkt->duration=(double)calc_duration/(double)(av_q2d(time_base1)*AV_TIME_BASE);
//帧的计数累加
frame_index++;
//pkt的pts dts是输入流的时间戳 要转换成 输出流的时间戳
av_packet_rescale_ts(pkt, in_stream->time_base, out_stream->time_base);
pkt->pos = -1;
pkt->stream_index=ost->st->index;
}
//写入到多媒体文件
ret = av_interleaved_write_frame(oc, pkt);
if (ret < 0) {
fprintf(stderr, "Error while writing output packet: %s\n", av_err2str(ret));
exit(1);
}
return ret == AVERROR_EOF ? 1 : 0;
}
av_read_frame会回调read_packet 获取一帧H264数据，再通过计算时间戳 pts dts 再转换对应的输出流时间戳才能写入多媒体文件
3.写入一帧Audio
//获取一帧原始的Audio PCM 数据
/* Prepare a 16 bit dummy audio frame of 'frame_size' samples and
* 'nb_channels' channels. */
static AVFrame *get_audio_frame(OutputStream *ost)
{
...
c = ost->enc;
for (j = 0; j nb_samples; j++) {
v = (int)(sin(ost->t) * 10000);
for (i = 0; i < ost->enc->ch_layout.nb_channels; i++)
*q++ = v;
ost->t += ost->tincr;
ost->tincr += ost->tincr2;
}
...
frame->pts = ost->next_pts;
ost->next_pts += frame->nb_samples;
count++;
return frame;
}
static int write_audio_frame(AVFormatContext *oc, OutputStream *ost)
{
....
//获取一帧原始的Audio PCM 数据
frame = get_audio_frame(ost);
if (frame) {
dst_nb_samples = av_rescale_rnd(swr_get_delay(ost->swr_ctx, c->sample_rate) + frame->nb_samples,
c->sample_rate, c->sample_rate, AV_ROUND_UP);
ret = av_frame_make_writable(ost->frame);
/* convert to destination format */
ret = swr_convert(ost->swr_ctx,
ost->frame->data, dst_nb_samples,
(const uint8_t **)frame->data, frame->nb_samples);
frame = ost->frame;
frame->pts = av_rescale_q(ost->samples_count, (AVRational){1, c->sample_rate}, c->time_base);
ost->samples_count += dst_nb_samples;
}
//先送去编码再写入多媒体文件
return write_frame(oc, c, ost, frame, ost->tmp_pkt);
}
static int write_frame(AVFormatContext *fmt_ctx, AVCodecContext *c,
OutputStream *ost, AVFrame *frame, AVPacket *pkt)
{
...
ret = avcodec_send_frame(c, frame);
...
while (ret >= 0) {
ret = avcodec_receive_packet(c, pkt);
...
/* rescale output packet timestamp values from codec to stream timebase */
av_packet_rescale_ts(pkt, c->time_base, st->time_base);
printf("%d %d\n", c->time_base.den, st->time_base.den);
pkt->stream_index = st->index;
ret = av_interleaved_write_frame(fmt_ctx, pkt);
...
count++;
}
return ret == AVERROR_EOF ? 1 : 0;
}
四.写入多媒体尾部结束：
av_write_trailer(oc);
一些BUG：
控制写入时间，可以在写入循环里添加break。写入数据过长会出现音视频不同步的情况，建议写入时间不超过30分钟
DEMO
有需要源码可以后台私信我
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