不同的视频封装格式有不同的特点，有的功能强大支持多个视频流、音频流和字幕文件比如MKV格式，有的形成的文件极小、加载速度极快，适合网络播放比如flv格式，有的体积小能跨平台播放比如MOV格式。为了适应不同场景下的播放需求，有时候我们需要将视频的封装格式进行转换。这里介绍一下如何通过FFmpeg库实现不同封装格式的转换。封装格式转换的流程如下所示:

FFmpeg库实现封装格式转换的示例如下所示:
(输出的封装格式通过文件的后缀名称来进行确定)

#include <libavutil/timestamp.h>
#include <libavformat/avformat.h>//输出包的详细信息(pts,dts,time_base)
static void log_packet(const AVFormatContext *fmt_ctx, const AVPacket *pkt, const char *tag)
{AVRational *time_base = &fmt_ctx->streams[pkt->stream_index]->time_base;printf("%s: pts:%s pts_time:%s dts:%s dts_time:%s duration:%s duration_time:%s stream_index:%d\n",tag,av_ts2str(pkt->pts), av_ts2timestr(pkt->pts, time_base),av_ts2str(pkt->dts), av_ts2timestr(pkt->dts, time_base),av_ts2str(pkt->duration), av_ts2timestr(pkt->duration, time_base),pkt->stream_index);
}//@1输入视频文件地址 @2输出视频文件地址
int main(int argc, char **argv)
{AVOutputFormat *ofmt = NULL;AVFormatContext *ifmt_ctx = NULL, *ofmt_ctx = NULL;AVPacket pkt;const char *in_filename, *out_filename;int ret, i;int stream_index = 0;int *stream_mapping = NULL;int stream_mapping_size = 0;if (argc < 3) {printf("usage: %s input output\n""argument1 input mp4 file path.\n""argument2 output flv file path.\n""\n", argv[0]);return 1;}in_filename = argv[1]; //input fileout_filename = argv[2];//output file//打开输入视频的上下文并获取流信息if ((ret = avformat_open_input(&ifmt_ctx, in_filename, 0, 0)) < 0) {fprintf(stderr, "Could not open input file '%s'", in_filename);goto end;}if ((ret = avformat_find_stream_info(ifmt_ctx, 0)) < 0) {fprintf(stderr, "Failed to retrieve input stream information");goto end;}av_dump_format(ifmt_ctx, 0, in_filename, 0);//创建输出上下文并关联输出文件avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, out_filename);if (!ofmt_ctx) {fprintf(stderr, "Could not create output context\n");ret = AVERROR_UNKNOWN;goto end;}stream_mapping_size = ifmt_ctx->nb_streams;stream_mapping = av_mallocz_array(stream_mapping_size, sizeof(*stream_mapping));if (!stream_mapping) {ret = AVERROR(ENOMEM);goto end;}ofmt = ofmt_ctx->oformat;//将输入流的参数拷贝到输出流中for (i = 0; i < ifmt_ctx->nb_streams; i++) {AVStream *out_stream;AVStream *in_stream = ifmt_ctx->streams[i];AVCodecParameters *in_codecpar = in_stream->codecpar;if (in_codecpar->codec_type != AVMEDIA_TYPE_AUDIO &&in_codecpar->codec_type != AVMEDIA_TYPE_VIDEO &&in_codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE) {stream_mapping[i] = -1;continue;}stream_mapping[i] = stream_index++;out_stream = avformat_new_stream(ofmt_ctx, NULL);if (!out_stream) {fprintf(stderr, "Failed allocating output stream\n");ret = AVERROR_UNKNOWN;goto end;}ret = avcodec_parameters_copy(out_stream->codecpar, in_codecpar);if (ret < 0) {fprintf(stderr, "Failed to copy codec parameters\n");goto end;}out_stream->codecpar->codec_tag = 0;}av_dump_format(ofmt_ctx, 0, out_filename, 1);//打开输出文件if (!(ofmt->flags & AVFMT_NOFILE)) {ret = avio_open(&ofmt_ctx->pb, out_filename, AVIO_FLAG_WRITE);if (ret < 0) {fprintf(stderr, "Could not open output file '%s'", out_filename);goto end;}}//写数据头ret = avformat_write_header(ofmt_ctx, NULL);if (ret < 0) {fprintf(stderr, "Error occurred when opening output file\n");goto end;}//解析转换数据while (1) {AVStream *in_stream, *out_stream;ret = av_read_frame(ifmt_ctx, &pkt);if (ret < 0)break;in_stream = ifmt_ctx->streams[pkt.stream_index];if (pkt.stream_index >= stream_mapping_size ||stream_mapping[pkt.stream_index] < 0) {av_packet_unref(&pkt);continue;}pkt.stream_index = stream_mapping[pkt.stream_index];out_stream = ofmt_ctx->streams[pkt.stream_index];log_packet(ifmt_ctx, &pkt, "in");//拷贝数据包,并转换时间基time_basepkt.pts = av_rescale_q_rnd(pkt.pts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX);pkt.dts = av_rescale_q_rnd(pkt.dts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX);pkt.duration = av_rescale_q(pkt.duration, in_stream->time_base, out_stream->time_base);pkt.pos = -1;log_packet(ofmt_ctx, &pkt, "out");ret = av_interleaved_write_frame(ofmt_ctx, &pkt);if (ret < 0) {fprintf(stderr, "Error muxing packet\n");break;}av_packet_unref(&pkt);}//写数据尾av_write_trailer(ofmt_ctx);end://关闭上下文并 释放资源avformat_close_input(&ifmt_ctx);if (ofmt_ctx && !(ofmt->flags & AVFMT_NOFILE))avio_closep(&ofmt_ctx->pb);avformat_free_context(ofmt_ctx);av_freep(&stream_mapping);if (ret < 0 && ret != AVERROR_EOF) {fprintf(stderr, "Error occurred: %s\n", av_err2str(ret));return 1;}return 0;
}