FFmpeg基础:封装格式转换

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 
#include 

//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 file
	out_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_base
		pkt.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;
}