最新版ffmpeg源码分析二:transcode()函数

还是先看一下主函数吧:(省略了很多无关大雅的代码)

int main(int argc, char **argv)
{
	OptionsContext o = { 0 };
	int64_t ti;

	//与命令行分析有关的结构的初始化,下面不再罗嗦
	reset_options(&o, 0);

	//设置日志级别
	av_log_set_flags(AV_LOG_SKIP_REPEATED);
	parse_loglevel(argc, argv, options);

	if (argc > 1 && !strcmp(argv[1], "-d"))	{
		run_as_daemon = 1;
		av_log_set_callback(log_callback_null);
		argc--;
		argv++;
	}

	//注册组件们
	avcodec_register_all();
#if CONFIG_AVDEVICE
	avdevice_register_all();
#endif
#if CONFIG_AVFILTER
	avfilter_register_all();
#endif
	av_register_all();
	//初始化网络,windows下需要
	avformat_network_init();

	show_banner();

	term_init();

	//分析命令行输入的参数们
	parse_options(&o, argc, argv, options, opt_output_file);

	//文件的转换就在此函数中发生
	if (transcode(output_files, nb_output_files, input_files, nb_input_files)< 0)
		exit_program(1);

	exit_program(0);
	return 0;
}

下面是transcode()函数,转换就发生在它里面.不废话,看注释吧,应很详细了

static int transcode(
		OutputFile *output_files,//输出文件数组
		int nb_output_files,//输出文件的数量
		InputFile *input_files,//输入文件数组
		int nb_input_files)//输入文件的数量
{
	int ret, i;
	AVFormatContext *is, *os;
	OutputStream *ost;
	InputStream *ist;
	uint8_t *no_packet;
	int no_packet_count = 0;
	int64_t timer_start;
	int key;

	if (!(no_packet = av_mallocz(nb_input_files)))
		exit_program(1);

	//设置编码参数,打开所有输出流的编码器,打开所有输入流的解码器,写入所有输出文件的文件头,于是准备好了
	ret = transcode_init(output_files, nb_output_files, input_files,nb_input_files);
	if (ret < 0)
		goto fail;

	if (!using_stdin){
		av_log(NULL, AV_LOG_INFO, "Press [q] to stop, [?] for help\n");
	}

	timer_start = av_gettime();

	//循环,直到收到系统信号才退出
	for (; received_sigterm == 0;)
	{
		int file_index, ist_index;
		AVPacket pkt;
		int64_t ipts_min;
		double opts_min;
		int64_t cur_time = av_gettime();

		ipts_min = INT64_MAX;
		opts_min = 1e100;
		/* if 'q' pressed, exits */
		if (!using_stdin)
		{
			//先查看用户按下了什么键,跟据键做出相应的反应
			static int64_t last_time;
			if (received_nb_signals)
				break;
			/* read_key() returns 0 on EOF */
			if (cur_time - last_time >= 100000 && !run_as_daemon){
				key = read_key();
				last_time = cur_time;
			}else{
			.................................
		}

		/* select the stream that we must read now by looking at the
		 smallest output pts */
		//下面这个循环的目的是找一个最小的输出pts(也就是离当前最近的)的输出流
		file_index = -1;
		for (i = 0; i < nb_output_streams; i++){
			OutputFile *of;
			int64_t ipts;
			double opts;
			ost = &output_streams[i];//循环每一个输出流
			of = &output_files[ost->file_index];//输出流对应的输出文件
			os = output_files[ost->file_index].ctx;//输出流对应的FormatContext
			ist = &input_streams[ost->source_index];//输出流对应的输入流

			if (ost->is_past_recording_time || //是否过了录制时间?(可能用户指定了一个录制时间段)
					no_packet[ist->file_index]|| //对应的输入流这个时间内没有数据?
					(os->pb && avio_tell(os->pb) >= of->limit_filesize))//是否超出了录制范围(也是用户指定的)
				continue;//是的,符合上面某一条,那么再看下一个输出流吧

			//判断当前输入流所在的文件是否可以使用(我也不很明白)
			opts = ost->st->pts.val * av_q2d(ost->st->time_base);
			ipts = ist->pts;
			if (!input_files[ist->file_index].eof_reached)	{
				if (ipts < ipts_min){
					//每找到一个pts更小的输入流就记录下来,这样循环完所有的输出流时就找到了
					//pts最小的输入流,及输入文件的序号
					ipts_min = ipts;
					if (input_sync)
						file_index = ist->file_index;
				}
				if (opts < opts_min){
					opts_min = opts;
					if (!input_sync)
						file_index = ist->file_index;
				}
			}

			//难道下面这句话的意思是:如果当前的输出流已接收的帧数,超出用户指定的输出最大帧数时,
			//则当前输出流所属的输出文件对应的所有输出流,都算超过了录像时间?
			if (ost->frame_number >= ost->max_frames){
				int j;
				for (j = 0; j < of->ctx->nb_streams; j++)
					output_streams[of->ost_index + j].is_past_recording_time =	1;
				continue;
			}
		}
		/* if none, if is finished */
		if (file_index < 0)	{
			//如果没有找到合适的输入文件
			if (no_packet_count){
				//如果是因为有的输入文件暂时得不到数据,则还不算是结束
				no_packet_count = 0;
				memset(no_packet, 0, nb_input_files);
				usleep(10000);
				continue;
			}
			//全部转换完成了,跳出大循环
			break;
		}

		//从找到的输入文件中读出一帧(可能是音频也可能是视频),并放到fifo队列中
		is = input_files[file_index].ctx;
		ret = av_read_frame(is, &pkt);
		if (ret == AVERROR(EAGAIN))	{
			//此时发生了暂时没数据的情况
			no_packet[file_index] = 1;
			no_packet_count++;
			continue;
		}

		//下文判断是否有输入文件到最后了
		if (ret < 0){
			input_files[file_index].eof_reached = 1;
			if (opt_shortest)
				break;
			else
				continue;
		}

		no_packet_count = 0;
		memset(no_packet, 0, nb_input_files);

		if (do_pkt_dump){
			av_pkt_dump_log2(NULL, AV_LOG_DEBUG, &pkt, do_hex_dump,
					is->streams[pkt.stream_index]);
		}
		/* the following test is needed in case new streams appear
		 dynamically in stream : we ignore them */
		//如果在输入文件中遇到一个忽然冒出的流,那么我们不鸟它
		if (pkt.stream_index >= input_files[file_index].nb_streams)
			goto discard_packet;

		//取得当前获得的帧对应的输入流
		ist_index = input_files[file_index].ist_index + pkt.stream_index;
		ist = &input_streams[ist_index];
		if (ist->discard)
			goto discard_packet;

		//重新鼓捣一下帧的时间戳
		if (pkt.dts != AV_NOPTS_VALUE)
			pkt.dts += av_rescale_q(input_files[ist->file_index].ts_offset,
					AV_TIME_BASE_Q, ist->st->time_base);
		if (pkt.pts != AV_NOPTS_VALUE)
			pkt.pts += av_rescale_q(input_files[ist->file_index].ts_offset,
					AV_TIME_BASE_Q, ist->st->time_base);

		if (pkt.pts != AV_NOPTS_VALUE)
			pkt.pts *= ist->ts_scale;
		if (pkt.dts != AV_NOPTS_VALUE)
			pkt.dts *= ist->ts_scale;

		if (pkt.dts != AV_NOPTS_VALUE && ist->next_pts != AV_NOPTS_VALUE
				&& (is->iformat->flags & AVFMT_TS_DISCONT))
		{
			int64_t pkt_dts = av_rescale_q(pkt.dts, ist->st->time_base,
					AV_TIME_BASE_Q);
			int64_t delta = pkt_dts - ist->next_pts;
			if ((delta < -1LL * dts_delta_threshold * AV_TIME_BASE
					|| (delta > 1LL * dts_delta_threshold * AV_TIME_BASE
							&& ist->st->codec->codec_type
									!= AVMEDIA_TYPE_SUBTITLE)
					|| pkt_dts + 1 < ist->pts) && !copy_ts)
			{
				input_files[ist->file_index].ts_offset -= delta;
				av_log(	NULL,	AV_LOG_DEBUG,
						"timestamp discontinuity %"PRId64", new offset= %"PRId64"\n",
						delta, input_files[ist->file_index].ts_offset);
				pkt.dts -= av_rescale_q(delta, AV_TIME_BASE_Q,	ist->st->time_base);
				if (pkt.pts != AV_NOPTS_VALUE)
					pkt.pts -= av_rescale_q(delta, AV_TIME_BASE_Q,	ist->st->time_base);
			}
		}

		//把这一帧转换并写入到输出文件中
		if (output_packet(ist, output_streams, nb_output_streams, &pkt) < 0){
			av_log(NULL, AV_LOG_ERROR,
					"Error while decoding stream #%d:%d\n",
					ist->file_index, ist->st->index);
			if (exit_on_error)
				exit_program(1);
			av_free_packet(&pkt);
			continue;
		}

discard_packet:
		av_free_packet(&pkt);

		/* dump report by using the output first video and audio streams */
		print_report(output_files, output_streams, nb_output_streams, 0,
				timer_start, cur_time);
	}

	//文件处理完了,把缓冲中剩余的数据写到输出文件中
	for (i = 0; i < nb_input_streams; i++){
		ist = &input_streams[i];
		if (ist->decoding_needed){
			output_packet(ist, output_streams, nb_output_streams, NULL);
		}
	}
	flush_encoders(output_streams, nb_output_streams);

	term_exit();

	//为输出文件写文件尾(有的不需要).
	for (i = 0; i < nb_output_files; i++){
		os = output_files[i].ctx;
		av_write_trailer(os);
	}

	/* dump report by using the first video and audio streams */
	print_report(output_files, output_streams, nb_output_streams, 1,
			timer_start, av_gettime());

	//关闭所有的编码器
	for (i = 0; i < nb_output_streams; i++){
		ost = &output_streams[i];
		if (ost->encoding_needed){
			av_freep(&ost->st->codec->stats_in);
			avcodec_close(ost->st->codec);
		}
#if CONFIG_AVFILTER
		avfilter_graph_free(&ost->graph);
#endif
	}

	//关闭所有的解码器
	for (i = 0; i < nb_input_streams; i++){
		ist = &input_streams[i];
		if (ist->decoding_needed){
			avcodec_close(ist->st->codec);
		}
	}

	/* finished ! */
	ret = 0;

	fail: av_freep(&bit_buffer);
	av_freep(&no_packet);

	if (output_streams)	{
		for (i = 0; i < nb_output_streams; i++)	{
			ost = &output_streams[i];
			if (ost)	{
				if (ost->stream_copy)
					av_freep(&ost->st->codec->extradata);
				if (ost->logfile){
					fclose(ost->logfile);
					ost->logfile = NULL;
				}
				av_fifo_free(ost->fifo); /* works even if fifo is not
				 initialized but set to zero */
				av_freep(&ost->st->codec->subtitle_header);
				av_free(ost->resample_frame.data[0]);
				av_free(ost->forced_kf_pts);
				if (ost->video_resample)
					sws_freeContext(ost->img_resample_ctx);
				swr_free(&ost->swr);
				av_dict_free(&ost->opts);
			}
		}
	}
	return ret;
}

原文链接： http://blog.csdn.net/nkmnkm/article/details/7175421