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_AVDEVICEavdevice_register_all();#endif#if CONFIG_AVFILTERavfilter_register_all();#endifav_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FormatContextist = &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;elsecontinue;}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_AVFILTERavfilter_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;}