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{ <span> </span>................................. } /* 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; }