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;
}