ffmpeg源码分析三：transcode_init函数
transcode_init()函数是在转换前做准备工作的。下面看看其源代码：
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transcode_init()函数是在转换前做准备工作的。下面看看其源代码：
static int transcode_init(void)
{
int ret = 0, i, j, k;
AVFormatContext *oc;
AVCodecContext *codec; //输出流的编解码器结构
OutputStream *ost; //输出流
InputStream *ist; //输入流
char error[1024];
int want_sdp = 1;
for (i = 0; i < nb_filtergraphs; i++) {
FilterGraph *fg = filtergraphs[i];
for (j = 0; j < fg->nb_outputs; j++) {
OutputFilter *ofilter = fg->outputs[j];
if (!ofilter->ost || ofilter->ost->source_index >= 0)
continue;
if (fg->nb_inputs != 1)
continue;
for (k = nb_input_streams-1; k >= 0 ; k--)
if (fg->inputs[0]->ist == input_streams[k])
break;
ofilter->ost->source_index = k;
}
}
/* init framerate emulation */
//初始化帧率仿真（转换时是不按帧率来的,但如果要求帧率仿真,就可以做到）
for (i = 0; i < nb_input_files; i++) {
InputFile *ifile = input_files[i];
//如果一个输入文件被要求帧率仿真(指的是即使是转换也像播放那样按照帧率来进行），则为这个文件中所有流记录下开始时间。
if (ifile->rate_emu)
for (j = 0; j < ifile->nb_streams; j++)
input_streams[j + ifile->ist_index]->start = av_gettime();
}
/* output stream init */
//什么也没做,只是做了个判断而已。
for (i = 0; i < nb_output_files; i++) {
oc = output_files[i]->ctx;
if (!oc->nb_streams && !(oc->oformat->flags & AVFMT_NOSTREAMS)) {
av_dump_format(oc, i, oc->filename, 1);
av_log(NULL, AV_LOG_ERROR, "Output file #%d does not contain any stream\n", i);
return AVERROR(EINVAL);
}
}
/* init complex filtergraphs */
//初始化滤波
for (i = 0; i < nb_filtergraphs; i++)
if ((ret = avfilter_graph_config(filtergraphs[i]->graph, NULL)) < 0)
return ret;
/* for each output stream, we compute the right encoding parameters */
//轮循所有的输出流，跟据对应的输入流，设置其编解码器的参数。
for (i = 0; i < nb_output_streams; i++) {
AVCodecContext *icodec = NULL; //输入流的编解码
ost = output_streams[i]; //轮循所有的输出流
oc = output_files[ost->file_index]->ctx; //输出流对应的FormatContext
ist = get_input_stream(ost); //取得输出流对应的输入流
if (ost->attachment_filename)
continue;
//先把能赋值的赋值
codec = ost->st->codec; //输出流的编解码器结构
if (ist) {
icodec = ist->st->codec; //输入流的编解码器结构
ost->st->disposition = ist->st->disposition;
codec->bits_per_raw_sample = icodec->bits_per_raw_sample;
codec->chroma_sample_location = icodec->chroma_sample_location;
} else {
for (j=0; j<oc->nb_streams; j++) {
AVStream *st = oc->streams[j];
if (st != ost->st && st->codec->codec_type == codec->codec_type)
break;
}
if (j == oc->nb_streams)
if (codec->codec_type == AVMEDIA_TYPE_AUDIO || codec->codec_type == AVMEDIA_TYPE_VIDEO)
ost->st->disposition = AV_DISPOSITION_DEFAULT;
}
//如果只是复制一个流(不用解码后再编码)，则把输入流的编码参数直接赋值给输出流
//此时是不需要解码也不需要编码，所以不需打开解码器和编码器
if (ost->stream_copy) {
AVRational sar;
uint64_t extra_size;
av_assert0(ist && !ost->filter);
//计算输出流的编解码器的extradata的大小，然后分配缓冲区
//然后把输入流的编解码器的extradata拷贝到输出流的编解码器中
extra_size = (uint64_t)icodec->extradata_size + FF_INPUT_BUFFER_PADDING_SIZE;
if (extra_size > INT_MAX) {
return AVERROR(EINVAL);
}
/* if stream_copy is selected, no need to decode or encode */
codec->codec_id = icodec->codec_id;
codec->codec_type = icodec->codec_type;
if (!codec->codec_tag) {
unsigned int codec_tag;
if (!oc->oformat->codec_tag ||
av_codec_get_id (oc->oformat->codec_tag, icodec->codec_tag) == codec->codec_id ||
!av_codec_get_tag2(oc->oformat->codec_tag, icodec->codec_id, &codec_tag))
codec->codec_tag = icodec->codec_tag;
}
codec->bit_rate = icodec->bit_rate;
codec->rc_max_rate = icodec->rc_max_rate;
codec->rc_buffer_size = icodec->rc_buffer_size;
codec->field_order = icodec->field_order;
codec->extradata = av_mallocz(extra_size);
if (!codec->extradata) {
return AVERROR(ENOMEM);
}
memcpy(codec->extradata, icodec->extradata, icodec->extradata_size);
codec->extradata_size= icodec->extradata_size;
codec->bits_per_coded_sample = icodec->bits_per_coded_sample;
//重新鼓捣一下time base(这家伙就是帧率)
codec->time_base = ist->st->time_base;
/*
* Avi is a special case here because it supports variable fps but
* having the fps and timebase differe significantly adds quite some
* overhead
*/
//如果输出文件是avi,做一点特殊处理
if(!strcmp(oc->oformat->name, "avi")) {
if ( copy_tb<0 && av_q2d(ist->st->r_frame_rate) >= av_q2d(ist->st->avg_frame_rate)
&& 0.5/av_q2d(ist->st->r_frame_rate) > av_q2d(ist->st->time_base)
&& 0.5/av_q2d(ist->st->r_frame_rate) > av_q2d(icodec->time_base)
&& av_q2d(ist->st->time_base) < 1.0/500 && av_q2d(icodec->time_base) < 1.0/500
|| copy_tb==2){
codec->time_base.num = ist->st->r_frame_rate.den;
codec->time_base.den = 2*ist->st->r_frame_rate.num;
codec->ticks_per_frame = 2;
} else if ( copy_tb<0 && av_q2d(icodec->time_base)*icodec->ticks_per_frame > 2*av_q2d(ist->st->time_base)
&& av_q2d(ist->st->time_base) < 1.0/500
|| copy_tb==0){
codec->time_base = icodec->time_base;
codec->time_base.num *= icodec->ticks_per_frame;
codec->time_base.den *= 2;
codec->ticks_per_frame = 2;
}
} else if(!(oc->oformat->flags & AVFMT_VARIABLE_FPS)
&& strcmp(oc->oformat->name, "mov") && strcmp(oc->oformat->name, "mp4") && strcmp(oc->oformat->name, "3gp")
&& strcmp(oc->oformat->name, "3g2") && strcmp(oc->oformat->name, "psp") && strcmp(oc->oformat->name, "ipod")
&& strcmp(oc->oformat->name, "f4v")
) {
if( copy_tb<0 && icodec->time_base.den
&& av_q2d(icodec->time_base)*icodec->ticks_per_frame > av_q2d(ist->st->time_base)
&& av_q2d(ist->st->time_base) < 1.0/500
|| copy_tb==0){
codec->time_base = icodec->time_base;
codec->time_base.num *= icodec->ticks_per_frame;
}
}
if ( codec->codec_tag == AV_RL32("tmcd")
&& icodec->time_base.num < icodec->time_base.den
&& icodec->time_base.num > 0
&& 121LL*icodec->time_base.num > icodec->time_base.den) {
codec->time_base = icodec->time_base;
}
if (ist && !ost->frame_rate.num)
ost->frame_rate = ist->framerate;
if(ost->frame_rate.num)
codec->time_base = av_inv_q(ost->frame_rate);
//再修正一下帧率
av_reduce(&codec->time_base.num, &codec->time_base.den,
codec->time_base.num, codec->time_base.den, INT_MAX);
ost->parser = av_parser_init(codec->codec_id);
//根据不同媒体类型，赋值自己对应的编码参数
switch (codec->codec_type) {
case AVMEDIA_TYPE_AUDIO: //音频的
if (audio_volume != 256) {
av_log(NULL, AV_LOG_FATAL, "-acodec copy and -vol are incompatible (frames are not decoded)\n");
exit_program(1);
}
codec->channel_layout = icodec->channel_layout;
codec->sample_rate = icodec->sample_rate;
codec->channels = icodec->channels;
codec->frame_size = icodec->frame_size;
codec->audio_service_type = icodec->audio_service_type;
codec->block_align = icodec->block_align;
codec->delay = icodec->delay;
if((codec->block_align == 1 || codec->block_align == 1152 || codec->block_align == 576) && codec->codec_id == AV_CODEC_ID_MP3)
codec->block_align= 0;
if(codec->codec_id == AV_CODEC_ID_AC3)
codec->block_align= 0;
break;
case AVMEDIA_TYPE_VIDEO: //视频的
codec->pix_fmt = icodec->pix_fmt;
codec->width = icodec->width;
codec->height = icodec->height;
codec->has_b_frames = icodec->has_b_frames;
if (ost->frame_aspect_ratio.num) { // overridden by the -aspect cli option
sar =
av_mul_q(ost->frame_aspect_ratio,
(AVRational){ codec->height, codec->width });
av_log(NULL, AV_LOG_WARNING, "Overriding aspect ratio "
"with stream copy may produce invalid files\n");
}
else if (ist->st->sample_aspect_ratio.num)
sar = ist->st->sample_aspect_ratio;
else
sar = icodec->sample_aspect_ratio;
ost->st->sample_aspect_ratio = codec->sample_aspect_ratio = sar;
ost->st->avg_frame_rate = ist->st->avg_frame_rate;
break;
case AVMEDIA_TYPE_SUBTITLE: //字幕的
codec->width = icodec->width;
codec->height = icodec->height;
break;
case AVMEDIA_TYPE_DATA:
case AVMEDIA_TYPE_ATTACHMENT: //其他
break;
default:
abort();
}
} else { //如果不是复制,就麻烦多了
//获取编码器
if (!ost->enc)
ost->enc = avcodec_find_encoder(codec->codec_id);
if (!ost->enc) {
/* should only happen when a default codec is not present. */
snprintf(error, sizeof(error), "Encoder (codec %s) not found for output stream #%d:%d",
avcodec_get_name(ost->st->codec->codec_id), ost->file_index, ost->index);
ret = AVERROR(EINVAL);
goto dump_format;
}
//因为需要转换,所以既需解码又需编码
if (ist)
ist->decoding_needed++;
ost->encoding_needed = 1;
if (!ost->filter &&
(codec->codec_type == AVMEDIA_TYPE_VIDEO ||
codec->codec_type == AVMEDIA_TYPE_AUDIO)) {
FilterGraph *fg;
fg = init_simple_filtergraph(ist, ost);
if (configure_filtergraph(fg)) {
av_log(NULL, AV_LOG_FATAL, "Error opening filters!\n");
exit_program(1);
}
}
if (codec->codec_type == AVMEDIA_TYPE_VIDEO) {
if (ost->filter && !ost->frame_rate.num)
ost->frame_rate = av_buffersink_get_frame_rate(ost->filter->filter);
if (ist && !ost->frame_rate.num)
ost->frame_rate = ist->framerate;
if (ist && !ost->frame_rate.num)
ost->frame_rate = ist->st->r_frame_rate;
if (ist && !ost->frame_rate.num) {
ost->frame_rate = (AVRational){25, 1};
av_log(NULL, AV_LOG_WARNING,
"No information "
"about the input framerate is available. Falling "
"back to a default value of 25fps for output stream #%d:%d. Use the -r option "
"if you want a different framerate.\n",
ost->file_index, ost->index);
}
// ost->frame_rate = ist->st->avg_frame_rate.num ? ist->st->avg_frame_rate : (AVRational){25, 1};
if (ost->enc && ost->enc->supported_framerates && !ost->force_fps) {
int idx = av_find_nearest_q_idx(ost->frame_rate, ost->enc->supported_framerates);
ost->frame_rate = ost->enc->supported_framerates[idx];
}
if (codec->codec_id == AV_CODEC_ID_MPEG4) {
av_reduce(&ost->frame_rate.num, &ost->frame_rate.den,
ost->frame_rate.num, ost->frame_rate.den, 65535);
}
}
switch (codec->codec_type) {
case AVMEDIA_TYPE_AUDIO: //设置音频编码器的参数
codec->sample_fmt = ost->filter->filter->inputs[0]->format; //样点格式
codec->sample_rate = ost->filter->filter->inputs[0]->sample_rate; //采样率
codec->channel_layout = ost->filter->filter->inputs[0]->channel_layout;
codec->channels = avfilter_link_get_channels(ost->filter->filter->inputs[0]); //声道
codec->time_base = (AVRational){ 1, codec->sample_rate };
break;
case AVMEDIA_TYPE_VIDEO: //设置视频编码器的参数
codec->time_base = av_inv_q(ost->frame_rate);
if (ost->filter && !(codec->time_base.num && codec->time_base.den))
codec->time_base = ost->filter->filter->inputs[0]->time_base;
if ( av_q2d(codec->time_base) < 0.001 && video_sync_method != VSYNC_PASSTHROUGH
&& (video_sync_method == VSYNC_CFR || video_sync_method == VSYNC_VSCFR || (video_sync_method == VSYNC_AUTO && !(oc->oformat->flags & AVFMT_VARIABLE_FPS)))){
av_log(oc, AV_LOG_WARNING, "Frame rate very high for a muxer not efficiently supporting it.\n"
"Please consider specifying a lower framerate, a different muxer or -vsync 2\n");
}
for (j = 0; j < ost->forced_kf_count; j++)
ost->forced_kf_pts[j] = av_rescale_q(ost->forced_kf_pts[j],
AV_TIME_BASE_Q,
codec->time_base);
//宽高
codec->width = ost->filter->filter->inputs[0]->w;
codec->height = ost->filter->filter->inputs[0]->h;
codec->sample_aspect_ratio = ost->st->sample_aspect_ratio =
ost->frame_aspect_ratio.num ? // overridden by the -aspect cli option
av_mul_q(ost->frame_aspect_ratio, (AVRational){ codec->height, codec->width }) :
ost->filter->filter->inputs[0]->sample_aspect_ratio;
if (!strncmp(ost->enc->name, "libx264", 7) &&
codec->pix_fmt == AV_PIX_FMT_NONE &&
ost->filter->filter->inputs[0]->format != AV_PIX_FMT_YUV420P)
av_log(NULL, AV_LOG_WARNING,
"No pixel format specified, %s for H.264 encoding chosen.\n"
"Use -pix_fmt yuv420p for compatibility with outdated media players.\n",
av_get_pix_fmt_name(ost->filter->filter->inputs[0]->format));
if (!strncmp(ost->enc->name, "mpeg2video", 10) &&
codec->pix_fmt == AV_PIX_FMT_NONE &&
ost->filter->filter->inputs[0]->format != AV_PIX_FMT_YUV420P)
av_log(NULL, AV_LOG_WARNING,
"No pixel format specified, %s for MPEG-2 encoding chosen.\n"
"Use -pix_fmt yuv420p for compatibility with outdated media players.\n",
av_get_pix_fmt_name(ost->filter->filter->inputs[0]->format));
codec->pix_fmt = ost->filter->filter->inputs[0]->format;
if (!icodec ||
codec->width != icodec->width ||
codec->height != icodec->height ||
codec->pix_fmt != icodec->pix_fmt) {
codec->bits_per_raw_sample = frame_bits_per_raw_sample;
}
if (ost->forced_keyframes) {
if (!strncmp(ost->forced_keyframes, "expr:", 5)) {
ret = av_expr_parse(&ost->forced_keyframes_pexpr, ost->forced_keyframes+5,
forced_keyframes_const_names, NULL, NULL, NULL, NULL, 0, NULL);
if (ret < 0) {
av_log(NULL, AV_LOG_ERROR,
"Invalid force_key_frames expression '%s'\n", ost->forced_keyframes+5);
return ret;
}
ost->forced_keyframes_expr_const_values[FKF_N] = 0;
ost->forced_keyframes_expr_const_values[FKF_N_FORCED] = 0;
ost->forced_keyframes_expr_const_values[FKF_PREV_FORCED_N] = NAN;
ost->forced_keyframes_expr_const_values[FKF_PREV_FORCED_T] = NAN;
} else {
parse_forced_key_frames(ost->forced_keyframes, ost, ost->st->codec);
}
}
break;
case AVMEDIA_TYPE_SUBTITLE: //设置字幕编码器的参数
codec->time_base = (AVRational){1, 1000};
if (!codec->width) {
codec->width = input_streams[ost->source_index]->st->codec->width;
codec->height = input_streams[ost->source_index]->st->codec->height;
}
break;
default:
abort();
break;
}
/* two pass mode */
if (codec->flags & (CODEC_FLAG_PASS1 | CODEC_FLAG_PASS2)) {
char logfilename[1024];
FILE *f;
snprintf(logfilename, sizeof(logfilename), "%s-%d.log",
ost->logfile_prefix ? ost->logfile_prefix :
DEFAULT_PASS_LOGFILENAME_PREFIX,
i);
if (!strcmp(ost->enc->name, "libx264")) {
av_dict_set(&ost->opts, "stats", logfilename, AV_DICT_DONT_OVERWRITE);
} else {
if (codec->flags & CODEC_FLAG_PASS2) {
char *logbuffer;
size_t logbuffer_size;
if (cmdutils_read_file(logfilename, &logbuffer, &logbuffer_size) < 0) {
av_log(NULL, AV_LOG_FATAL, "Error reading log file '%s' for pass-2 encoding\n",
logfilename);
exit_program(1);
}
codec->stats_in = logbuffer;
}
if (codec->flags & CODEC_FLAG_PASS1) {
f = av_fopen_utf8(logfilename, "wb");
if (!f) {
av_log(NULL, AV_LOG_FATAL, "Cannot write log file '%s' for pass-1 encoding: %s\n",
logfilename, strerror(errno));
exit_program(1);
}
ost->logfile = f;
}
}
}
}
}
/* open each encoder */
//轮循所有输出流,打开每个输出流的编码器
for (i = 0; i < nb_output_streams; i++) {
ost = output_streams[i];
if (ost->encoding_needed) {
AVCodec *codec = ost->enc;
AVCodecContext *dec = NULL;
if ((ist = get_input_stream(ost)))
dec = ist->st->codec;
if (dec && dec->subtitle_header) {
/* ASS code assumes this buffer is null terminated so add extra byte. */
ost->st->codec->subtitle_header = av_mallocz(dec->subtitle_header_size + 1);
if (!ost->st->codec->subtitle_header) {
ret = AVERROR(ENOMEM);
goto dump_format;
}
memcpy(ost->st->codec->subtitle_header, dec->subtitle_header, dec->subtitle_header_size);
ost->st->codec->subtitle_header_size = dec->subtitle_header_size;
}
if (!av_dict_get(ost->opts, "threads", NULL, 0))
av_dict_set(&ost->opts, "threads", "auto", 0);
//打开
if ((ret = avcodec_open2(ost->st->codec, codec, &ost->opts)) < 0) {
if (ret == AVERROR_EXPERIMENTAL)
abort_codec_experimental(codec, 1);
snprintf(error, sizeof(error), "Error while opening encoder for output stream #%d:%d - maybe incorrect parameters such as bit_rate, rate, width or height",
ost->file_index, ost->index);
goto dump_format;
}
if (ost->enc->type == AVMEDIA_TYPE_AUDIO &&
!(ost->enc->capabilities & CODEC_CAP_VARIABLE_FRAME_SIZE))
av_buffersink_set_frame_size(ost->filter->filter,
ost->st->codec->frame_size);
assert_avoptions(ost->opts);
if (ost->st->codec->bit_rate && ost->st->codec->bit_rate < 1000)
av_log(NULL, AV_LOG_WARNING, "The bitrate parameter is set too low."
" It takes bits/s as argument, not kbits/s\n");
extra_size += ost->st->codec->extradata_size;
} else {
av_opt_set_dict(ost->st->codec, &ost->opts);
}
}
/* init input streams */
//初始化所有的输入流
for (i = 0; i < nb_input_streams; i++)
if ((ret = init_input_stream(i, error, sizeof(error))) < 0) {
for (i = 0; i < nb_output_streams; i++) {
ost = output_streams[i];
avcodec_close(ost->st->codec);
}
goto dump_format;
}
/* discard unused programs */
for (i = 0; i < nb_input_files; i++) {
InputFile *ifile = input_files[i];
for (j = 0; j < ifile->ctx->nb_programs; j++) {
AVProgram *p = ifile->ctx->programs[j];
int discard = AVDISCARD_ALL;
for (k = 0; k < p->nb_stream_indexes; k++)
if (!input_streams[ifile->ist_index + p->stream_index[k]]->discard) {
discard = AVDISCARD_DEFAULT;
break;
}
p->discard = discard;
}
}
/* open files and write file headers */
//打开所有输出文件，写入媒体文件头
for (i = 0; i < nb_output_files; i++) {
oc = output_files[i]->ctx;
oc->interrupt_callback = int_cb;
if ((ret = avformat_write_header(oc, &output_files[i]->opts)) < 0) {
snprintf(error, sizeof(error),
"Could not write header for output file #%d "
"(incorrect codec parameters ?): %s",
i, av_err2str(ret));
ret = AVERROR(EINVAL);
goto dump_format;
}
// assert_avoptions(output_files[i]->opts);
if (strcmp(oc->oformat->name, "rtp")) {
want_sdp = 0;
}
}
dump_format:
/* dump the file output parameters - cannot be done before in case
of stream copy */
for (i = 0; i < nb_output_files; i++) {
av_dump_format(output_files[i]->ctx, i, output_files[i]->ctx->filename, 1);
}
/* dump the stream mapping */
av_log(NULL, AV_LOG_INFO, "Stream mapping:\n");
for (i = 0; i < nb_input_streams; i++) {
ist = input_streams[i];
for (j = 0; j < ist->nb_filters; j++) {
if (ist->filters[j]->graph->graph_desc) {
av_log(NULL, AV_LOG_INFO, " Stream #%d:%d (%s) -> %s",
ist->file_index, ist->st->index, ist->dec ? ist->dec->name : "?",
ist->filters[j]->name);
if (nb_filtergraphs > 1)
av_log(NULL, AV_LOG_INFO, " (graph %d)", ist->filters[j]->graph->index);
av_log(NULL, AV_LOG_INFO, "\n");
}
}
}
for (i = 0; i < nb_output_streams; i++) {
ost = output_streams[i];
if (ost->attachment_filename) {
/* an attached file */
av_log(NULL, AV_LOG_INFO, " File %s -> Stream #%d:%d\n",
ost->attachment_filename, ost->file_index, ost->index);
continue;
}
if (ost->filter && ost->filter->graph->graph_desc) {
/* output from a complex graph */
av_log(NULL, AV_LOG_INFO, " %s", ost->filter->name);
if (nb_filtergraphs > 1)
av_log(NULL, AV_LOG_INFO, " (graph %d)", ost->filter->graph->index);
av_log(NULL, AV_LOG_INFO, " -> Stream #%d:%d (%s)\n", ost->file_index,
ost->index, ost->enc ? ost->enc->name : "?");
continue;
}
av_log(NULL, AV_LOG_INFO, " Stream #%d:%d -> #%d:%d",
input_streams[ost->source_index]->file_index,
input_streams[ost->source_index]->st->index,
ost->file_index,
ost->index);
if (ost->sync_ist != input_streams[ost->source_index])
av_log(NULL, AV_LOG_INFO, " [sync #%d:%d]",
ost->sync_ist->file_index,
ost->sync_ist->st->index);
if (ost->stream_copy)
av_log(NULL, AV_LOG_INFO, " (copy)");
else
av_log(NULL, AV_LOG_INFO, " (%s -> %s)", input_streams[ost->source_index]->dec ?
input_streams[ost->source_index]->dec->name : "?",
ost->enc ? ost->enc->name : "?");
av_log(NULL, AV_LOG_INFO, "\n");
}
if (ret) {
av_log(NULL, AV_LOG_ERROR, "%s\n", error);
return ret;
}
if (want_sdp) {
print_sdp();
}
transcode_init_done = 1; //初始化完成
return 0;
}
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