最新版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.其大体要完成的任务在第一篇中已做了猜测.此处看一下它的真面目,不废话,看注释吧:

//为转换过程做准备
static int transcode_init(OutputFile *output_files,
int nb_output_files,
InputFile *input_files,
int nb_input_files)
{
int ret = 0, i, j, k;
AVFormatContext *oc;
AVCodecContext *codec, *icodec;
OutputStream *ost;
InputStream *ist;
char error[1024];
int want_sdp = 1;

/* 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 */
for (i = 0; i < nb_output_files; i++)
{
//什么也没做,只是做了个判断而已
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);
}
}

//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++)
{
//轮循所有的输出流
ost = &output_streams[i];
//输出流对应的FormatContext
oc = output_files[ost->file_index].ctx;
//取得输出流对应的输入流
ist = &input_streams[ost->source_index];

//attachment_filename是不是这样的东西:一个文件,它单独容纳一个输出流?此处不懂
if (ost->attachment_filename)
continue;

codec = ost->st->codec;//输出流的编解码器结构
icodec = ist->st->codec;//输入流的编解码器结构

//先把能复制的复制一下
ost->st->disposition = ist->st->disposition;
codec->bits_per_raw_sample = icodec->bits_per_raw_sample;
codec->chroma_sample_location = icodec->chroma_sample_location;

//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复制给输出流
//此时是不需要解码也不需要编码的，所以不需打开解码器和编码器
if (ost->stream_copy)
{
//计算输出流的编解码器的extradata的大小,然后分配容纳extradata的缓冲
//然后把输入流的编解码器的extradata复制到输出流的编解码器中
uint64_t 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){
if (!oc->oformat->codec_tag
||av_codec_get_id(oc->oformat->codec_tag,icodec->codec_tag) == codec->codec_id
||av_codec_get_tag(oc->oformat->codec_tag,icodec->codec_id) <= 0)
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->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;

//重新鼓捣一下time base(这家伙就是帧率)
codec->time_base = ist->st->time_base;
//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(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;
}
}
else if (!(oc->oformat->flags & AVFMT_VARIABLE_FPS))
{
if (copy_tb < 0
&& 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;
}
}

//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);

//单独复制各不同媒体自己的编码参数
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;
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 (!codec->sample_aspect_ratio.num){
codec->sample_aspect_ratio = ost->st->sample_aspect_ratio =
ist->st->sample_aspect_ratio.num ?ist->st->sample_aspect_ratio :
ist->st->codec->sample_aspect_ratio.num ?ist->st->codec->sample_aspect_ratio :(AVRational){0, 1};
}
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:
//??的
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(ost->st->codec->codec_id);

//因为需要转换,所以既需解码又需编码
ist->decoding_needed = 1;
ost->encoding_needed = 1;

switch(codec->codec_type)
{
case AVMEDIA_TYPE_AUDIO:
//鼓捣音频编码器的参数,基本上是把一些不合适的参数替换掉
ost->fifo = av_fifo_alloc(1024);//音频数据所在的缓冲
if (!ost->fifo)	{
return AVERROR(ENOMEM);
}

//采样率
if (!codec->sample_rate)
codec->sample_rate = icodec->sample_rate;
choose_sample_rate(ost->st, ost->enc);
codec->time_base = (AVRational){1, codec->sample_rate};

//样点格式
if (codec->sample_fmt == AV_SAMPLE_FMT_NONE)
codec->sample_fmt = icodec->sample_fmt;
choose_sample_fmt(ost->st, ost->enc);

//声道
if (ost->audio_channels_mapped)	{
/* the requested output channel is set to the number of
* -map_channel only if no -ac are specified */
if (!codec->channels)	{
codec->channels = ost->audio_channels_mapped;
codec->channel_layout = av_get_default_channel_layout(codec->channels);
if (!codec->channel_layout)	{
av_log(NULL, AV_LOG_FATAL, "Unable to find an appropriate channel layout for requested number of channel\n);
exit_program(1);
}
}
/* fill unused channel mapping with -1 (which means a muted
* channel in case the number of output channels is bigger
* than the number of mapped channel) */
for (j = ost->audio_channels_mapped; j < FF_ARRAY_ELEMS(ost->audio_channels_map); j++)
ost->audio_channels_map[j] = -1;
}else if (!codec->channels){
codec->channels = icodec->channels;
codec->channel_layout = icodec->channel_layout;
}
if (av_get_channel_layout_nb_channels(codec->channel_layout) != codec->channels)
codec->channel_layout = 0;

//是否需要重采样
ost->audio_resample = codec->sample_rate != icodec->sample_rate || audio_sync_method > 1;
ost->audio_resample |= codec->sample_fmt != icodec->sample_fmt	||
codec->channel_layout != icodec->channel_layout;
icodec->request_channels = codec->channels;
ost->resample_sample_fmt = icodec->sample_fmt;
ost->resample_sample_rate = icodec->sample_rate;
ost->resample_channels = icodec->channels;
break;
case AVMEDIA_TYPE_VIDEO:
//鼓捣视频编码器的参数,基本上是把一些不合适的参数替换掉
if (codec->pix_fmt == PIX_FMT_NONE)
codec->pix_fmt = icodec->pix_fmt;
choose_pixel_fmt(ost->st, ost->enc);
if (ost->st->codec->pix_fmt == PIX_FMT_NONE){
av_log(NULL, AV_LOG_FATAL, "Video pixel format is unknown, stream cannot be encoded\n");
exit_program(1);
}

//宽高
if (!codec->width || !codec->height){
codec->width = icodec->width;
codec->height = icodec->height;
}

//视频是否需要重采样
ost->video_resample = codec->width != icodec->width ||
codec->height != icodec->height ||
codec->pix_fmt != icodec->pix_fmt;
if (ost->video_resample){
codec->bits_per_raw_sample= frame_bits_per_raw_sample;
}

ost->resample_height = icodec->height;
ost->resample_width = icodec->width;
ost->resample_pix_fmt = icodec->pix_fmt;

//计算帧率
if (!ost->frame_rate.num)
ost->frame_rate = ist->st->r_frame_rate.num ?
ist->st->r_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];
}
codec->time_base = (AVRational)	{ost->frame_rate.den, ost->frame_rate.num};
if( av_q2d(codec->time_base) < 0.001 &&
video_sync_method &&
(video_sync_method==1 ||
(video_sync_method<0 &&	!
(oc->oformat->flags & AVFMT_VARIABLE_FPS))))
{
av_log(oc, AV_LOG_WARNING, "Frame rate very high for a muxer not effciciently supporting it.\n"
"Please consider specifiying 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);
break;
case AVMEDIA_TYPE_SUBTITLE:
break;
default:
abort();
break;
}
/* two pass mode */
if (codec->codec_id != CODEC_ID_H264 &&
(codec->flags & (CODEC_FLAG_PASS1 | CODEC_FLAG_PASS2)))
{
char logfilename[1024];
FILE *f;

snprintf(logfilename, sizeof(logfilename), "%s-%d.log",
pass_logfilename_prefix ? pass_logfilename_prefix : DEFAULT_PASS_LOGFILENAME_PREFIX,
i);
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 = fopen(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;
}
}
}
if (codec->codec_type == AVMEDIA_TYPE_VIDEO){
/* maximum video buffer size is 6-bytes per pixel, plus DPX header size (1664)*/
//计算编码输出缓冲的大小,计算一个最大值
int size = codec->width * codec->height;
bit_buffer_size = FFMAX(bit_buffer_size, 7 * size + 10000);
}
}

//分配编码后数据所在的缓冲
if (!bit_buffer)
bit_buffer = av_malloc(bit_buffer_size);
if (!bit_buffer){
av_log(NULL, AV_LOG_ERROR,
"Cannot allocate %d bytes output buffer\n",
bit_buffer_size);
return AVERROR(ENOMEM);
}

//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){
//当然,只有在需要编码时才打开编码器
AVCodec *codec = ost->enc;
AVCodecContext *dec = input_streams[ost->source_index].st->codec;
if (!codec)	{
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;
}
if (dec->subtitle_header){
ost->st->codec->subtitle_header = av_malloc(dec->subtitle_header_size);
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;
}
//打开啦
if (avcodec_open2(ost->st->codec, codec, &ost->opts) < 0)	{
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);
ret = AVERROR(EINVAL);
goto dump_format;
}
assert_codec_experimental(ost->st->codec, 1);
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;

if (ost->st->codec->me_threshold)
input_streams[ost->source_index].st->codec->debug |= FF_DEBUG_MV;
}
}

//初始化所有的输入流(主要做的就是在需要时打开解码器)
for (i = 0; i < nb_input_streams; i++)
if ((ret = init_input_stream(i, output_streams, nb_output_streams,
error, sizeof(error))) < 0)
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;
}
}

//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 (avformat_write_header(oc, &output_files[i].opts) < 0){
snprintf(error,	sizeof(error),
"Could not write header for output file #%d (incorrect codec parameters ?)",
i);
ret = AVERROR(EINVAL);
goto dump_format;
}
//        assert_avoptions(output_files[i].opts);
if (strcmp(oc->oformat->name, "rtp")){
want_sdp = 0;
}
}

return 0;
}

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原文链接：
http://blog.csdn.net/nkmnkm/article/details/7177877