ffmpeg源码分析：transcode_init()函数

transcode_init()函数是在转换前做准备工作的.此处看一下它的真面目,不废话,看注释吧:

[cpp] view plaincopy
  1. //为转换过程做准备  
  2. static int transcode_init(OutputFile *output_files,  
  3.         int nb_output_files,  
  4.         InputFile *input_files,  
  5.         int nb_input_files)  
  6. {  
  7.     int ret = 0, i, j, k;  
  8.     AVFormatContext *oc;  
  9.     AVCodecContext *codec, *icodec;  
  10.     OutputStream *ost;  
  11.     InputStream *ist;  
  12.     char error[1024];  
  13.     int want_sdp = 1;  
  14.   
  15.     /* init framerate emulation */  
  16.     //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)  
  17.     for (i = 0; i < nb_input_files; i++)  
  18.     {  
  19.         InputFile *ifile = &input_files[i];  
  20.         //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),  
  21.         //则为这个文件中所有流记录下开始时间  
  22.         if (ifile->rate_emu)  
  23.             for (j = 0; j < ifile->nb_streams; j++)  
  24.                 input_streams[j + ifile->ist_index].start = av_gettime();  
  25.     }  
  26.   
  27.     /* output stream init */  
  28.     for (i = 0; i < nb_output_files; i++)  
  29.     {  
  30.         //什么也没做,只是做了个判断而已  
  31.         oc = output_files[i].ctx;  
  32.         if (!oc->nb_streams && !(oc->oformat->flags & AVFMT_NOSTREAMS))  
  33.         {  
  34.             av_dump_format(oc, i, oc->filename, 1);  
  35.             av_log(NULL, AV_LOG_ERROR,  
  36.                     "Output file #%d does not contain any stream ", i);  
  37.             return AVERROR(EINVAL);  
  38.         }  
  39.     }  
  40.   
  41.     //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  
  42.     for (i = 0; i < nb_output_streams; i++)  
  43.     {  
  44.         //轮循所有的输出流  
  45.         ost = &output_streams[i];  
  46.         //输出流对应的FormatContext  
  47.         oc = output_files[ost->file_index].ctx;  
  48.         //取得输出流对应的输入流  
  49.         ist = &input_streams[ost->source_index];  
  50.   
  51.         //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  
  52.         if (ost->attachment_filename)  
  53.             continue;  
  54.   
  55.         codec = ost->st->codec;//输出流的编解码器结构  
  56.         icodec = ist->st->codec;//输入流的编解码器结构  
  57.   
  58.         //先把能复制的复制一下  
  59.         ost->st->disposition = ist->st->disposition;  
  60.         codec->bits_per_raw_sample = icodec->bits_per_raw_sample;  
  61.         codec->chroma_sample_location = icodec->chroma_sample_location;  
  62.   
  63.         //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  
  64.         //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  
  65.         if (ost->stream_copy)  
  66.         {  
  67.             //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  
  68.             //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  
  69.             uint64_t extra_size = (uint64_t) icodec->extradata_size  
  70.                     + FF_INPUT_BUFFER_PADDING_SIZE;  
  71.   
  72.             if (extra_size > INT_MAX)    {  
  73.                 return AVERROR(EINVAL);  
  74.             }  
  75.   
  76.             /* if stream_copy is selected, no need to decode or encode */  
  77.             codec->codec_id = icodec->codec_id;  
  78.             codec->codec_type = icodec->codec_type;  
  79.   
  80.             if (!codec->codec_tag){  
  81.                 if (!oc->oformat->codec_tag  
  82.                     ||av_codec_get_id(oc->oformat->codec_tag,icodec->codec_tag) == codec->codec_id  
  83.                     ||av_codec_get_tag(oc->oformat->codec_tag,icodec->codec_id) <= 0)  
  84.                     codec->codec_tag = icodec->codec_tag;  
  85.             }  
  86.   
  87.             codec->bit_rate = icodec->bit_rate;  
  88.             codec->rc_max_rate = icodec->rc_max_rate;  
  89.             codec->rc_buffer_size = icodec->rc_buffer_size;  
  90.             codec->extradata = av_mallocz(extra_size);  
  91.             if (!codec->extradata){  
  92.                 return AVERROR(ENOMEM);  
  93.             }  
  94.             memcpy(codec->extradata, icodec->extradata, icodec->extradata_size);  
  95.             codec->extradata_size = icodec->extradata_size;  
  96.   
  97.             //重新鼓捣一下time base(这家伙就是帧率)  
  98.             codec->time_base = ist->st->time_base;  
  99.             //如果输出文件是avi,做一点特殊处理  
  100.             if (!strcmp(oc->oformat->name, "avi"))    {  
  101.                 if (copy_tb < 0  
  102.                         && av_q2d(icodec->time_base) * icodec->ticks_per_frame    >  
  103.                             2 * av_q2d(ist->st->time_base)  
  104.                         && av_q2d(ist->st->time_base) < 1.0 / 500  
  105.                         || copy_tb == 0)  
  106.                 {  
  107.                     codec->time_base = icodec->time_base;  
  108.                     codec->time_base.num *= icodec->ticks_per_frame;  
  109.                     codec->time_base.den *= 2;  
  110.                 }  
  111.             }  
  112.             else if (!(oc->oformat->flags & AVFMT_VARIABLE_FPS))  
  113.             {  
  114.                 if (copy_tb < 0  
  115.                         && av_q2d(icodec->time_base) * icodec->ticks_per_frame  
  116.                                 > av_q2d(ist->st->time_base)  
  117.                         && av_q2d(ist->st->time_base) < 1.0 / 500  
  118.                         || copy_tb == 0)  
  119.                 {  
  120.                     codec->time_base = icodec->time_base;  
  121.                     codec->time_base.num *= icodec->ticks_per_frame;  
  122.                 }  
  123.             }  
  124.   
  125.             //再修正一下帧率  
  126.             av_reduce(&codec->time_base.num, &codec->time_base.den,  
  127.                     codec->time_base.num, codec->time_base.den, INT_MAX);  
  128.   
  129.             //单独复制各不同媒体自己的编码参数  
  130.             switch (codec->codec_type)  
  131.             {  
  132.             case AVMEDIA_TYPE_AUDIO:  
  133.                 //音频的  
  134.                 if (audio_volume != 256){  
  135.                     av_log( NULL,AV_LOG_FATAL,  
  136.                             "-acodec copy and -vol are incompatible (frames are not decoded) ");  
  137.                     exit_program(1);  
  138.                 }  
  139.                 codec->channel_layout = icodec->channel_layout;  
  140.                 codec->sample_rate = icodec->sample_rate;  
  141.                 codec->channels = icodec->channels;  
  142.                 codec->frame_size = icodec->frame_size;  
  143.                 codec->audio_service_type = icodec->audio_service_type;  
  144.                 codec->block_align = icodec->block_align;  
  145.                 break;  
  146.             case AVMEDIA_TYPE_VIDEO:  
  147.                 //视频的  
  148.                 codec->pix_fmt = icodec->pix_fmt;  
  149.                 codec->width = icodec->width;  
  150.                 codec->height = icodec->height;  
  151.                 codec->has_b_frames = icodec->has_b_frames;  
  152.                 if (!codec->sample_aspect_ratio.num){  
  153.                     codec->sample_aspect_ratio = ost->st->sample_aspect_ratio =  
  154.                             ist->st->sample_aspect_ratio.num ?ist->st->sample_aspect_ratio :  
  155.                                     ist->st->codec->sample_aspect_ratio.num ?ist->st->codec->sample_aspect_ratio :(AVRational){0, 1};  
  156.                 }  
  157.                 ost->st->avg_frame_rate = ist->st->avg_frame_rate;  
  158.                 break;  
  159.             case AVMEDIA_TYPE_SUBTITLE:  
  160.                 //字幕的  
  161.                 codec->width  = icodec->width;  
  162.                 codec->height = icodec->height;  
  163.                 break;  
  164.             case AVMEDIA_TYPE_DATA:  
  165.             case AVMEDIA_TYPE_ATTACHMENT:  
  166.                 //??的  
  167.                 break;  
  168.             default:  
  169.                 abort();  
  170.             }  
  171.     }  
  172.     else  
  173.     {  
  174.         //如果不是复制,就麻烦多了  
  175.   
  176.         //获取编码器  
  177.         if (!ost->enc)  
  178.             ost->enc = avcodec_find_encoder(ost->st->codec->codec_id);  
  179.   
  180.         //因为需要转换,所以既需解码又需编码  
  181.         ist->decoding_needed = 1;  
  182.         ost->encoding_needed = 1;  
  183.   
  184.         switch(codec->codec_type)  
  185.         {  
  186.         case AVMEDIA_TYPE_AUDIO:  
  187.             //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  
  188.             ost->fifo = av_fifo_alloc(1024);//音频数据所在的缓冲  
  189.             if (!ost->fifo)  {  
  190.                 return AVERROR(ENOMEM);  
  191.             }  
  192.   
  193.             //采样率  
  194.             if (!codec->sample_rate)  
  195.                 codec->sample_rate = icodec->sample_rate;  
  196.             choose_sample_rate(ost->st, ost->enc);  
  197.             codec->time_base = (AVRational){1, codec->sample_rate};  
  198.   
  199.             //样点格式  
  200.             if (codec->sample_fmt == AV_SAMPLE_FMT_NONE)  
  201.                 codec->sample_fmt = icodec->sample_fmt;  
  202.             choose_sample_fmt(ost->st, ost->enc);  
  203.   
  204.             //声道  
  205.             if (ost->audio_channels_mapped)  {  
  206.                 /* the requested output channel is set to the number of 
  207.                  * -map_channel only if no -ac are specified */  
  208.                 if (!codec->channels)    {  
  209.                     codec->channels = ost->audio_channels_mapped;  
  210.                     codec->channel_layout = av_get_default_channel_layout(codec->channels);  
  211.                     if (!codec->channel_layout)  {  
  212.                         av_log(NULL, AV_LOG_FATAL, "Unable to find an appropriate channel layout for requested number of channel );  
  213.                         exit_program(1);  
  214.                     }  
  215.                 }  
  216.                 /* fill unused channel mapping with -1 (which means a muted 
  217.                  * channel in case the number of output channels is bigger 
  218.                  * than the number of mapped channel) */  
  219.                 for (j = ost->audio_channels_mapped; j < FF_ARRAY_ELEMS(ost->audio_channels_map); j++)  
  220.                 <span>  </span>ost->audio_channels_map[j] = -1;  
  221.             }else if (!codec->channels){  
  222.                 codec->channels = icodec->channels;  
  223.                 codec->channel_layout = icodec->channel_layout;  
  224.             }  
  225.             if (av_get_channel_layout_nb_channels(codec->channel_layout) != codec->channels)  
  226.                 codec->channel_layout = 0;  
  227.   
  228.             //是否需要重采样  
  229.             ost->audio_resample = codec->sample_rate != icodec->sample_rate || audio_sync_method > 1;  
  230.             ost->audio_resample |= codec->sample_fmt != icodec->sample_fmt ||  
  231.                     codec->channel_layout != icodec->channel_layout;  
  232.             icodec->request_channels = codec->channels;  
  233.             ost->resample_sample_fmt = icodec->sample_fmt;  
  234.             ost->resample_sample_rate = icodec->sample_rate;  
  235.             ost->resample_channels = icodec->channels;  
  236.             break;  
  237.         case AVMEDIA_TYPE_VIDEO:  
  238.             //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  
  239.             if (codec->pix_fmt == PIX_FMT_NONE)  
  240.                 codec->pix_fmt = icodec->pix_fmt;  
  241.             choose_pixel_fmt(ost->st, ost->enc);  
  242.             if (ost->st->codec->pix_fmt == PIX_FMT_NONE){  
  243.                 av_log(NULL, AV_LOG_FATAL, "Video pixel format is unknown, stream cannot be encoded ");  
  244.                 exit_program(1);  
  245.             }  
  246.   
  247.             //宽高  
  248.             if (!codec->width || !codec->height){  
  249.                 codec->width = icodec->width;  
  250.                 codec->height = icodec->height;  
  251.             }  
  252.   
  253.             //视频是否需要重采样  
  254.             ost->video_resample = codec->width != icodec->width ||  
  255.                     codec->height != icodec->height ||  
  256.                     codec->pix_fmt != icodec->pix_fmt;  
  257.             if (ost->video_resample){  
  258.                 codec->bits_per_raw_sample= frame_bits_per_raw_sample;  
  259.             }  
  260.   
  261.             ost->resample_height = icodec->height;  
  262.             ost->resample_width = icodec->width;  
  263.             ost->resample_pix_fmt = icodec->pix_fmt;  
  264.   
  265.             //计算帧率  
  266.             if (!ost->frame_rate.num)  
  267.                 ost->frame_rate = ist->st->r_frame_rate.num ?  
  268.                         ist->st->r_frame_rate : (AVRational){25,1};  
  269.             if (ost->enc && ost->enc->supported_framerates && !ost->force_fps)  {  
  270.                 int idx = av_find_nearest_q_idx(ost->frame_rate,ost->enc->supported_framerates);  
  271.                 ost->frame_rate = ost->enc->supported_framerates[idx];  
  272.             }  
  273.             codec->time_base = (AVRational)  {ost->frame_rate.den, ost->frame_rate.num};  
  274.             if( av_q2d(codec->time_base) < 0.001 &&  
  275.                     video_sync_method &&  
  276.                     (video_sync_method==1 ||  
  277.                         (video_sync_method<0 &&  !  
  278.                             (oc->oformat->flags & AVFMT_VARIABLE_FPS))))  
  279.             {  
  280.                 av_log(oc, AV_LOG_WARNING, "Frame rate very high for a muxer not effciciently supporting it. "  
  281.                         "Please consider specifiying a lower framerate, a different muxer or -vsync 2 ");  
  282.             }  
  283.         <span>  </span>for (j = 0; j < ost->forced_kf_count; j++)  
  284.                 ost->forced_kf_pts[j] = av_rescale_q(ost->forced_kf_pts[j],  
  285.                         AV_TIME_BASE_Q, codec->time_base);  
  286.                 break;  
  287.             case AVMEDIA_TYPE_SUBTITLE:  
  288.                 break;  
  289.             default:  
  290.                 abort();  
  291.                 break;  
  292.             }  
  293.             /* two pass mode */  
  294.             if (codec->codec_id != CODEC_ID_H264 &&  
  295.                     (codec->flags & (CODEC_FLAG_PASS1 | CODEC_FLAG_PASS2)))  
  296.             {  
  297.                 char logfilename[1024];  
  298.                 FILE *f;  
  299.   
  300.                 snprintf(logfilename, sizeof(logfilename), "%s-%d.log",  
  301.                         pass_logfilename_prefix ? pass_logfilename_prefix : DEFAULT_PASS_LOGFILENAME_PREFIX,  
  302.                         i);  
  303.                 if (codec->flags & CODEC_FLAG_PASS2){  
  304.                     char *logbuffer;  
  305.                     size_t logbuffer_size;  
  306.                     if (cmdutils_read_file(logfilename, &logbuffer, &logbuffer_size) < 0){  
  307.                         av_log(NULL, AV_LOG_FATAL,  
  308.                                 "Error reading log file '%s' for pass-2 encoding ",  
  309.                                 logfilename);  
  310.                         exit_program(1);  
  311.                     }  
  312.                     codec->stats_in = logbuffer;  
  313.                 }  
  314.                 if (codec->flags & CODEC_FLAG_PASS1){  
  315.                     f = fopen(logfilename, "wb");  
  316.                     if (!f) {  
  317.                         av_log(NULL, AV_LOG_FATAL, "Cannot write log file '%s' for pass-1 encoding: %s ",  
  318.                                 logfilename, strerror(errno));  
  319.                         exit_program(1);  
  320.                     }  
  321.                     ost->logfile = f;  
  322.                 }  
  323.             }  
  324.         }  
  325.         if (codec->codec_type == AVMEDIA_TYPE_VIDEO){  
  326.             /* maximum video buffer size is 6-bytes per pixel, plus DPX header size (1664)*/  
  327.             //计算编码输出缓冲的大小,计算一个最大值  
  328.             int size = codec->width * codec->height;  
  329.             bit_buffer_size = FFMAX(bit_buffer_size, 7 * size + 10000);  
  330.         }  
  331.     }  
  332.   
  333.     //分配编码后数据所在的缓冲  
  334.     if (!bit_buffer)  
  335.         bit_buffer = av_malloc(bit_buffer_size);  
  336.     if (!bit_buffer){  
  337.         av_log(NULL, AV_LOG_ERROR,  
  338.                 "Cannot allocate %d bytes output buffer ",  
  339.                 bit_buffer_size);  
  340.         return AVERROR(ENOMEM);  
  341.     }  
  342.   
  343.     //轮循所有输出流,打开每个输出流的编码器  
  344.     for (i = 0; i < nb_output_streams; i++)  
  345.     {  
  346.         ost = &output_streams[i];  
  347.         if (ost->encoding_needed){  
  348.             //当然,只有在需要编码时才打开编码器  
  349.             AVCodec *codec = ost->enc;  
  350.             AVCodecContext *dec = input_streams[ost->source_index].st->codec;  
  351.             if (!codec) {  
  352.                 snprintf(error, sizeof(error),  
  353.                         "Encoder (codec %s) not found for output stream #%d:%d",  
  354.                         avcodec_get_name(ost->st->codec->codec_id),  
  355.                         ost->file_index, ost->index);  
  356.                 ret = AVERROR(EINVAL);  
  357.                 goto dump_format;  
  358.             }  
  359.             if (dec->subtitle_header){  
  360.                 ost->st->codec->subtitle_header = av_malloc(dec->subtitle_header_size);  
  361.                 if (!ost->st->codec->subtitle_header){  
  362.                     ret = AVERROR(ENOMEM);  
  363.                     goto dump_format;  
  364.                 }  
  365.                 memcpy(ost->st->codec->subtitle_header,  
  366.                         dec->subtitle_header,dec->subtitle_header_size);  
  367.                 ost->st->codec->subtitle_header_size = dec->subtitle_header_size;  
  368.             }  
  369.             //打开啦  
  370.             if (avcodec_open2(ost->st->codec, codec, &ost->opts) < 0)   {  
  371.                 snprintf(error, sizeof(error),  
  372.                         "Error while opening encoder for output stream #%d:%d - maybe incorrect parameters such as bit_rate, rate, width or height",  
  373.                         ost->file_index, ost->index);  
  374.                 ret = AVERROR(EINVAL);  
  375.                 goto dump_format;  
  376.             }  
  377.             assert_codec_experimental(ost->st->codec, 1);  
  378.             assert_avoptions(ost->opts);  
  379.             if (ost->st->codec->bit_rate && ost->st->codec->bit_rate < 1000)  
  380.                 av_log(NULL, AV_LOG_WARNING,  
  381.                         "The bitrate parameter is set too low."  
  382.                         " It takes bits/s as argument, not kbits/s ");  
  383.             extra_size += ost->st->codec->extradata_size;  
  384.   
  385.             if (ost->st->codec->me_threshold)  
  386.                 input_streams[ost->source_index].st->codec->debug |= FF_DEBUG_MV;  
  387.         }  
  388.     }  
  389.   
  390.     //初始化所有的输入流(主要做的就是在需要时打开解码器)  
  391.     for (i = 0; i < nb_input_streams; i++)  
  392.         if ((ret = init_input_stream(i, output_streams, nb_output_streams,  
  393.                 error, sizeof(error))) < 0)  
  394.             goto dump_format;  
  395.   
  396.     /* discard unused programs */  
  397.     for (i = 0; i < nb_input_files; i++){  
  398.         InputFile *ifile = &input_files[i];  
  399.         for (j = 0; j < ifile->ctx->nb_programs; j++){  
  400.             AVProgram *p = ifile->ctx->programs[j];  
  401.             int discard = AVDISCARD_ALL;  
  402.   
  403.             for (k = 0; k < p->nb_stream_indexes; k++){  
  404.                 if (!input_streams[ifile->ist_index + p->stream_index[k]].discard){  
  405.                     discard = AVDISCARD_DEFAULT;  
  406.                     break;  
  407.                 }  
  408.             }  
  409.             p->discard = discard;  
  410.         }  
  411.     }  
  412.   
  413.     //打开所有输出文件，写入媒体文件头  
  414.     for (i = 0; i < nb_output_files; i++){  
  415.         oc = output_files[i].ctx;  
  416.         oc->interrupt_callback = int_cb;  
  417.         if (avformat_write_header(oc, &output_files[i].opts) < 0){  
  418.             snprintf(error, sizeof(error),  
  419.                     "Could not write header for output file #%d (incorrect codec parameters ?)",  
  420.                     i);  
  421.             ret = AVERROR(EINVAL);  
  422.             goto dump_format;  
  423.         }  
  424. //        assert_avoptions(output_files[i].opts);  
  425.         if (strcmp(oc->oformat->name, "rtp")){  
  426.             want_sdp = 0;  
  427.         }  
  428.     }  
  429.   
  430.     return 0;  
  431. }  

原文地址：http://blog.csdn.net/niu_gao/article/details/7175421

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原文地址：https://www.cnblogs.com/leixiaohua1020/p/3902205.html