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  • 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
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