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      =http://www.mamicode.com/ 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;}