原帖地址：http://blog.csdn.net/austinblog/article/details/25061945
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; }
至此，初始化部分就分析完了，后面将继续分析其他关键函数。
