本文主要是介绍ffmpeg源码分析三：transcode_init函数，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
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FormatContextist = 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 optionsar =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;elsesar = 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 optionav_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 caseof 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)");elseav_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;
}
这篇关于ffmpeg源码分析三：transcode_init函数的文章就介绍到这儿，希望我们推荐的文章对编程师们有所帮助！
