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从ffmpeg源代码分析如何解决ffmpeg编码的延迟问题(如何解决编码 0 延时)

發(fā)布時(shí)間：2024/2/28 编程问答 37 豆豆

生活随笔收集整理的這篇文章主要介紹了从ffmpeg源代码分析如何解决ffmpeg编码的延迟问题(如何解决编码 0 延时) 小編覺得挺不錯(cuò)的,現(xiàn)在分享給大家,幫大家做個(gè)參考.

在看到這篇文章前我已經(jīng)解決了這個(gè)問題，但是知其然不知其所以然，看到原作者寫的文章很詳細(xì)。

于是轉(zhuǎn)之，感謝原作者。

原文鏈接：http://blog.csdn.net/ymsdu2004/article/details/8565822

FFMPEG中如果你編碼完一段視頻發(fā)現(xiàn)視頻變短了，那很有可能就是沒有設(shè)置0延時(shí)的原因。

近日在做一個(gè)分布式轉(zhuǎn)碼服務(wù)器，解碼器是采用開源的ffmpeg，在開發(fā)的過程中遇到一個(gè)問題：編碼延遲多大5、6秒鐘，也就是最初編碼的幾十幀并不能馬上取出，而我們的要求是實(shí)時(shí)編碼！雖然我對(duì)視頻編碼方面不是很熟悉，但根據(jù)開發(fā)的經(jīng)驗(yàn)，我想必定可以通過設(shè)置一些參數(shù)來改變這些情況。但我本人接觸ffmpeg項(xiàng)目時(shí)間并不長，對(duì)很多與編解碼方面參數(shù)的設(shè)置并不熟悉，于是google了很久，網(wǎng)上也有相關(guān)方面的討論，說什么的都有，但我試了不行，更有甚者說修改源代碼的，這個(gè)可能能夠解決問題，但修改源代碼畢竟不是解決問題的最佳途徑。于是決定分析一下源代碼，跟蹤源碼來找出問題的根源。

? ? 首先我使用的ffmpeg源代碼版本是1.0.3，同時(shí)給出我的測(cè)試代碼，項(xiàng)目中的代碼就不給出來了，我給個(gè)簡單的玩具代碼：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">/**?*?@file?*?libavcodec?API?use?example.?*?*?Note?that?libavcodec?only?handles?codecs?(mpeg,?mpeg4,?etc...),?*?not?file?formats?(avi,?vob,?mp4,?mov,?mkv,?mxf,?flv,?mpegts,?mpegps,?etc...).?See?library?'libavformat'?for?the?*?format?handling?*/??#if?_MSC_VER?? #define?snprintf?_snprintf?? #endif??#include?<stdio.h>?? #include?<math.h>??extern?"C"?{?? #include?<libavutil/opt.h>????????//?for?av_opt_set?? #include?<libavcodec/avcodec.h>?? #include?<libavutil/imgutils.h>?? };??/*?*?Video?encoding?example?*/?? static?void?video_encode_example(const?char?*filename,?int?codec_id)?? {??AVCodec?*codec;??AVCodecContext?*c=?NULL;??int?i,?ret,?x,?y,?got_output;??FILE?*f;??AVFrame?*picture;??AVPacket?pkt;??uint8_t?endcode[]?=?{?0,?0,?1,?0xb7?};??printf("Encode?video?file?%s\n",?filename);??/*?find?the?mpeg1?video?encoder?*/??codec?=?avcodec_find_encoder((AVCodecID)codec_id);??if?(!codec)?{??fprintf(stderr,?"codec?not?found\n");??exit(1);??}??c?=?avcodec_alloc_context3(codec);??/*?put?sample?parameters?*/??c->bit_rate?=?400000;??/*?resolution?must?be?a?multiple?of?two?*/??c->width?=?800/*352*/;??c->height?=?500/*288*/;??/*?frames?per?second?*/??c->time_base.den?=?1;??c->time_base.num?=?25;??c->gop_size?=?10;?/*?emit?one?intra?frame?every?ten?frames?*/??c->max_b_frames=1;??c->pix_fmt?=?PIX_FMT_YUV420P;??/*?open?it?*/??if?(avcodec_open2(c,?codec,?NULL)?<?0)?{??fprintf(stderr,?"could?not?open?codec\n");??exit(1);??}??f?=?fopen(filename,?"wb");??if?(!f)?{??fprintf(stderr,?"could?not?open?%s\n",?filename);??exit(1);??}??picture?=?avcodec_alloc_frame();??if?(!picture)?{??fprintf(stderr,?"Could?not?allocate?video?frame\n");??exit(1);??}??picture->format?=?c->pix_fmt;??picture->width??=?c->width;??picture->height?=?c->height;??/*?the?image?can?be?allocated?by?any?means?and?av_image_alloc()?is?*?just?the?most?convenient?way?if?av_malloc()?is?to?be?used?*/??ret?=?av_image_alloc(picture->data,?picture->linesize,?c->width,?c->height,??c->pix_fmt,?32);??if?(ret?<?0)?{??fprintf(stderr,?"could?not?alloc?raw?picture?buffer\n");??exit(1);??}??static?int?delayedFrame?=?0;??/*?encode?1?second?of?video?*/??for(i=0;i<25;i++)?{??av_init_packet(&pkt);??pkt.data?=?NULL;????//?packet?data?will?be?allocated?by?the?encoder??pkt.size?=?0;??fflush(stdout);??/*?prepare?a?dummy?image?*/??/*?Y?*/??for(y=0;y<c->height;y++)?{??for(x=0;x<c->width;x++)?{??picture->data[0][y?*?picture->linesize[0]?+?x]?=?x?+?y?+?i?*?3;??}??}??/*?Cb?and?Cr?*/??for(y=0;y<c->height/2;y++)?{??for(x=0;x<c->width/2;x++)?{??picture->data[1][y?*?picture->linesize[1]?+?x]?=?128?+?y?+?i?*?2;??picture->data[2][y?*?picture->linesize[2]?+?x]?=?64?+?x?+?i?*?5;??}??}??picture->pts?=?i;??printf("encoding?frame?%3d----",?i);??/*?encode?the?image?*/??ret?=?avcodec_encode_video2(c,?&pkt,?picture,?&got_output);??if?(ret?<?0)?{??fprintf(stderr,?"error?encoding?frame\n");??exit(1);??}??if?(got_output)?{??printf("output?frame?%3d?(size=%5d)\n",?i-delayedFrame,?pkt.size);??fwrite(pkt.data,?1,?pkt.size,?f);??av_free_packet(&pkt);??}??else?{??delayedFrame++;??printf("no?output?frame\n");??}??}??/*?get?the?delayed?frames?*/??for?(got_output?=?1;?got_output;?i++)?{??fflush(stdout);??ret?=?avcodec_encode_video2(c,?&pkt,?NULL,?&got_output);??if?(ret?<?0)?{??fprintf(stderr,?"error?encoding?frame\n");??exit(1);??}??if?(got_output)?{??printf("output?delayed?frame?%3d?(size=%5d)\n",?i-delayedFrame,?pkt.size);??fwrite(pkt.data,?1,?pkt.size,?f);??av_free_packet(&pkt);??}??}??/*?add?sequence?end?code?to?have?a?real?mpeg?file?*/??fwrite(endcode,?1,?sizeof(endcode),?f);??fclose(f);??avcodec_close(c);??av_free(c);??av_freep(&picture->data[0]);??av_free(picture);??printf("\n");?? }??int?main(int?argc,?char?**argv)?? {??/*?register?all?the?codecs?*/??avcodec_register_all();??video_encode_example("test.h264",?AV_CODEC_ID_H264);??system("pause");??return?0;?? }??

??? 運(yùn)行上面的代碼，我們編碼25幀，發(fā)現(xiàn)延遲了多達(dá)18幀，如下圖所示：

? ? 現(xiàn)在我們開始分析ffmpeg的源代碼（因?yàn)閒fmpeg的編碼器是基于X264項(xiàng)目的，所以我們的代碼還要追蹤帶X264中去）：

avcodec_register_all()做了些什么

? ? 因?yàn)槲覀冴P(guān)心的是H.264的編碼所以我們只關(guān)心該函數(shù)中對(duì)X264編碼器做了些什么，該函數(shù)主要是注冊(cè)ffmpeg提供的所有編解碼器，由于該函數(shù)較長，但都是相同的動(dòng)作（注冊(cè)編解碼器），所以我們只列出部分的代碼：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">void?avcodec_register_all(void)?? {??static?int?initialized;??if?(initialized)??return;??initialized?=?1;??/*?hardware?accelerators?*/??REGISTER_HWACCEL?(H263_VAAPI,?h263_vaapi);??REGISTER_HWACCEL?(H264_DXVA2,?h264_dxva2);??......??/*?video?codecs?*/??REGISTER_ENCODER?(A64MULTI,?a64multi);??REGISTER_ENCODER?(A64MULTI5,?a64multi5);??......??/*?audio?codecs?*/??REGISTER_ENCDEC??(AAC,?aac);??REGISTER_DECODER?(AAC_LATM,?aac_latm);??......??/*?PCM?codecs?*/??REGISTER_ENCDEC??(PCM_ALAW,?pcm_alaw);??REGISTER_DECODER?(PCM_BLURAY,?pcm_bluray);??......??/*?DPCM?codecs?*/??REGISTER_DECODER?(INTERPLAY_DPCM,?interplay_dpcm);??REGISTER_ENCDEC??(ROQ_DPCM,?roq_dpcm);??......??/*?ADPCM?codecs?*/??REGISTER_DECODER?(ADPCM_4XM,?adpcm_4xm);??REGISTER_ENCDEC??(ADPCM_ADX,?adpcm_adx);??......??/*?subtitles?*/??REGISTER_ENCDEC??(ASS,?ass);??REGISTER_ENCDEC??(DVBSUB,?dvbsub);??......??/*?external?libraries?*/??REGISTER_DECODER?(LIBCELT,?libcelt);??......??<span?style="color:#ff6666;">????//??//?這是我們關(guān)注的libx264編碼器??REGISTER_ENCODER?(LIBX264,?libx264);??......??/*?text?*/??REGISTER_DECODER?(BINTEXT,?bintext);??......??/*?parsers?*/??REGISTER_PARSER??(AAC,?aac);??REGISTER_PARSER??(AAC_LATM,?aac_latm);??......??/*?bitstream?filters?*/??REGISTER_BSF?????(AAC_ADTSTOASC,?aac_adtstoasc);??REGISTER_BSF?????(CHOMP,?chomp);??......?? }??

? ? 很顯然，我們關(guān)心的是REGISTER_ENCODER (LIBX264, libx264)，這里是注冊(cè)libx264編碼器：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">#define?REGISTER_ENCODER(X,x)?{?\??extern?AVCodec?ff_##x##_encoder;?\??if(CONFIG_##X##_ENCODER)??avcodec_register(&ff_##x##_encoder);?}??

將宏以參數(shù)LIBX264和libx264展開得到：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">{??extern?AVCodec?ff_libx264_encoder;??if?(CONFIG_LIBX264_ENCODER)??avcodec_register(&ff_libx264_encoder);?? }??

在ffmpeg中查找ff_libx264_encoder變量：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">AVCodec?ff_libx264_encoder?=?{??.name?????????????=?"libx264",??.type?????????????=?AVMEDIA_TYPE_VIDEO,??.id???????????????=?AV_CODEC_ID_H264,??.priv_data_size???=?sizeof(X264Context),??<span?style="color:#ff6666;">.init?????????????=?X264_init,??.encode2??????????=?X264_frame,??.close????????????=?X264_close,??.capabilities?????=?CODEC_CAP_DELAY?|?CODEC_CAP_AUTO_THREADS,??.long_name????????=?NULL_IF_CONFIG_SMALL("libx264?H.264?/?AVC?/?MPEG-4?AVC?/?MPEG-4?part?10"),??.priv_class???????=?&class,??.defaults?????????=?x264_defaults,??.init_static_data?=?X264_init_static,?? };??

看到這個(gè)結(jié)構(gòu)體中的init成員，我們可以推測(cè)這個(gè)成員注冊(cè)的X264_init函數(shù)一定是對(duì)X264編碼器的各項(xiàng)參數(shù)做初始化工作，這給我們提供了繼續(xù)查找下去的線索，稍后我們來分析，這里有個(gè)條件判斷CONFIG_LIBX264_ENCODER，我們?cè)趂fmpeg工程中查找，發(fā)現(xiàn)在它Config.mak中，這個(gè)文件是在咱們編譯ffmpeg工程時(shí)由./configure根據(jù)你的選項(xiàng)自動(dòng)生成的，還記得我們編譯時(shí)用了--enable-libx264選項(xiàng)嗎(我們要使用X264編碼器，當(dāng)然要指定該選項(xiàng))？所以有CONFIG_LIBX264_ENCODER=yes，因此這里可以成功注冊(cè)x264編碼器，如果當(dāng)初沒有指定該選項(xiàng)，編碼器是不會(huì)注冊(cè)進(jìn)去的。

? ? 而avcodec_register則是將具體的codec注冊(cè)到編解碼器鏈表中去：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">void?avcodec_register(AVCodec?*codec)?? {??AVCodec?**p;??avcodec_init();??p?=?&first_avcodec;??while?(*p?!=?NULL)?p?=?&(*p)->next;??*p?=?codec;??codec->next?=?NULL;??if?(codec->init_static_data)??codec->init_static_data(codec);?? }??

這里first_avcodec是一個(gè)全局變量，作為編解碼器鏈表的起始位置，之后注冊(cè)的編解碼器都加入到這個(gè)鏈表中去。

avcodec_find_encoder

該函數(shù)就是在編解碼器鏈表中找出你需要的codec，如果你之前沒有注冊(cè)該device，將會(huì)查找失敗，從代碼中可以看出，它就是中first_avcodec開始查找每個(gè)節(jié)點(diǎn)，比較每個(gè)device的id是否與你參數(shù)給的一直，如果是，則找到了，并返回之：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">AVCodec?*avcodec_find_encoder(enum?AVCodecID?id)?? {??AVCodec?*p,?*experimental=NULL;??p?=?first_avcodec;??id=?remap_deprecated_codec_id(id);??while?(p)?{??if?(av_codec_is_encoder(p)?&&?p->id?==?id)?{??if?(p->capabilities?&?CODEC_CAP_EXPERIMENTAL?&&?!experimental)?{??experimental?=?p;??}?else??return?p;??}??p?=?p->next;??}??return?experimental;?? }??

至此你應(yīng)該理解了為什么每次使用編碼器前，我們都會(huì)先調(diào)用avcodec_register_all或者avcodec_register，你也了解到了為什么你調(diào)用了avcodec_register_all，但查找AV_CODEC_ID_H264編碼器時(shí)會(huì)還是會(huì)失敗（因?yàn)槟憔幾gffmpeg時(shí)未指定--enable-libx264）。

打開編碼器，avcodec_open2

這個(gè)函數(shù)主要是打開你找到的編碼器，所謂打開其實(shí)是設(shè)置編碼器的各項(xiàng)參數(shù)，要設(shè)置的參數(shù)數(shù)據(jù)則是從我么設(shè)置的AVCodecContext來獲得的。

[cpp]?view plaincopy<span?style="font-family:Courier?New;">int?attribute_align_arg?avcodec_open2(AVCodecContext?*avctx,?const?AVCodec?*codec,?AVDictionary?**options)?? {??int?ret?=?0;??AVDictionary?*tmp?=?NULL;??if?(avcodec_is_open(avctx))??return?0;??if?((!codec?&&?!avctx->codec))?{??av_log(avctx,?AV_LOG_ERROR,?"No?codec?provided?to?avcodec_open2().\n");??return?AVERROR(EINVAL);??}??if?((codec?&&?avctx->codec?&&?codec?!=?avctx->codec))?{??av_log(avctx,?AV_LOG_ERROR,?"This?AVCodecContext?was?allocated?for?%s,?"??"but?%s?passed?to?avcodec_open2().\n",?avctx->codec->name,?codec->name);??return?AVERROR(EINVAL);??}??if?(!codec)??codec?=?avctx->codec;??if?(avctx->extradata_size?<?0?||?avctx->extradata_size?>=?FF_MAX_EXTRADATA_SIZE)??return?AVERROR(EINVAL);??if?(options)??av_dict_copy(&tmp,?*options,?0);??/*?If?there?is?a?user-supplied?mutex?locking?routine,?call?it.?*/??if?(ff_lockmgr_cb)?{??if?((*ff_lockmgr_cb)(&codec_mutex,?AV_LOCK_OBTAIN))??return?-1;??}??entangled_thread_counter++;??if(entangled_thread_counter?!=?1){??av_log(avctx,?AV_LOG_ERROR,?"insufficient?thread?locking?around?avcodec_open/close()\n");??ret?=?-1;??goto?end;??}??avctx->internal?=?av_mallocz(sizeof(AVCodecInternal));??if?(!avctx->internal)?{??ret?=?AVERROR(ENOMEM);??goto?end;??}??if?(codec->priv_data_size?>?0)?{??if(!avctx->priv_data){??avctx->priv_data?=?av_mallocz(codec->priv_data_size);??if?(!avctx->priv_data)?{??ret?=?AVERROR(ENOMEM);??goto?end;??}??if?(codec->priv_class)?{??*(const?AVClass**)avctx->priv_data=?codec->priv_class;??av_opt_set_defaults(avctx->priv_data);??}??}??if?(codec->priv_class?&&?(ret?=?av_opt_set_dict(avctx->priv_data,?&tmp))?<?0)??goto?free_and_end;??}?else?{??avctx->priv_data?=?NULL;??}??if?((ret?=?av_opt_set_dict(avctx,?&tmp))?<?0)??goto?free_and_end;??if?(codec->capabilities?&?CODEC_CAP_EXPERIMENTAL)??if?(avctx->strict_std_compliance?>?FF_COMPLIANCE_EXPERIMENTAL)?{??av_log(avctx,?AV_LOG_ERROR,?"Codec?is?experimental?but?experimental?codecs?are?not?enabled,?try?-strict?-2\n");??ret?=?-1;??goto?free_and_end;??}??//We?only?call?avcodec_set_dimensions()?for?non?h264?codecs?so?as?not?to?overwrite?previously?setup?dimensions??if(!(?avctx->coded_width?&&?avctx->coded_height?&&?avctx->width?&&?avctx->height?&&?avctx->codec_id?==?AV_CODEC_ID_H264)){??if(avctx->coded_width?&&?avctx->coded_height)??avcodec_set_dimensions(avctx,?avctx->coded_width,?avctx->coded_height);??else?if(avctx->width?&&?avctx->height)??avcodec_set_dimensions(avctx,?avctx->width,?avctx->height);??}??if?((avctx->coded_width?||?avctx->coded_height?||?avctx->width?||?avctx->height)??&&?(??av_image_check_size(avctx->coded_width,?avctx->coded_height,?0,?avctx)?<?0??||?av_image_check_size(avctx->width,???????avctx->height,???????0,?avctx)?<?0))?{??av_log(avctx,?AV_LOG_WARNING,?"ignoring?invalid?width/height?values\n");??avcodec_set_dimensions(avctx,?0,?0);??}??/*?if?the?decoder?init?function?was?already?called?previously,?free?the?already?allocated?subtitle_header?before?overwriting?it?*/??if?(av_codec_is_decoder(codec))??av_freep(&avctx->subtitle_header);??#define?SANE_NB_CHANNELS?128U??if?(avctx->channels?>?SANE_NB_CHANNELS)?{??ret?=?AVERROR(EINVAL);??goto?free_and_end;??}??avctx->codec?=?codec;??if?((avctx->codec_type?==?AVMEDIA_TYPE_UNKNOWN?||?avctx->codec_type?==?codec->type)?&&??avctx->codec_id?==?AV_CODEC_ID_NONE)?{??avctx->codec_type?=?codec->type;??avctx->codec_id???=?codec->id;??}??if?(avctx->codec_id?!=?codec->id?||?(avctx->codec_type?!=?codec->type??&&?avctx->codec_type?!=?AVMEDIA_TYPE_ATTACHMENT))?{??av_log(avctx,?AV_LOG_ERROR,?"codec?type?or?id?mismatches\n");??ret?=?AVERROR(EINVAL);??goto?free_and_end;??}??avctx->frame_number?=?0;??avctx->codec_descriptor?=?avcodec_descriptor_get(avctx->codec_id);??if?(avctx->codec_type?==?AVMEDIA_TYPE_AUDIO?&&??(!avctx->time_base.num?||?!avctx->time_base.den))?{??avctx->time_base.num?=?1;??avctx->time_base.den?=?avctx->sample_rate;??}??if?(!HAVE_THREADS)??av_log(avctx,?AV_LOG_WARNING,?"Warning:?not?compiled?with?thread?support,?using?thread?emulation\n");??if?(HAVE_THREADS)?{??entangled_thread_counter--;?//we?will?instanciate?a?few?encoders?thus?kick?the?counter?to?prevent?false?detection?of?a?problem??ret?=?ff_frame_thread_encoder_init(avctx,?options???*options?:?NULL);??entangled_thread_counter++;??if?(ret?<?0)??goto?free_and_end;??}??if?(HAVE_THREADS?&&?!avctx->thread_opaque??&&?!(avctx->internal->frame_thread_encoder?&&?(avctx->active_thread_type&FF_THREAD_FRAME)))?{??ret?=?ff_thread_init(avctx);??if?(ret?<?0)?{??goto?free_and_end;??}??}??if?(!HAVE_THREADS?&&?!(codec->capabilities?&?CODEC_CAP_AUTO_THREADS))??avctx->thread_count?=?1;??if?(avctx->codec->max_lowres?<?avctx->lowres?||?avctx->lowres?<?0)?{??av_log(avctx,?AV_LOG_ERROR,?"The?maximum?value?for?lowres?supported?by?the?decoder?is?%d\n",??avctx->codec->max_lowres);??ret?=?AVERROR(EINVAL);??goto?free_and_end;??}??if?(av_codec_is_encoder(avctx->codec))?{??int?i;??if?(avctx->codec->sample_fmts)?{??for?(i?=?0;?avctx->codec->sample_fmts[i]?!=?AV_SAMPLE_FMT_NONE;?i++)??if?(avctx->sample_fmt?==?avctx->codec->sample_fmts[i])??break;??if?(avctx->codec->sample_fmts[i]?==?AV_SAMPLE_FMT_NONE)?{??av_log(avctx,?AV_LOG_ERROR,?"Specified?sample_fmt?is?not?supported.\n");??ret?=?AVERROR(EINVAL);??goto?free_and_end;??}??}??if?(avctx->codec->pix_fmts)?{??for?(i?=?0;?avctx->codec->pix_fmts[i]?!=?PIX_FMT_NONE;?i++)??if?(avctx->pix_fmt?==?avctx->codec->pix_fmts[i])??break;??if?(avctx->codec->pix_fmts[i]?==?PIX_FMT_NONE??&&?!((avctx->codec_id?==?AV_CODEC_ID_MJPEG?||?avctx->codec_id?==?AV_CODEC_ID_LJPEG)??&&?avctx->strict_std_compliance?<=?FF_COMPLIANCE_UNOFFICIAL))?{??av_log(avctx,?AV_LOG_ERROR,?"Specified?pix_fmt?is?not?supported\n");??ret?=?AVERROR(EINVAL);??goto?free_and_end;??}??}??if?(avctx->codec->supported_samplerates)?{??for?(i?=?0;?avctx->codec->supported_samplerates[i]?!=?0;?i++)??if?(avctx->sample_rate?==?avctx->codec->supported_samplerates[i])??break;??if?(avctx->codec->supported_samplerates[i]?==?0)?{??av_log(avctx,?AV_LOG_ERROR,?"Specified?sample_rate?is?not?supported\n");??ret?=?AVERROR(EINVAL);??goto?free_and_end;??}??}??if?(avctx->codec->channel_layouts)?{??if?(!avctx->channel_layout)?{??av_log(avctx,?AV_LOG_WARNING,?"channel_layout?not?specified\n");??}?else?{??for?(i?=?0;?avctx->codec->channel_layouts[i]?!=?0;?i++)??if?(avctx->channel_layout?==?avctx->codec->channel_layouts[i])??break;??if?(avctx->codec->channel_layouts[i]?==?0)?{??av_log(avctx,?AV_LOG_ERROR,?"Specified?channel_layout?is?not?supported\n");??ret?=?AVERROR(EINVAL);??goto?free_and_end;??}??}??}??if?(avctx->channel_layout?&&?avctx->channels)?{??if?(av_get_channel_layout_nb_channels(avctx->channel_layout)?!=?avctx->channels)?{??av_log(avctx,?AV_LOG_ERROR,?"channel?layout?does?not?match?number?of?channels\n");??ret?=?AVERROR(EINVAL);??goto?free_and_end;??}??}?else?if?(avctx->channel_layout)?{??avctx->channels?=?av_get_channel_layout_nb_channels(avctx->channel_layout);??}??}??avctx->pts_correction_num_faulty_pts?=??avctx->pts_correction_num_faulty_dts?=?0;??avctx->pts_correction_last_pts?=??avctx->pts_correction_last_dts?=?INT64_MIN;??<span?style="color:#ff6666;">??????//?這里會(huì)調(diào)用編碼器的中指定的初始化函數(shù)init,?對(duì)于x264編碼器，也就是調(diào)用ff_libx264_encoder中指定的X264_init??if(avctx->codec->init?&&?(!(avctx->active_thread_type&FF_THREAD_FRAME)?||?avctx->internal->frame_thread_encoder)){??ret?=?avctx->codec->init(avctx);??if?(ret?<?0)?{??goto?free_and_end;??}??}????ret=0;??if?(av_codec_is_decoder(avctx->codec))?{??if?(!avctx->bit_rate)??avctx->bit_rate?=?get_bit_rate(avctx);??/*?validate?channel?layout?from?the?decoder?*/??if?(avctx->channel_layout?&&??av_get_channel_layout_nb_channels(avctx->channel_layout)?!=?avctx->channels)?{??av_log(avctx,?AV_LOG_WARNING,?"channel?layout?does?not?match?number?of?channels\n");??avctx->channel_layout?=?0;??}??}?? end:??entangled_thread_counter--;??/*?Release?any?user-supplied?mutex.?*/??if?(ff_lockmgr_cb)?{??(*ff_lockmgr_cb)(&codec_mutex,?AV_LOCK_RELEASE);??}??if?(options)?{??av_dict_free(options);??*options?=?tmp;??}??return?ret;?? free_and_end:??av_dict_free(&tmp);??av_freep(&avctx->priv_data);??av_freep(&avctx->internal);??avctx->codec=?NULL;??goto?end;?? }??

看看我們?cè)诖a中標(biāo)注的那幾行代碼：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">if(avctx->codec->init?&&?(!(avctx->active_thread_type&FF_THREAD_FRAME)?||?avctx->internal->frame_thread_encoder)){??ret?=?avctx->codec->init(avctx);??if?(ret?<?0)?{??goto?free_and_end;??}??}??

這里如果codec的init成員指定了對(duì)codec的初始化函數(shù)時(shí)，它會(huì)調(diào)用該初始化函數(shù)，通過前面的分析我們知道，X264編碼器的初始化函數(shù)指定為X264_init，該函數(shù)的參數(shù)即是我們給定的AVCodecContext，下面我們來看看X264_init做了些什么。

X264編碼器的初始化，X264_init

首先列出源代碼：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">static?av_cold?int?X264_init(AVCodecContext?*avctx)?? {??X264Context?*x4?=?avctx->priv_data;??int?sw,sh;??x264_param_default(&x4->params);??x4->params.b_deblocking_filter?????????=?avctx->flags?&?CODEC_FLAG_LOOP_FILTER;??x4->params.rc.f_ip_factor?????????????=?1?/?fabs(avctx->i_quant_factor);??x4->params.rc.f_pb_factor?????????????=?avctx->b_quant_factor;??x4->params.analyse.i_chroma_qp_offset?=?avctx->chromaoffset;??if?(x4->preset?||?x4->tune)??<span?style="color:#ff6666;">??????????//?在這里面會(huì)設(shè)置很多關(guān)鍵的參數(shù)，這個(gè)函數(shù)式X264提供的，接下來我們要到X264中查看其源代碼??if?(x264_param_default_preset(&x4->params,?x4->preset,?x4->tune)?<?0)?{??int?i;??av_log(avctx,?AV_LOG_ERROR,?"Error?setting?preset/tune?%s/%s.\n",?x4->preset,?x4->tune);??av_log(avctx,?AV_LOG_INFO,?"Possible?presets:");??for?(i?=?0;?x264_preset_names[i];?i++)??av_log(avctx,?AV_LOG_INFO,?"?%s",?x264_preset_names[i]);??av_log(avctx,?AV_LOG_INFO,?"\n");??av_log(avctx,?AV_LOG_INFO,?"Possible?tunes:");??for?(i?=?0;?x264_tune_names[i];?i++)??av_log(avctx,?AV_LOG_INFO,?"?%s",?x264_tune_names[i]);??av_log(avctx,?AV_LOG_INFO,?"\n");??return?AVERROR(EINVAL);??}??/??if?(avctx->level?>?0)??x4->params.i_level_idc?=?avctx->level;??x4->params.pf_log???????????????=?X264_log;??x4->params.p_log_private????????=?avctx;??x4->params.i_log_level??????????=?X264_LOG_DEBUG;??x4->params.i_csp????????????????=?convert_pix_fmt(avctx->pix_fmt);??OPT_STR("weightp",?x4->wpredp);??if?(avctx->bit_rate)?{??x4->params.rc.i_bitrate???=?avctx->bit_rate?/?1000;??x4->params.rc.i_rc_method?=?X264_RC_ABR;??}??x4->params.rc.i_vbv_buffer_size?=?avctx->rc_buffer_size?/?1000;??x4->params.rc.i_vbv_max_bitrate?=?avctx->rc_max_rate????/?1000;??x4->params.rc.b_stat_write??????=?avctx->flags?&?CODEC_FLAG_PASS1;??if?(avctx->flags?&?CODEC_FLAG_PASS2)?{??x4->params.rc.b_stat_read?=?1;??}?else?{??if?(x4->crf?>=?0)?{??x4->params.rc.i_rc_method???=?X264_RC_CRF;??x4->params.rc.f_rf_constant?=?x4->crf;??}?else?if?(x4->cqp?>=?0)?{??x4->params.rc.i_rc_method???=?X264_RC_CQP;??x4->params.rc.i_qp_constant?=?x4->cqp;??}??if?(x4->crf_max?>=?0)??x4->params.rc.f_rf_constant_max?=?x4->crf_max;??}??if?(avctx->rc_buffer_size?&&?avctx->rc_initial_buffer_occupancy?&&??(avctx->rc_initial_buffer_occupancy?<=?avctx->rc_buffer_size))?{??x4->params.rc.f_vbv_buffer_init?=??(float)avctx->rc_initial_buffer_occupancy?/?avctx->rc_buffer_size;??}??OPT_STR("level",?x4->level);??if(x4->x264opts){??const?char?*p=?x4->x264opts;??while(p){??char?param[256]={0},?val[256]={0};??if(sscanf(p,?"%255[^:=]=%255[^:]",?param,?val)?==?1){??OPT_STR(param,?"1");??}else??OPT_STR(param,?val);??p=?strchr(p,?':');??p+=!!p;??}??}??if?(avctx->me_method?==?ME_EPZS)??x4->params.analyse.i_me_method?=?X264_ME_DIA;??else?if?(avctx->me_method?==?ME_HEX)??x4->params.analyse.i_me_method?=?X264_ME_HEX;??else?if?(avctx->me_method?==?ME_UMH)??x4->params.analyse.i_me_method?=?X264_ME_UMH;??else?if?(avctx->me_method?==?ME_FULL)??x4->params.analyse.i_me_method?=?X264_ME_ESA;??else?if?(avctx->me_method?==?ME_TESA)??x4->params.analyse.i_me_method?=?X264_ME_TESA;??if?(avctx->gop_size?>=?0)??x4->params.i_keyint_max?????????=?avctx->gop_size;??if?(avctx->max_b_frames?>=?0)??x4->params.i_bframe?????????????=?avctx->max_b_frames;??if?(avctx->scenechange_threshold?>=?0)??x4->params.i_scenecut_threshold?=?avctx->scenechange_threshold;??if?(avctx->qmin?>=?0)??x4->params.rc.i_qp_min??????????=?avctx->qmin;??if?(avctx->qmax?>=?0)??x4->params.rc.i_qp_max??????????=?avctx->qmax;??if?(avctx->max_qdiff?>=?0)??x4->params.rc.i_qp_step?????????=?avctx->max_qdiff;??if?(avctx->qblur?>=?0)??x4->params.rc.f_qblur???????????=?avctx->qblur;?????/*?temporally?blur?quants?*/??if?(avctx->qcompress?>=?0)??x4->params.rc.f_qcompress???????=?avctx->qcompress;?/*?0.0?=>?cbr,?1.0?=>?constant?qp?*/??if?(avctx->refs?>=?0)??x4->params.i_frame_reference????=?avctx->refs;??if?(avctx->trellis?>=?0)??x4->params.analyse.i_trellis????=?avctx->trellis;??if?(avctx->me_range?>=?0)??x4->params.analyse.i_me_range???=?avctx->me_range;??if?(avctx->noise_reduction?>=?0)??x4->params.analyse.i_noise_reduction?=?avctx->noise_reduction;??if?(avctx->me_subpel_quality?>=?0)??x4->params.analyse.i_subpel_refine???=?avctx->me_subpel_quality;??if?(avctx->b_frame_strategy?>=?0)??x4->params.i_bframe_adaptive?=?avctx->b_frame_strategy;??if?(avctx->keyint_min?>=?0)??x4->params.i_keyint_min?=?avctx->keyint_min;??if?(avctx->coder_type?>=?0)??x4->params.b_cabac?=?avctx->coder_type?==?FF_CODER_TYPE_AC;??if?(avctx->me_cmp?>=?0)??x4->params.analyse.b_chroma_me?=?avctx->me_cmp?&?FF_CMP_CHROMA;??if?(x4->aq_mode?>=?0)??x4->params.rc.i_aq_mode?=?x4->aq_mode;??if?(x4->aq_strength?>=?0)??x4->params.rc.f_aq_strength?=?x4->aq_strength;??PARSE_X264_OPT("psy-rd",?psy_rd);??PARSE_X264_OPT("deblock",?deblock);??PARSE_X264_OPT("partitions",?partitions);??PARSE_X264_OPT("stats",?stats);??if?(x4->psy?>=?0)??x4->params.analyse.b_psy??=?x4->psy;??if?(x4->rc_lookahead?>=?0)??x4->params.rc.i_lookahead?=?x4->rc_lookahead;??if?(x4->weightp?>=?0)??x4->params.analyse.i_weighted_pred?=?x4->weightp;??if?(x4->weightb?>=?0)??x4->params.analyse.b_weighted_bipred?=?x4->weightb;??if?(x4->cplxblur?>=?0)??x4->params.rc.f_complexity_blur?=?x4->cplxblur;??if?(x4->ssim?>=?0)??x4->params.analyse.b_ssim?=?x4->ssim;??if?(x4->intra_refresh?>=?0)??x4->params.b_intra_refresh?=?x4->intra_refresh;??if?(x4->b_bias?!=?INT_MIN)??x4->params.i_bframe_bias??????????????=?x4->b_bias;??if?(x4->b_pyramid?>=?0)??x4->params.i_bframe_pyramid?=?x4->b_pyramid;??if?(x4->mixed_refs?>=?0)??x4->params.analyse.b_mixed_references?=?x4->mixed_refs;??if?(x4->dct8x8?>=?0)??x4->params.analyse.b_transform_8x8????=?x4->dct8x8;??if?(x4->fast_pskip?>=?0)??x4->params.analyse.b_fast_pskip???????=?x4->fast_pskip;??if?(x4->aud?>=?0)??x4->params.b_aud??????????????????????=?x4->aud;??if?(x4->mbtree?>=?0)??x4->params.rc.b_mb_tree???????????????=?x4->mbtree;??if?(x4->direct_pred?>=?0)??x4->params.analyse.i_direct_mv_pred???=?x4->direct_pred;??if?(x4->slice_max_size?>=?0)??x4->params.i_slice_max_size?=??x4->slice_max_size;??if?(x4->fastfirstpass)??x264_param_apply_fastfirstpass(&x4->params);??if?(x4->profile)??if?(x264_param_apply_profile(&x4->params,?x4->profile)?<?0)?{??int?i;??av_log(avctx,?AV_LOG_ERROR,?"Error?setting?profile?%s.\n",?x4->profile);??av_log(avctx,?AV_LOG_INFO,?"Possible?profiles:");??for?(i?=?0;?x264_profile_names[i];?i++)??av_log(avctx,?AV_LOG_INFO,?"?%s",?x264_profile_names[i]);??av_log(avctx,?AV_LOG_INFO,?"\n");??return?AVERROR(EINVAL);??}??x4->params.i_width??????????=?avctx->width;??x4->params.i_height?????????=?avctx->height;??av_reduce(&sw,?&sh,?avctx->sample_aspect_ratio.num,?avctx->sample_aspect_ratio.den,?4096);??x4->params.vui.i_sar_width??=?sw;??x4->params.vui.i_sar_height?=?sh;??x4->params.i_fps_num?=?x4->params.i_timebase_den?=?avctx->time_base.den;??x4->params.i_fps_den?=?x4->params.i_timebase_num?=?avctx->time_base.num;??x4->params.analyse.b_psnr?=?avctx->flags?&?CODEC_FLAG_PSNR;??x4->params.i_threads??????=?avctx->thread_count;??if?(avctx->thread_type)??x4->params.b_sliced_threads?=?avctx->thread_type?==?FF_THREAD_SLICE;??x4->params.b_interlaced???=?avctx->flags?&?CODEC_FLAG_INTERLACED_DCT;??//????x4->params.b_open_gop?????=?!(avctx->flags?&?CODEC_FLAG_CLOSED_GOP);??x4->params.i_slice_count??=?avctx->slices;??x4->params.vui.b_fullrange?=?avctx->pix_fmt?==?PIX_FMT_YUVJ420P;??if?(avctx->flags?&?CODEC_FLAG_GLOBAL_HEADER)??x4->params.b_repeat_headers?=?0;??//?update?AVCodecContext?with?x264?parameters??avctx->has_b_frames?=?x4->params.i_bframe????x4->params.i_bframe_pyramid???2?:?1?:?0;??if?(avctx->max_b_frames?<?0)??avctx->max_b_frames?=?0;??avctx->bit_rate?=?x4->params.rc.i_bitrate*1000;??x4->enc?=?x264_encoder_open(&x4->params);??if?(!x4->enc)??return?-1;??avctx->coded_frame?=?&x4->out_pic;??if?(avctx->flags?&?CODEC_FLAG_GLOBAL_HEADER)?{??x264_nal_t?*nal;??uint8_t?*p;??int?nnal,?s,?i;??s?=?x264_encoder_headers(x4->enc,?&nal,?&nnal);??avctx->extradata?=?p?=?av_malloc(s);??for?(i?=?0;?i?<?nnal;?i++)?{??/*?Don't?put?the?SEI?in?extradata.?*/??if?(nal[i].i_type?==?NAL_SEI)?{??av_log(avctx,?AV_LOG_INFO,?"%s\n",?nal[i].p_payload+25);??x4->sei_size?=?nal[i].i_payload;??x4->sei??????=?av_malloc(x4->sei_size);??memcpy(x4->sei,?nal[i].p_payload,?nal[i].i_payload);??continue;??}??memcpy(p,?nal[i].p_payload,?nal[i].i_payload);??p?+=?nal[i].i_payload;??}??avctx->extradata_size?=?p?-?avctx->extradata;??}??return?0;?? }?? ??

看看我們做出標(biāo)記的那幾行代碼，這里它調(diào)用了x264_param_default_preset(&x4->params, x4->preset, x4->tune)，所以我們接下來當(dāng)然是看看這個(gè)函數(shù)了。

對(duì)編碼器參數(shù)的設(shè)置，x264_param_default_preset

? ? 這個(gè)函數(shù)的定義并不在ffmpeg中，因?yàn)檫@是X264提供給外界對(duì)編碼器做設(shè)置API函數(shù)，于是我們?cè)赬264項(xiàng)目中查找該函數(shù)，它定義在Common.c中，代碼如下：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">int?x264_param_default_preset(?x264_param_t?*param,?const?char?*preset,?const?char?*tune?)?? {??x264_param_default(?param?);??if(?preset?&&?<span?style="color:#ff6666;">x264_param_apply_preset(?param,?preset?)?<?0?)??return?-1;??if(?tune?&&?<span?style="color:#ff6666;">x264_param_apply_tune(?param,?tune?)?<?0?)??return?-1;??return?0;?? }??

它首先調(diào)用下x264_param_default設(shè)置默認(rèn)參數(shù)，這在用戶沒有指定額外設(shè)置時(shí)，設(shè)置就是使用該函數(shù)默認(rèn)參數(shù)，但如果用戶指定了preset和(或者)tune參數(shù)時(shí)，它就會(huì)進(jìn)行額外參數(shù)的設(shè)置。

? ? 首先看一下應(yīng)用模式的設(shè)置：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">static?int?x264_param_apply_preset(?x264_param_t?*param,?const?char?*preset?)?? {??char?*end;??int?i?=?strtol(?preset,?&end,?10?);??if(?*end?==?0?&&?i?>=?0?&&?i?<?sizeof(x264_preset_names)/sizeof(*x264_preset_names)-1?)??preset?=?x264_preset_names[i];??if(?!strcasecmp(?preset,?"ultrafast"?)?)??{??param->i_frame_reference?=?1;??param->i_scenecut_threshold?=?0;??param->b_deblocking_filter?=?0;??param->b_cabac?=?0;??param->i_bframe?=?0;??param->analyse.intra?=?0;??param->analyse.inter?=?0;??param->analyse.b_transform_8x8?=?0;??param->analyse.i_me_method?=?X264_ME_DIA;??param->analyse.i_subpel_refine?=?0;??param->rc.i_aq_mode?=?0;??param->analyse.b_mixed_references?=?0;??param->analyse.i_trellis?=?0;??param->i_bframe_adaptive?=?X264_B_ADAPT_NONE;??param->rc.b_mb_tree?=?0;??param->analyse.i_weighted_pred?=?X264_WEIGHTP_NONE;??param->analyse.b_weighted_bipred?=?0;??param->rc.i_lookahead?=?0;??}??else?if(?!strcasecmp(?preset,?"superfast"?)?)??{??param->analyse.inter?=?X264_ANALYSE_I8x8|X264_ANALYSE_I4x4;??param->analyse.i_me_method?=?X264_ME_DIA;??param->analyse.i_subpel_refine?=?1;??param->i_frame_reference?=?1;??param->analyse.b_mixed_references?=?0;??param->analyse.i_trellis?=?0;??param->rc.b_mb_tree?=?0;??param->analyse.i_weighted_pred?=?X264_WEIGHTP_SIMPLE;??param->rc.i_lookahead?=?0;??}??else?if(?!strcasecmp(?preset,?"veryfast"?)?)??{??param->analyse.i_me_method?=?X264_ME_HEX;??param->analyse.i_subpel_refine?=?2;??param->i_frame_reference?=?1;??param->analyse.b_mixed_references?=?0;??param->analyse.i_trellis?=?0;??param->analyse.i_weighted_pred?=?X264_WEIGHTP_SIMPLE;??param->rc.i_lookahead?=?10;??}??else?if(?!strcasecmp(?preset,?"faster"?)?)??{??param->analyse.b_mixed_references?=?0;??param->i_frame_reference?=?2;??param->analyse.i_subpel_refine?=?4;??param->analyse.i_weighted_pred?=?X264_WEIGHTP_SIMPLE;??param->rc.i_lookahead?=?20;??}??else?if(?!strcasecmp(?preset,?"fast"?)?)??{??param->i_frame_reference?=?2;??param->analyse.i_subpel_refine?=?6;??param->analyse.i_weighted_pred?=?X264_WEIGHTP_SIMPLE;??param->rc.i_lookahead?=?30;??}??else?if(?!strcasecmp(?preset,?"medium"?)?)??{??/*?Default?is?medium?*/??}??else?if(?!strcasecmp(?preset,?"slow"?)?)??{??param->analyse.i_me_method?=?X264_ME_UMH;??param->analyse.i_subpel_refine?=?8;??param->i_frame_reference?=?5;??param->i_bframe_adaptive?=?X264_B_ADAPT_TRELLIS;??param->analyse.i_direct_mv_pred?=?X264_DIRECT_PRED_AUTO;??param->rc.i_lookahead?=?50;??}??else?if(?!strcasecmp(?preset,?"slower"?)?)??{??param->analyse.i_me_method?=?X264_ME_UMH;??param->analyse.i_subpel_refine?=?9;??param->i_frame_reference?=?8;??param->i_bframe_adaptive?=?X264_B_ADAPT_TRELLIS;??param->analyse.i_direct_mv_pred?=?X264_DIRECT_PRED_AUTO;??param->analyse.inter?|=?X264_ANALYSE_PSUB8x8;??param->analyse.i_trellis?=?2;??param->rc.i_lookahead?=?60;??}??else?if(?!strcasecmp(?preset,?"veryslow"?)?)??{??param->analyse.i_me_method?=?X264_ME_UMH;??param->analyse.i_subpel_refine?=?10;??param->analyse.i_me_range?=?24;??param->i_frame_reference?=?16;??param->i_bframe_adaptive?=?X264_B_ADAPT_TRELLIS;??param->analyse.i_direct_mv_pred?=?X264_DIRECT_PRED_AUTO;??param->analyse.inter?|=?X264_ANALYSE_PSUB8x8;??param->analyse.i_trellis?=?2;??param->i_bframe?=?8;??param->rc.i_lookahead?=?60;??}??else?if(?!strcasecmp(?preset,?"placebo"?)?)??{??param->analyse.i_me_method?=?X264_ME_TESA;??param->analyse.i_subpel_refine?=?11;??param->analyse.i_me_range?=?24;??param->i_frame_reference?=?16;??param->i_bframe_adaptive?=?X264_B_ADAPT_TRELLIS;??param->analyse.i_direct_mv_pred?=?X264_DIRECT_PRED_AUTO;??param->analyse.inter?|=?X264_ANALYSE_PSUB8x8;??param->analyse.b_fast_pskip?=?0;??param->analyse.i_trellis?=?2;??param->i_bframe?=?16;??param->rc.i_lookahead?=?60;??}??else??{??x264_log(?NULL,?X264_LOG_ERROR,?"invalid?preset?'%s'\n",?preset?);??return?-1;??}??return?0;?? }??

這里定義了幾種模式供用戶選擇，經(jīng)過測(cè)試，這些模式是會(huì)影響到編碼的延遲時(shí)間的，越快的模式，其延遲越小，對(duì)于"ultralfast"模式，我們發(fā)現(xiàn)延遲幀減少了許多，同時(shí)發(fā)現(xiàn)越快的模式相對(duì)于其他模式會(huì)有些花屏，但此時(shí)我發(fā)現(xiàn)所有模式都沒有使得延遲為0的情況（此時(shí)我是直接修改源代碼來固定設(shè)置為特定模式的，后面我們會(huì)講到如何通過ffmpeg中的API來設(shè)置），于是我將希望寄托于下面的x264_param_apply_tune，我感覺這可能是我最后的救命稻草了！下面我們來看一下這個(gè)函數(shù)的源代碼：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">static?int?x264_param_apply_tune(?x264_param_t?*param,?const?char?*tune?)?? {??char?*tmp?=?x264_malloc(?strlen(?tune?)?+?1?);??if(?!tmp?)??return?-1;??tmp?=?strcpy(?tmp,?tune?);??char?*s?=?strtok(?tmp,?",./-+"?);??int?psy_tuning_used?=?0;??while(?s?)??{??if(?!strncasecmp(?s,?"film",?4?)?)??{??if(?psy_tuning_used++?)?goto?psy_failure;??param->i_deblocking_filter_alphac0?=?-1;??param->i_deblocking_filter_beta?=?-1;??param->analyse.f_psy_trellis?=?0.15;??}??else?if(?!strncasecmp(?s,?"animation",?9?)?)??{??if(?psy_tuning_used++?)?goto?psy_failure;??param->i_frame_reference?=?param->i_frame_reference?>?1???param->i_frame_reference*2?:?1;??param->i_deblocking_filter_alphac0?=?1;??param->i_deblocking_filter_beta?=?1;??param->analyse.f_psy_rd?=?0.4;??param->rc.f_aq_strength?=?0.6;??param->i_bframe?+=?2;??}??else?if(?!strncasecmp(?s,?"grain",?5?)?)??{??if(?psy_tuning_used++?)?goto?psy_failure;??param->i_deblocking_filter_alphac0?=?-2;??param->i_deblocking_filter_beta?=?-2;??param->analyse.f_psy_trellis?=?0.25;??param->analyse.b_dct_decimate?=?0;??param->rc.f_pb_factor?=?1.1;??param->rc.f_ip_factor?=?1.1;??param->rc.f_aq_strength?=?0.5;??param->analyse.i_luma_deadzone[0]?=?6;??param->analyse.i_luma_deadzone[1]?=?6;??param->rc.f_qcompress?=?0.8;??}??else?if(?!strncasecmp(?s,?"stillimage",?5?)?)??{??if(?psy_tuning_used++?)?goto?psy_failure;??param->i_deblocking_filter_alphac0?=?-3;??param->i_deblocking_filter_beta?=?-3;??param->analyse.f_psy_rd?=?2.0;??param->analyse.f_psy_trellis?=?0.7;??param->rc.f_aq_strength?=?1.2;??}??else?if(?!strncasecmp(?s,?"psnr",?4?)?)??{??if(?psy_tuning_used++?)?goto?psy_failure;??param->rc.i_aq_mode?=?X264_AQ_NONE;??param->analyse.b_psy?=?0;??}??else?if(?!strncasecmp(?s,?"ssim",?4?)?)??{??if(?psy_tuning_used++?)?goto?psy_failure;??param->rc.i_aq_mode?=?X264_AQ_AUTOVARIANCE;??param->analyse.b_psy?=?0;??}??else?if(?!strncasecmp(?s,?"fastdecode",?10?)?)??{??param->b_deblocking_filter?=?0;??param->b_cabac?=?0;??param->analyse.b_weighted_bipred?=?0;??param->analyse.i_weighted_pred?=?X264_WEIGHTP_NONE;??}??<span?style="color:#ff6666;">else?if(?!strncasecmp(?s,?"zerolatency",?11?)?)??{??param->rc.i_lookahead?=?0;??param->i_sync_lookahead?=?0;??param->i_bframe?=?0;??param->b_sliced_threads?=?1;??param->b_vfr_input?=?0;??param->rc.b_mb_tree?=?0;??}??else?if(?!strncasecmp(?s,?"touhou",?6?)?)??{??if(?psy_tuning_used++?)?goto?psy_failure;??param->i_frame_reference?=?param->i_frame_reference?>?1???param->i_frame_reference*2?:?1;??param->i_deblocking_filter_alphac0?=?-1;??param->i_deblocking_filter_beta?=?-1;??param->analyse.f_psy_trellis?=?0.2;??param->rc.f_aq_strength?=?1.3;??if(?param->analyse.inter?&?X264_ANALYSE_PSUB16x16?)??param->analyse.inter?|=?X264_ANALYSE_PSUB8x8;??}??else??{??x264_log(?NULL,?X264_LOG_ERROR,?"invalid?tune?'%s'\n",?s?);??x264_free(?tmp?);??return?-1;??}??if(?0?)??{??psy_failure:??x264_log(?NULL,?X264_LOG_WARNING,?"only?1?psy?tuning?can?be?used:?ignoring?tune?%s\n",?s?);??}??s?=?strtok(?NULL,?",./-+"?);??}??x264_free(?tmp?);??return?0;?? }??

我們?cè)诖a中也看到了有幾種模式供選擇，每種模式都是對(duì)一些參數(shù)的具體設(shè)置，當(dāng)然這些參數(shù)的意義我也不是很清楚，有待后面繼續(xù)的研究，但我卻驚喜地發(fā)現(xiàn)了一個(gè)“zerolatency”模式，這不就是我要找的實(shí)時(shí)編碼模式嗎，至少從字面上來講是！于是修改源代碼寫死為“zerolatency”模式，編譯、運(yùn)行，我的天哪，終于找到了！

? ? 另外，我了解到，其實(shí)在工程編譯出的可執(zhí)行文件運(yùn)行時(shí)也是可以指定這些運(yùn)行參數(shù)的，這更加證實(shí)了我的想法。于是我得出了一個(gè)結(jié)論：

x264_param_apply_preset和x264_param_apply_tune的參數(shù)決定了編碼器的全部運(yùn)作方式（當(dāng)然包括是否編碼延遲，以及延遲多長）！

如何不修改ffmpeg或者x264工程源代碼來達(dá)到實(shí)時(shí)編碼

? ? 知道了影響編碼延遲的原因后，我們又要上溯到ffmpeg中的X264_init代碼中去了，看看該函數(shù)是如何指定x264_param_default_preset函數(shù)的參數(shù)的，為了便于講解，我們?cè)俅瘟谐霾糠执a：

? ? 這里調(diào)用x264_param_default_preset(&x4->params, x4->preset, x4->tune) ，而x4變量的類型是X264Context ，這個(gè)結(jié)構(gòu)體中的參數(shù)是最終要傳給X264來設(shè)置編碼器參數(shù)的，我們還可以從X264Context *x4 = avctx->priv_data;中看到，x4變量其實(shí)是有AVCodecContext中的priv_data成員指定的，在AVCodecContext中priv_data是void*類型，而AVCodecContext正是我們傳進(jìn)來的，也就是說，我們現(xiàn)在終于可以想辦法控制這些參數(shù)了----這要把這些參數(shù)指定給priv_data成員即可了。

? ? 現(xiàn)在我們還是先看看X264Context 中那些成員指定了控制得到實(shí)時(shí)編碼的的參數(shù)：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">typedef?struct?X264Context?{??AVClass????????*class;??x264_param_t????params;??x264_t?????????*enc;??x264_picture_t??pic;??uint8_t????????*sei;??int?????????????sei_size;??AVFrame?????????out_pic;??char?*preset;??char?*tune;??char?*profile;??char?*level;??int?fastfirstpass;??char?*wpredp;??char?*x264opts;??float?crf;??float?crf_max;??int?cqp;??int?aq_mode;??float?aq_strength;??char?*psy_rd;??int?psy;??int?rc_lookahead;??int?weightp;??int?weightb;??int?ssim;??int?intra_refresh;??int?b_bias;??int?b_pyramid;??int?mixed_refs;??int?dct8x8;??int?fast_pskip;??int?aud;??int?mbtree;??char?*deblock;??float?cplxblur;??char?*partitions;??int?direct_pred;??int?slice_max_size;??char?*stats;?? }?X264Context;??

? ? 出于本能，我第一時(shí)間發(fā)現(xiàn)了兩個(gè)我最關(guān)心的兩個(gè)參數(shù)：preset和tune，這正是(x264_param_default_preset要用到的兩個(gè)參數(shù)。

? ? 至此，我認(rèn)為已經(jīng)想到了解決編碼延遲的解決方案了（離完美還差那么一步），于是我立馬在將測(cè)試代碼中做出如下的修改：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">???......????c?=?avcodec_alloc_context3(codec);??/*?put?sample?parameters?*/??c->bit_rate?=?400000;??/*?resolution?must?be?a?multiple?of?two?*/??c->width?=?800/*352*/;??c->height?=?500/*288*/;??/*?frames?per?second?*/??c->time_base.den?=?1;??c->time_base.num?=?25;??c->gop_size?=?10;?/*?emit?one?intra?frame?every?ten?frames?*/??c->max_b_frames=1;??c->pix_fmt?=?PIX_FMT_YUV420P;??<span?style="color:#ff6666;">//?新增語句，設(shè)置為編碼延遲??if?(c->priv_data)?{??((X264Context*)(c->priv_data))->preset?=?"superfast";??((X264Context*)(c->priv_data))->tune?=?"zerolatency";??}??......??......??

? ?編譯......error！原來編譯器不認(rèn)識(shí)X264Context，一定是忘了包含頭文件，查看源代碼，發(fā)現(xiàn)這是一個(gè)對(duì)外不公開的結(jié)構(gòu)體，我無法通過包含頭文件來包含該結(jié)構(gòu)體，于是抱怨ffmpeg怎么搞的！也許是給予解決問題，同時(shí)驗(yàn)證之前的的理解正確與否，我采用了最粗暴的方法，直接將該結(jié)構(gòu)體復(fù)制到我的文件中，當(dāng)然這個(gè)結(jié)構(gòu)體中有一個(gè)名為class的成員需要更改一下名字，因?yàn)槲业捻?xiàng)目是C++開發(fā)的，這個(gè)而class是C++的關(guān)鍵字，同時(shí)也要將x264.h和x264_config.h頭文件復(fù)制到你的工程中，因?yàn)閄264Context中的幾個(gè)成員類型如x264_param_t、x264_t等是在x264.h中定義的，而x264.h又包含x264_config.h，好在x264_config.h沒有在繼續(xù)包含別的文件了（這也再次證明了我們?cè)陂_發(fā)的一條規(guī)范的好處：盡量在頭文件中不再包含其他頭文件，而是盡量使用向前聲明，這樣方便代碼的移植）。折騰一番之后，編譯代碼，終于順利通過了，此時(shí)，正如我的想象一樣，編碼果然沒有任何延遲了！（在我的工程代碼中卻是沒有哪怕一幀的延遲，但在這個(gè)測(cè)試代碼中卻存在一幀的延遲，當(dāng)然一幀的延遲幾乎沒有任何影響），見下圖運(yùn)行效果：

我的目的終于達(dá)到了，同時(shí)驗(yàn)證我的理解也是正確的，一時(shí)間海闊天空！

但歡呼勝利之后，我卻看著自己定義的那個(gè)X264Context非常別扭，于是我想，ffmpeg不會(huì)肯定提供了其他的途徑來設(shè)置我們想要的這些參數(shù)，而不至于用戶自己手工去配置priv_data，要知道這是一個(gè)void*指針！而且ffmpeg并沒有將X264Context開放給外部使用者，這讓我更加懷疑我的設(shè)置方式是否合理？是否存在接口讓我方便地設(shè)置priv_data?

讓我歡喜的av_opt_set

? ? 帶著對(duì)自己的懷疑，我繼續(xù)查找資料，看源代碼......終于我在一個(gè)官方的例子代碼中發(fā)現(xiàn)了新大陸，在decoding_encdoing.cpp的視頻編碼例子（我的測(cè)試?yán)诱菑脑摾游募刑崛〕鰜淼?#xff0c;見該文件中的video_encode_example函數(shù)）中，我發(fā)現(xiàn)了下面一條語句（其實(shí)以前看例子就看到過這條語句，但當(dāng)時(shí)由于沒有細(xì)細(xì)研究，也沒有管它的用意何在）：

[cpp]?view plaincopy

if(codec_id?==?AV_CODEC_ID_H264)???

????????av_opt_set(c->priv_data,?"preset",?"slow",?0);??

于是趕緊查查看源代碼：

[cpp]?view plaincopyint?av_opt_set(void?*obj,?const?char?*name,?const?char?*val,?int?search_flags)?? {??int?ret;??void?*dst,?*target_obj;??const?AVOption?*o?=?av_opt_find2(obj,?name,?NULL,?0,?search_flags,?&target_obj);??if?(!o?||?!target_obj)??return?AVERROR_OPTION_NOT_FOUND;??if?(!val?&&?(o->type?!=?AV_OPT_TYPE_STRING?&&?o->type?!=?AV_OPT_TYPE_PIXEL_FMT?&&?o->type?!=?AV_OPT_TYPE_IMAGE_SIZE))??return?AVERROR(EINVAL);??dst?=?((uint8_t*)target_obj)?+?o->offset;??switch?(o->type)?{??case?AV_OPT_TYPE_STRING:???return?set_string(obj,?o,?val,?dst);??case?AV_OPT_TYPE_BINARY:???return?set_string_binary(obj,?o,?val,?dst);??case?AV_OPT_TYPE_FLAGS:??case?AV_OPT_TYPE_INT:??case?AV_OPT_TYPE_INT64:??case?AV_OPT_TYPE_FLOAT:??case?AV_OPT_TYPE_DOUBLE:??case?AV_OPT_TYPE_RATIONAL:?return?set_string_number(obj,?o,?val,?dst);??case?AV_OPT_TYPE_IMAGE_SIZE:??if?(!val?||?!strcmp(val,?"none"))?{??*(int?*)dst?=?*((int?*)dst?+?1)?=?0;??return?0;??}??ret?=?av_parse_video_size(dst,?((int?*)dst)?+?1,?val);??if?(ret?<?0)??av_log(obj,?AV_LOG_ERROR,?"Unable?to?parse?option?value?\"%s\"?as?image?size\n",?val);??return?ret;??case?AV_OPT_TYPE_PIXEL_FMT:??if?(!val?||?!strcmp(val,?"none"))??ret?=?PIX_FMT_NONE;??else?{??ret?=?av_get_pix_fmt(val);??if?(ret?==?PIX_FMT_NONE)?{??char?*tail;??ret?=?strtol(val,?&tail,?0);??if?(*tail?||?(unsigned)ret?>=?PIX_FMT_NB)?{??av_log(obj,?AV_LOG_ERROR,?"Unable?to?parse?option?value?\"%s\"?as?pixel?format\n",?val);??return?AVERROR(EINVAL);??}??}??}??*(enum?PixelFormat?*)dst?=?ret;??return?0;??}??av_log(obj,?AV_LOG_ERROR,?"Invalid?option?type.\n");??return?AVERROR(EINVAL);?? }??

果然不出我所料，就是它了！

于是迫不及待地重新修改我的測(cè)試代碼，將原先的修改全刪掉，修改為如下：

[cpp]?view plaincopy<span?style="font-family:Courier?New;">????......????c?=?avcodec_alloc_context3(codec);??/*?put?sample?parameters?*/??c->bit_rate?=?400000;??/*?resolution?must?be?a?multiple?of?two?*/??c->width?=?800/*352*/;??c->height?=?500/*288*/;??/*?frames?per?second?*/??c->time_base.den?=?1;??c->time_base.num?=?25;??c->gop_size?=?10;?/*?emit?one?intra?frame?every?ten?frames?*/??c->max_b_frames=1;??c->pix_fmt?=?PIX_FMT_YUV420P;??<span?style="color:#ff6666;">????//?新增語句，設(shè)置為編碼延遲??av_opt_set(c->priv_data,?"preset",?"superfast",?0);??//?實(shí)時(shí)編碼關(guān)鍵看這句，上面那條無所謂??av_opt_set(c->priv_data,?"tune",?"zerolatency",?0);??......??......??

編譯運(yùn)行.......果然OK，見下圖測(cè)試結(jié)果（在我的項(xiàng)目中沒有延遲哪怕一幀，但這個(gè)例子代碼中有一幀延遲，但一幀無傷大雅）：

困擾我兩天的問題終于圓滿解決了！萬歲！

總結(jié)

? ? 我在想，如果我之前在網(wǎng)上或者別的什么地方看到了用av_op_set這樣一條簡單的語句就解決了我的問題，那么我節(jié)省了兩天時(shí)間，但僅此而已，我也僅僅是知其然而不知其所以然。但在我在網(wǎng)上沒有尋找到滿意的答案之后，我決定自己閱讀源代碼，刨根問底，我花費(fèi)了兩天時(shí)間，但是換回的不僅僅是知道要這么做，也知道了為什么這樣做可以有效果！

? ? 所以，開源項(xiàng)目對(duì)已我們每個(gè)IT從業(yè)人員都是一筆寶貴的財(cái)富，開源項(xiàng)目源代碼的閱讀是提高我們軟件開發(fā)能力的一條最佳途徑之一。

? ? 這篇文章記錄了我閱讀ffmpeg源代碼解決問題的各個(gè)環(huán)節(jié)，以備后用，也希望我的這些文字可以給遇到同樣問題的人提供一點(diǎn)點(diǎn)幫助。

? ? 最后列出一些我在解決這個(gè)問題中參看的一些網(wǎng)頁：

? ??http://bbs.csdn.net/topics/370233998

? ??http://x264-settings.wikispaces.com/x264_Encoding_Suggestions

總結(jié)

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