最简单的基于FFMPEG的转码程序

本文介绍一个简单的基于FFmpeg的转码器。它可以将一种视频格式（包括封转格式和编码格式）转换为另一种视频格式。转码器在视音频编解码处理的程序中，属于一个比较复杂的东西。因为它结合了视频的解码和编码。一个视频播放器，一般只包含解码功能；一个视频编码工具，一般只包含编码功能；而一个视频转码器，则需要先对视频进行解码，然后再对视频进行编码，因而相当于解码器和编码器的结合。下图例举了一个视频的转码流程。输入视频的封装格式是FLV，视频编码标准是H.264，音频编码标准是AAC；输出视频的封装格式是AVI，视频编码标准是MPEG2，音频编码标准是MP3。从流程中可以看出，首先从输入视频中分离出视频码流和音频压缩码流，然后分别将视频码流和音频码流进行解码，获取到非压缩的像素数据/音频采样数据，接着将非压缩的像素数据/音频采样数据重新进行编码，获得重新编码后的视频码流和音频码流，最后将视频码流和音频码流重新封装成一个文件。

本文介绍的视频转码器正是使用FFMPEG类库从编程的角度实现了上述流程。该例子是从FFmpeg的例子改编的，平台是VC2010，类库版本是2014.5.6。

流程图（2014.9.29更新）

下面附两张使用FFmpeg转码视频的流程图。图中使用浅绿色标出了视频的编码、解码函数。从代码中可以看出，使用了AVFilter的不少东西，因此建议先学习AVFilter的内容后再看这个转码器的源代码。

PS：实际上，转码器不是一定依赖AVFilter的。因此打算有时间对这个转码器进行进一步的简化，使学习的人无需AVFilter的基础也可以理解转码器。

简单介绍一下流程中各个函数的意义：
open_input_file()：打开输入文件，并初始化相关的结构体。
open_output_file()：打开输出文件，并初始化相关的结构体。
init_filters()：初始化AVFilter相关的结构体。
av_read_frame()：从输入文件中读取一个AVPacket。
avcodec_decode_video2()：解码一个视频AVPacket（存储H.264等压缩码流数据）为AVFrame（存储YUV等非压缩的像素数据）。
avcodec_decode_video4()：解码一个音频AVPacket（存储MP3等压缩码流数据）为AVFrame（存储PCM采样数据）。
filter_encode_write_frame()：编码一个AVFrame。
flush_encoder()：输入文件读取完毕后，输出编码器中剩余的AVPacket。

以上函数中open_input_file()，open_output_file()，init_filters()中的函数在其他文章中都有所叙述，在这里不再重复：

open_input_file()可参考：100行代码实现最简单的基于FFMPEG+SDL的视频播放器（SDL1.x）

open_output_file()可参考：最简单的基于FFMPEG的视频编码器（YUV编码为H.264）

init_filters()可参考：最简单的基于FFmpeg的AVfilter例子（水印叠加）

在这里介绍一下其中编码的函数filter_encode_write_frame()。filter_encode_write_frame()函数的流程如下图所示，它完成了视频/音频的编码功能。

PS：视频和音频的编码流程中除了编码函数avcodec_encode_video2()和avcodec_encode_audio2()不一样之外，其他部分几乎完全一样。

简单介绍一下filter_encode_write_frame()中各个函数的意义：

av_buffersrc_add_frame()：将解码后的AVFrame加入Filtergraph。

av_buffersink_get_buffer_ref()：从Filtergraph中取一个AVFrame。

avcodec_encode_video2()：编码一个视频AVFrame为AVPacket。

avcodec_encode_audio2()：编码一个音频AVFrame为AVPacket。

av_interleaved_write_frame()：将编码后的AVPacket写入文件。

代码

贴上代码

/**最简单的基于FFmpeg的转码器*Simplest FFmpeg Transcoder**雷霄骅 Lei Xiaohua*leixiaohua1020@126.com*中国传媒大学/数字电视技术*Communication University of China / DigitalTV Technology*http://blog.csdn.net/leixiaohua1020**本程序实现了视频格式之间的转换。是一个最简单的视频转码程序。**/#include "stdafx.h"
extern "C"
{
#include "libavcodec/avcodec.h"
#include "libavformat/avformat.h"
#include "libavfilter/avfiltergraph.h"
#include "libavfilter/avcodec.h"
#include "libavfilter/buffersink.h"
#include "libavfilter/buffersrc.h"
#include "libavutil/avutil.h"
#include "libavutil/opt.h"
#include "libavutil/pixdesc.h"
};static AVFormatContext *ifmt_ctx;
static AVFormatContext *ofmt_ctx;
typedef struct FilteringContext{AVFilterContext*buffersink_ctx;AVFilterContext*buffersrc_ctx;AVFilterGraph*filter_graph;
} FilteringContext;
static FilteringContext *filter_ctx;
static int open_input_file(const char *filename)
{int ret;unsigned int i;ifmt_ctx =NULL;if ((ret = avformat_open_input(&ifmt_ctx,filename, NULL, NULL)) < 0) {av_log(NULL, AV_LOG_ERROR, "Cannot openinput file\n");return ret;}if ((ret = avformat_find_stream_info(ifmt_ctx, NULL))< 0) {av_log(NULL, AV_LOG_ERROR, "Cannot findstream information\n");return ret;}for (i = 0; i < ifmt_ctx->nb_streams; i++) {AVStream*stream;AVCodecContext *codec_ctx;stream =ifmt_ctx->streams[i];codec_ctx =stream->codec;/* Reencode video & audio and remux subtitles etc. */if (codec_ctx->codec_type == AVMEDIA_TYPE_VIDEO||codec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {/* Open decoder */ret =avcodec_open2(codec_ctx,avcodec_find_decoder(codec_ctx->codec_id), NULL);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Failed toopen decoder for stream #%u\n", i);return ret;}}}av_dump_format(ifmt_ctx, 0, filename, 0);return 0;
}
static int open_output_file(const char *filename)
{AVStream*out_stream;AVStream*in_stream;AVCodecContext*dec_ctx, *enc_ctx;AVCodec*encoder;int ret;unsigned int i;ofmt_ctx =NULL;avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, filename);if (!ofmt_ctx) {av_log(NULL, AV_LOG_ERROR, "Could notcreate output context\n");return AVERROR_UNKNOWN;}for (i = 0; i < ifmt_ctx->nb_streams; i++) {out_stream= avformat_new_stream(ofmt_ctx, NULL);if (!out_stream) {av_log(NULL, AV_LOG_ERROR, "Failedallocating output stream\n");return AVERROR_UNKNOWN;}in_stream =ifmt_ctx->streams[i];dec_ctx =in_stream->codec;enc_ctx =out_stream->codec;if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO||dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {/* in this example, we choose transcoding to same codec */encoder= avcodec_find_encoder(dec_ctx->codec_id);/* In this example, we transcode to same properties(picture size,* sample rate etc.). These properties can be changed for output* streams easily using filters */if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {enc_ctx->height = dec_ctx->height;enc_ctx->width = dec_ctx->width;enc_ctx->sample_aspect_ratio = dec_ctx->sample_aspect_ratio;/* take first format from list of supported formats */enc_ctx->pix_fmt = encoder->pix_fmts[0];/* video time_base can be set to whatever is handy andsupported by encoder */enc_ctx->time_base = dec_ctx->time_base;} else {enc_ctx->sample_rate = dec_ctx->sample_rate;enc_ctx->channel_layout = dec_ctx->channel_layout;enc_ctx->channels = av_get_channel_layout_nb_channels(enc_ctx->channel_layout);/* take first format from list of supported formats */enc_ctx->sample_fmt = encoder->sample_fmts[0];AVRationaltime_base={1, enc_ctx->sample_rate};enc_ctx->time_base = time_base;}/* Third parameter can be used to pass settings to encoder*/ret =avcodec_open2(enc_ctx, encoder, NULL);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Cannot openvideo encoder for stream #%u\n", i);return ret;}} else if(dec_ctx->codec_type == AVMEDIA_TYPE_UNKNOWN) {av_log(NULL, AV_LOG_FATAL, "Elementarystream #%d is of unknown type, cannot proceed\n", i);return AVERROR_INVALIDDATA;} else {/* if this stream must be remuxed */ret =avcodec_copy_context(ofmt_ctx->streams[i]->codec,ifmt_ctx->streams[i]->codec);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Copyingstream context failed\n");return ret;}}if (ofmt_ctx->oformat->flags &AVFMT_GLOBALHEADER)enc_ctx->flags |= CODEC_FLAG_GLOBAL_HEADER;}av_dump_format(ofmt_ctx, 0, filename, 1);if (!(ofmt_ctx->oformat->flags &AVFMT_NOFILE)) {ret =avio_open(&ofmt_ctx->pb, filename, AVIO_FLAG_WRITE);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Could notopen output file '%s'", filename);return ret;}}/* init muxer, write output file header */ret =avformat_write_header(ofmt_ctx, NULL);if (ret < 0) {av_log(NULL,AV_LOG_ERROR, "Error occurred when openingoutput file\n");return ret;}return 0;
}
static intinit_filter(FilteringContext* fctx, AVCodecContext *dec_ctx,AVCodecContext *enc_ctx, const char *filter_spec)
{char args[512];int ret = 0;AVFilter*buffersrc = NULL;AVFilter*buffersink = NULL;AVFilterContext*buffersrc_ctx = NULL;AVFilterContext*buffersink_ctx = NULL;AVFilterInOut*outputs = avfilter_inout_alloc();AVFilterInOut*inputs  = avfilter_inout_alloc();AVFilterGraph*filter_graph = avfilter_graph_alloc();if (!outputs || !inputs || !filter_graph) {ret =AVERROR(ENOMEM);goto end;}if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {buffersrc =avfilter_get_by_name("buffer");buffersink= avfilter_get_by_name("buffersink");if (!buffersrc || !buffersink) {av_log(NULL, AV_LOG_ERROR, "filteringsource or sink element not found\n");ret = AVERROR_UNKNOWN;goto end;}_snprintf(args, sizeof(args),"video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",dec_ctx->width, dec_ctx->height, dec_ctx->pix_fmt,dec_ctx->time_base.num,dec_ctx->time_base.den,dec_ctx->sample_aspect_ratio.num,dec_ctx->sample_aspect_ratio.den);ret =avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",args, NULL, filter_graph);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Cannotcreate buffer source\n");goto end;}ret =avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",NULL, NULL, filter_graph);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Cannotcreate buffer sink\n");goto end;}ret =av_opt_set_bin(buffersink_ctx, "pix_fmts",(uint8_t*)&enc_ctx->pix_fmt, sizeof(enc_ctx->pix_fmt),AV_OPT_SEARCH_CHILDREN);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Cannot setoutput pixel format\n");goto end;}} else if(dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {buffersrc = avfilter_get_by_name("abuffer");buffersink= avfilter_get_by_name("abuffersink");if (!buffersrc || !buffersink) {av_log(NULL, AV_LOG_ERROR, "filteringsource or sink element not found\n");ret =AVERROR_UNKNOWN;goto end;}if (!dec_ctx->channel_layout)dec_ctx->channel_layout =av_get_default_channel_layout(dec_ctx->channels);_snprintf(args, sizeof(args),"time_base=%d/%d:sample_rate=%d:sample_fmt=%s:channel_layout=0x%I64x",dec_ctx->time_base.num, dec_ctx->time_base.den,dec_ctx->sample_rate,av_get_sample_fmt_name(dec_ctx->sample_fmt),dec_ctx->channel_layout);ret =avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",args, NULL, filter_graph);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Cannotcreate audio buffer source\n");goto end;}ret =avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",NULL, NULL, filter_graph);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Cannotcreate audio buffer sink\n");goto end;}ret = av_opt_set_bin(buffersink_ctx, "sample_fmts",(uint8_t*)&enc_ctx->sample_fmt, sizeof(enc_ctx->sample_fmt),AV_OPT_SEARCH_CHILDREN);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Cannot setoutput sample format\n");goto end;}ret =av_opt_set_bin(buffersink_ctx, "channel_layouts",(uint8_t*)&enc_ctx->channel_layout,sizeof(enc_ctx->channel_layout),AV_OPT_SEARCH_CHILDREN);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Cannot setoutput channel layout\n");goto end;}ret =av_opt_set_bin(buffersink_ctx, "sample_rates",(uint8_t*)&enc_ctx->sample_rate, sizeof(enc_ctx->sample_rate),AV_OPT_SEARCH_CHILDREN);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Cannot setoutput sample rate\n");goto end;}} else {ret =AVERROR_UNKNOWN;goto end;}/* Endpoints for the filter graph. */outputs->name       =av_strdup("in");outputs->filter_ctx = buffersrc_ctx;outputs->pad_idx    = 0;outputs->next       = NULL;inputs->name       = av_strdup("out");inputs->filter_ctx = buffersink_ctx;inputs->pad_idx    = 0;inputs->next       = NULL;if (!outputs->name || !inputs->name) {ret =AVERROR(ENOMEM);goto end;}if ((ret = avfilter_graph_parse_ptr(filter_graph,filter_spec,&inputs, &outputs, NULL)) < 0)goto end;if ((ret = avfilter_graph_config(filter_graph, NULL))< 0)goto end;/* Fill FilteringContext */fctx->buffersrc_ctx = buffersrc_ctx;fctx->buffersink_ctx = buffersink_ctx;fctx->filter_graph= filter_graph;
end:avfilter_inout_free(&inputs);avfilter_inout_free(&outputs);return ret;
}
static int init_filters(void)
{const char*filter_spec;unsigned int i;int ret;filter_ctx =(FilteringContext *)av_malloc_array(ifmt_ctx->nb_streams, sizeof(*filter_ctx));if (!filter_ctx)return AVERROR(ENOMEM);for (i = 0; i < ifmt_ctx->nb_streams; i++) {filter_ctx[i].buffersrc_ctx  =NULL;filter_ctx[i].buffersink_ctx= NULL;filter_ctx[i].filter_graph   =NULL;if(!(ifmt_ctx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO||ifmt_ctx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO))continue;if (ifmt_ctx->streams[i]->codec->codec_type== AVMEDIA_TYPE_VIDEO)filter_spec = "null"; /* passthrough (dummy) filter for video */elsefilter_spec = "anull"; /* passthrough (dummy) filter for audio */ret =init_filter(&filter_ctx[i], ifmt_ctx->streams[i]->codec,ofmt_ctx->streams[i]->codec, filter_spec);if (ret)return ret;}return 0;
}
static intencode_write_frame(AVFrame *filt_frame, unsignedint stream_index, int*got_frame) {int ret;int got_frame_local;AVPacketenc_pkt;int (*enc_func)(AVCodecContext *, AVPacket *, const AVFrame *, int*) =(ifmt_ctx->streams[stream_index]->codec->codec_type ==AVMEDIA_TYPE_VIDEO) ? avcodec_encode_video2 : avcodec_encode_audio2;if (!got_frame)got_frame =&got_frame_local;av_log(NULL,AV_LOG_INFO, "Encoding frame\n");/* encode filtered frame */enc_pkt.data =NULL;enc_pkt.size =0;av_init_packet(&enc_pkt);ret =enc_func(ofmt_ctx->streams[stream_index]->codec, &enc_pkt,filt_frame, got_frame);av_frame_free(&filt_frame);if (ret < 0)return ret;if (!(*got_frame))return 0;/* prepare packet for muxing */enc_pkt.stream_index = stream_index;enc_pkt.dts =av_rescale_q_rnd(enc_pkt.dts,ofmt_ctx->streams[stream_index]->codec->time_base,ofmt_ctx->streams[stream_index]->time_base,(AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));enc_pkt.pts =av_rescale_q_rnd(enc_pkt.pts,ofmt_ctx->streams[stream_index]->codec->time_base,ofmt_ctx->streams[stream_index]->time_base,(AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));enc_pkt.duration = av_rescale_q(enc_pkt.duration,ofmt_ctx->streams[stream_index]->codec->time_base,ofmt_ctx->streams[stream_index]->time_base);av_log(NULL,AV_LOG_DEBUG, "Muxing frame\n");/* mux encoded frame */ret =av_interleaved_write_frame(ofmt_ctx, &enc_pkt);return ret;
}
static intfilter_encode_write_frame(AVFrame *frame, unsignedint stream_index)
{int ret;AVFrame*filt_frame;av_log(NULL,AV_LOG_INFO, "Pushing decoded frame tofilters\n");/* push the decoded frame into the filtergraph */ret =av_buffersrc_add_frame_flags(filter_ctx[stream_index].buffersrc_ctx,frame,0);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Error whilefeeding the filtergraph\n");return ret;}/* pull filtered frames from the filtergraph */while (1) {filt_frame= av_frame_alloc();if (!filt_frame) {ret =AVERROR(ENOMEM);break;}av_log(NULL, AV_LOG_INFO, "Pullingfiltered frame from filters\n");ret =av_buffersink_get_frame(filter_ctx[stream_index].buffersink_ctx,filt_frame);if (ret < 0) {/* if nomore frames for output - returns AVERROR(EAGAIN)* if flushed and no more frames for output - returns AVERROR_EOF* rewrite retcode to 0 to show it as normal procedure completion*/if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)ret= 0;av_frame_free(&filt_frame);break;}filt_frame->pict_type = AV_PICTURE_TYPE_NONE;ret =encode_write_frame(filt_frame, stream_index, NULL);if (ret < 0)break;}return ret;
}
static int flush_encoder(unsigned intstream_index)
{int ret;int got_frame;if(!(ofmt_ctx->streams[stream_index]->codec->codec->capabilities&CODEC_CAP_DELAY))return 0;while (1) {av_log(NULL, AV_LOG_INFO, "Flushingstream #%u encoder\n", stream_index);ret =encode_write_frame(NULL, stream_index, &got_frame);if (ret < 0)break;if (!got_frame)return 0;}return ret;
}int_tmain(int argc, _TCHAR* argv[])
{int ret;AVPacketpacket;AVFrame *frame= NULL;enum AVMediaType type;unsigned intstream_index;unsigned int i;int got_frame;int (*dec_func)(AVCodecContext *, AVFrame *, int *, const AVPacket*);if (argc != 3) {av_log(NULL, AV_LOG_ERROR, "Usage: %s<input file> <output file>\n", argv[0]);return 1;}av_register_all();avfilter_register_all();if ((ret = open_input_file(argv[1])) < 0)goto end;if ((ret = open_output_file(argv[2])) < 0)goto end;if ((ret = init_filters()) < 0)goto end;/* read all packets */while (1) {if ((ret= av_read_frame(ifmt_ctx, &packet)) < 0)break;stream_index = packet.stream_index;type =ifmt_ctx->streams[packet.stream_index]->codec->codec_type;av_log(NULL, AV_LOG_DEBUG, "Demuxergave frame of stream_index %u\n",stream_index);if (filter_ctx[stream_index].filter_graph) {av_log(NULL, AV_LOG_DEBUG, "Going toreencode&filter the frame\n");frame =av_frame_alloc();if (!frame) {ret = AVERROR(ENOMEM);break;}packet.dts = av_rescale_q_rnd(packet.dts,ifmt_ctx->streams[stream_index]->time_base,ifmt_ctx->streams[stream_index]->codec->time_base,(AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));packet.pts = av_rescale_q_rnd(packet.pts,ifmt_ctx->streams[stream_index]->time_base,ifmt_ctx->streams[stream_index]->codec->time_base,(AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));dec_func = (type == AVMEDIA_TYPE_VIDEO) ? avcodec_decode_video2 :avcodec_decode_audio4;ret =dec_func(ifmt_ctx->streams[stream_index]->codec, frame,&got_frame, &packet);if (ret < 0) {av_frame_free(&frame);av_log(NULL, AV_LOG_ERROR, "Decodingfailed\n");break;}if (got_frame) {frame->pts = av_frame_get_best_effort_timestamp(frame);ret= filter_encode_write_frame(frame, stream_index);av_frame_free(&frame);if (ret< 0)goto end;} else {av_frame_free(&frame);}} else {/* remux this frame without reencoding */packet.dts = av_rescale_q_rnd(packet.dts,ifmt_ctx->streams[stream_index]->time_base,ofmt_ctx->streams[stream_index]->time_base,(AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));packet.pts = av_rescale_q_rnd(packet.pts,ifmt_ctx->streams[stream_index]->time_base,ofmt_ctx->streams[stream_index]->time_base,(AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));ret =av_interleaved_write_frame(ofmt_ctx, &packet);if (ret < 0)goto end;}av_free_packet(&packet);}/* flush filters and encoders */for (i = 0; i < ifmt_ctx->nb_streams; i++) {/* flush filter */if (!filter_ctx[i].filter_graph)continue;ret =filter_encode_write_frame(NULL, i);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Flushingfilter failed\n");goto end;}/* flush encoder */ret = flush_encoder(i);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Flushingencoder failed\n");goto end;}}av_write_trailer(ofmt_ctx);
end:av_free_packet(&packet);av_frame_free(&frame);for (i = 0; i < ifmt_ctx->nb_streams; i++) {avcodec_close(ifmt_ctx->streams[i]->codec);if (ofmt_ctx && ofmt_ctx->nb_streams >i && ofmt_ctx->streams[i] &&ofmt_ctx->streams[i]->codec)avcodec_close(ofmt_ctx->streams[i]->codec);if(filter_ctx && filter_ctx[i].filter_graph)avfilter_graph_free(&filter_ctx[i].filter_graph);}av_free(filter_ctx);avformat_close_input(&ifmt_ctx);if (ofmt_ctx &&!(ofmt_ctx->oformat->flags & AVFMT_NOFILE))avio_close(ofmt_ctx->pb);avformat_free_context(ofmt_ctx);if (ret < 0)av_log(NULL, AV_LOG_ERROR, "Erroroccurred\n");return (ret? 1:0);
}

程序运行截图：

默认情况下运行程序，会将“cuc_ieschool.ts”转换为“cuc_ieschool.avi”。调试的时候，可以修改“配置属性->调试->命令参数”中的参数，即可改变转码的输入输出文件。

工程下载地址（VC2010）：http://download.csdn.net/detail/leixiaohua1020/7394649

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流程图（2014.9.29更新）

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