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
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/* *最简单的基于FFmpeg的转码器 *Simplest FFmpeg Transcoder * *雷霄骅 Lei Xiaohua *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。 * */ #define __STDC_CONSTANT_MACROS 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]; AVRational time_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 int init_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 */ else filter_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 int encode_write_frame(AVFrame *filt_frame, unsigned int stream_index, int*got_frame) { int ret; int got_frame_local; AVPacket enc_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 int filter_encode_write_frame(AVFrame *frame, unsigned int 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 int stream_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 main(int argc, char* argv[]) { int ret; AVPacket packet; AVFrame *frame= NULL; enum AVMediaType type; unsigned int stream_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）：simplest_ffmpeg_transcoder
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