本文介绍一个简单的基于FFmpeg的转码器。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。
下面贴上代码，代码是从FFmpeg的例子改编的，平台是VC2010，类库版本是2014.5.6。
/*
*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
");
return ret;
}
if ((ret = avformat_find_stream_info(ifmt_ctx, NULL))< 0) {
av_log(NULL, AV_LOG_ERROR, "Cannot findstream information
");
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
", 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
");
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
");
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
", 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
", 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
");
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
");
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
");
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
");
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
");
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
");
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
");
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
");
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
");
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
");
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
");
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
");
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 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
");
/* 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
");
/* 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
");
/* 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
");
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
");
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
", 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>
", 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
",
stream_index);
if (filter_ctx[stream_index].filter_graph) {
av_log(NULL, AV_LOG_DEBUG, "Going toreencode&filter the frame
");
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
");
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
");
goto end;
}
/* flush encoder */
ret = flush_encoder(i);
if (ret < 0) {
av_log(NULL, AV_LOG_ERROR, "Flushingencoder failed
");
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
");
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