FFMPEG实现的转码程序

本例子是由FFEMPG的doc/example例子transcode.c修改而来，可以根据需求任意转换音视频的编码。

原来的例子的作用更类似于remux，并没有实现转码的功能，只是实现了格式转换，比如ts转avi等。并不能实现音视频编码格式的转换，比如将h264转为mpeg2。

FFMPEG转码的实现有多种方式：

一种方式是：流解复用->视频+音频流->解码->YUV/PCM等->视音频编码->重新生成的音视频流->复用->流

另一种方式依赖AVFilter，这一部分在另外的几篇文章中解释怎么用。虽然AVFilter学习起来可能比较困难，但是在实际的编程应用中，依靠AVFilter做转码效率比第一种方式高，并且解码的CPU和时间消耗也少的多。所以，还是建议好好学习这部分的，毕竟我一直觉得FFMPEG的强项就是解码和转码。

本例子是视频mpeg2转h264，音频mpegaudio转g711。

<span style="font-family:SimHei;font-size:18px;">/** based on FFMPEG transcode.c* modified by tongli*/#include <stdio.h>#include "snprintf.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/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 open input file\n");return ret;}if ((ret = avformat_find_stream_info(ifmt_ctx, NULL)) < 0) {av_log(NULL, AV_LOG_ERROR, "Cannot find stream 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 to open 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 not create 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, "Failed allocating 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) {encoder = avcodec_find_encoder(AV_CODEC_ID_H264);if (!encoder) {av_log(NULL, AV_LOG_FATAL, "Neccessary encoder not found\n");return AVERROR_INVALIDDATA;}enc_ctx->height = dec_ctx->height;enc_ctx->width = dec_ctx->width;enc_ctx->sample_aspect_ratio = dec_ctx->sample_aspect_ratio;enc_ctx->pix_fmt = encoder->pix_fmts[0];enc_ctx->time_base = dec_ctx->time_base;enc_ctx->me_range = 16;enc_ctx->max_qdiff = 4;enc_ctx->qmin = 10;enc_ctx->qmax = 51;enc_ctx->qcompress = 0.6;enc_ctx->refs = 3;enc_ctx->bit_rate = 500000;ret = avcodec_open2(enc_ctx, encoder, NULL);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Cannot open video encoder for stream #%u\n", i);return ret;}}else if (dec_ctx->codec_type == AVMEDIA_TYPE_UNKNOWN) {av_log(NULL, AV_LOG_FATAL, "Elementary stream #%d is of unknown type, cannot proceed\n", i);return AVERROR_INVALIDDATA;}else if (dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO){encoder = avcodec_find_encoder(AV_CODEC_ID_PCM_ALAW);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);enc_ctx->sample_fmt = encoder->sample_fmts[0];AVRational ar = { 1, enc_ctx->sample_rate };enc_ctx->time_base = ar;ret = avcodec_open2(enc_ctx, encoder, NULL);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Cannot open video encoder for stream #%u\n", i);return ret;}}else {ret = avcodec_copy_context(ofmt_ctx->streams[i]->codec,ifmt_ctx->streams[i]->codec);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Copying stream 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 not open 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 opening output 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, "filtering source 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, "Cannot create 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, "Cannot create 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 set output 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, "filtering source 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%"PRIx64,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, "Cannot create 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, "Cannot create 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 set output 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 set output 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 set output 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 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;av_packet_rescale_ts(&enc_pkt,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 to filters\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 while feeding 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, "Pulling filtered frame from filters\n");ret = av_buffersink_get_frame(filter_ctx[stream_index].buffersink_ctx,filt_frame);if (ret < 0) {/* if no more 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, "Flushing stream #%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; //= { .data = NULL, .size = 0 };packet.data = NULL;packet.size = 0;AVFrame *frame = NULL;enum AVMediaType type;unsigned int stream_index;unsigned int i;int got_frame;int(*dec_func)(AVCodecContext *, AVFrame *, int *, const AVPacket *);av_register_all();avfilter_register_all();if ((ret = open_input_file("test.ts")) < 0)goto end;if ((ret = open_output_file("test.avi")) < 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, "Demuxer gave frame of stream_index %u\n",stream_index);if (filter_ctx[stream_index].filter_graph) {av_log(NULL, AV_LOG_DEBUG, "Going to reencode&filter the frame\n");frame = av_frame_alloc();if (!frame) {ret = AVERROR(ENOMEM);break;}av_packet_rescale_ts(&packet,ifmt_ctx->streams[stream_index]->time_base,ifmt_ctx->streams[stream_index]->codec->time_base);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, "Decoding failed\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 */av_packet_rescale_ts(&packet,ifmt_ctx->streams[stream_index]->time_base,ofmt_ctx->streams[stream_index]->time_base);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, "Flushing filter failed\n");goto end;}/* flush encoder */ret = flush_encoder(i);if (ret < 0) {av_log(NULL, AV_LOG_ERROR, "Flushing encoder 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_closep(&ofmt_ctx->pb);avformat_free_context(ofmt_ctx);if (ret < 0)av_log(NULL, AV_LOG_ERROR, "Error occurred: %s\n"); //av_err2str(ret));return ret ? 1 : 0;}</span>

源代码下载：

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