本文主要是介绍ffmpeg学习十四:转码,希望对大家解决编程问题提供一定的参考价值,需要的开发者们随着小编来一起学习吧!
转码指的是把一种音视频文件的格式(封装格式+编码格式)转换为另一种音视频文件的格式。其过程如图所示:
从图中可以得知,转码涉及了解封装,解编码,编码,再封装的过程。这个过程基本涵盖了之前文章的所有内容。对于一个视频文件而言,要把它转为另一种格式,意味着首先要改变它的封装格式,其次,视频文件可能包含多个流,比如视频流,音频流,字幕流等都需要重新编码。回顾之前的文章,我们单独分析了解封装,封装,编码音频,编码视频,解码视频的过程,唯独没有做解码音频的单独分析,但只要对解码视频比较了解,解码音频也很容易理解。因此,这篇文章分析的转码是之前所有文章的学习的知识的一次综合运用。
ffmpeg已经为我们提供了转码的例程,它是很好的学习教材。其路径为:doc/examples/transcoding.c
我下载的ffmpeg的版本是2.7版本,这个版本的这个例程能顺利编译,但是执行这个程序的时候,会报如下错误:
[libx264 @ 00021bb0]broken ffmpeg default settings detected
[libx264 @ 00021bb0]use an encoding preset (vpre)
解决方法:
参考ffmpeg x264编译与使用介绍这篇博客,
在avcodec_open函数之间增加如下几个AVCodecContext 的初始化:
/default settings for x264/
ctx->me_range = 16;
ctx->max_qdiff = 4;
ctx->qmin = 10;
ctx->qmax = 51;
ctx->qcompress = 0.6;
也就是在open_input_file和open_output_file函数的avcodec_open2函数之前,为AVCodecContext的实例做如上的附加配置,这样本人亲测可以解决这个问题。修改过的完整的代码如下:
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavfilter/avfiltergraph.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|| dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {/* in this example, we choose transcoding to same codec */encoder = avcodec_find_encoder(dec_ctx->codec_id);if (!encoder) {av_log(NULL, AV_LOG_FATAL, "Necessary encoder not found\n");return AVERROR_INVALIDDATA;}/* 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 */if (encoder->pix_fmts)enc_ctx->pix_fmt = encoder->pix_fmts[0];elseenc_ctx->pix_fmt = dec_ctx->pix_fmt;/* video time_base can be set to whatever is handy and supported 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];enc_ctx->time_base = (AVRational){1, enc_ctx->sample_rate};}/* 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 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 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, "Copying stream context failed\n");return ret;}}if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)enc_ctx->flags |= AV_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 = 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 &AV_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 };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, "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_packet_unref(&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_packet_unref(&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;
}
转码的整个过程可用如下流程图来概述:
重要函数的简要介绍:
open_input_file():打开输入文件,并初始化相关的结构体。
其中的avformat_open_input()函数可参考 ffmpeg学习五:avformat_open_input函数源码分析(以mp4文件为例)一文。avformat_find_stream_info()可参考ffmpeg学习七:avformat_find_stream_info函数源码分析一文。avcodec_open2()函数可参考 ffmpeg学习六:avcodec_open2函数源码分析一文。
open_output_file():打开输出文件,并初始化相关的结构体。会写入格式相关的文件头部信息
init_filters():初始化AVFilter相关的结构体。可参考ffmpeg学习十二:滤镜(实现视频缩放,裁剪,水印等)一文。
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():首先会对AVFrame做滤镜处理,然后编码一个AVFrame,最后写入文件。
此外,音频编码可参考ffmpeg学习十:将pcm格式的音频编码为aac格式一文。
视频编码可参考ffmpeg学习八:软件生成yuv420p视频并将其编码为H264格式一文。
封装可参考 ffmpeg学习十一:封装音视频到同一个文件(muxing.c源码分析)一文。
解封装与解码可参考ffmpeg学习四:写第一个程序-视频解封装与解码一文。
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