[编辑]概述

• Output_example.c是ffmpeg项目中很典型的一个例程，演示了如何利用ffmpeg提供的库（API）进行音频与视频的编码并生成文件。

• Output_example.c的主要功能是根据给定的文件名（通过命令行参数给定）生成一段对应的多媒体文件，文件的格式与给定的文件名相关。这里有一点需要特别的说明，就是音频与视频数据的来源问题，因为在ffmpeg工程组论坛里面经常会有初学者问及。在生成的多媒体文件中包括音频与视频，其中音频是通过"get_audio_frame"方法根据一个简单的算法产生的音频数据，视频则是通过"fill_yuv_image"方法根据一个简单的算法产生的视频数据。

• Output_example.c向AVFormatContext结构体中添加流等相应的信息，接着使用库的函数保存相应的内容，如何保存都设置在这个结构体中，这个结构本就是一个中介它在每一个函数的参数中可以体现这一点。

/*

* Libavformat API example: Output a media file in any supported
* libavformat format. The default codecs are used.
*
* Copyright (c) 2003 Fabrice Bellard
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*/
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <math.h>#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif#include "avformat.h"
#include "swscale.h"/* 5 seconds stream duration */
#define STREAM_DURATION   5.0
#define STREAM_FRAME_RATE 25 /* 25 images/s */
#define STREAM_NB_FRAMES  ((int)(STREAM_DURATION * STREAM_FRAME_RATE))
#define STREAM_PIX_FMT PIX_FMT_YUV420P /* default pix_fmt */static int sws_flags = SWS_BICUBIC;

[编辑]音频输出

/**************************************************************/ /* audio output */

float t, tincr, tincr2; int16_t *samples; uint8_t *audio_outbuf; int audio_outbuf_size; int audio_input_frame_size;

[编辑]add_audio_stream

//add an audio output stream static AVStream * add_audio_stream(AVFormatContext *oc, int codec_id) {

   AVCodecContext *c;AVStream *st;st = av_new_stream(oc, 1);if (!st) {fprintf(stderr, "Could not alloc stream\n");exit(1);}c = st->codec;c->codec_id = codec_id;c->codec_type = CODEC_TYPE_AUDIO;/* put sample parameters */c->bit_rate = 64000;c->sample_rate = 44100;c->channels = 2;return st;

}

[编辑]open_audio

static void open_audio(AVFormatContext *oc, AVStream *st) {

   AVCodecContext *c;AVCodec *codec;c = st->codec;/* find the audio encoder */codec = avcodec_find_encoder(c->codec_id);if (!codec) {fprintf(stderr, "codec not found\n");exit(1);}/* open it */if (avcodec_open(c, codec) < 0) {fprintf(stderr, "could not open codec\n");exit(1);}/* init signal generator */t = 0;tincr = 2 * M_PI * 110.0 / c->sample_rate;/* increment frequency by 110 Hz per second */tincr2 = 2 * M_PI * 110.0 / c->sample_rate / c->sample_rate;audio_outbuf_size = 10000;audio_outbuf = av_malloc(audio_outbuf_size);/* ugly hack for PCM codecs (will be removed ASAP with new PCMsupport to compute the input frame size in samples */if (c->frame_size <= 1) {audio_input_frame_size = audio_outbuf_size / c->channels;switch(st->codec->codec_id) {case CODEC_ID_PCM_S16LE:case CODEC_ID_PCM_S16BE:case CODEC_ID_PCM_U16LE:case CODEC_ID_PCM_U16BE:audio_input_frame_size >>= 1;break;default:break;}} else {audio_input_frame_size = c->frame_size;}samples = av_malloc(audio_input_frame_size * 2 * c->channels);

}

[编辑]get_audio_frame

/* prepare a 16 bit dummy audio frame of 'frame_size' samples and

  'nb_channels' channels */

static void get_audio_frame(int16_t *samples, int frame_size, int nb_channels) {

   int j, i, v;int16_t *q;q = samples;for(j=0;j<frame_size;j++) {v = (int)(sin(t) * 10000);for(i = 0; i < nb_channels; i++)*q++ = v;t += tincr;tincr += tincr2;}

}

[编辑]write_audio_frame

static void write_audio_frame(AVFormatContext *oc, AVStream *st) {

   AVCodecContext *c;AVPacket pkt;av_init_packet(&pkt);c = st->codec;get_audio_frame(samples, audio_input_frame_size, c->channels);pkt.size= avcodec_encode_audio(c, audio_outbuf, audio_outbuf_size, samples);pkt.pts= av_rescale_q(c->coded_frame->pts, c->time_base, st->time_base);pkt.flags |= PKT_FLAG_KEY;pkt.stream_index= st->index;pkt.data= audio_outbuf;/* write the compressed frame in the media file */if (av_write_frame(oc, &pkt) != 0) {fprintf(stderr, "Error while writing audio frame\n");exit(1);}

}

[编辑]close_audio

static void close_audio(AVFormatContext *oc, AVStream *st) {

   avcodec_close(st->codec);av_free(samples);av_free(audio_outbuf);

}

[编辑]视频输出

/**************************************************************/ /* video output */

AVFrame *picture, *tmp_picture; uint8_t *video_outbuf; int frame_count, video_outbuf_size;

[编辑]add_video_stream

/* add a video output stream */ static AVStream *add_video_stream(AVFormatContext *oc, int codec_id) {

   AVCodecContext *c;AVStream *st;st = av_new_stream(oc, 0);if (!st) {fprintf(stderr, "Could not alloc stream\n");exit(1);}c = st->codec;c->codec_id = codec_id;c->codec_type = CODEC_TYPE_VIDEO;

   /* put sample parameters */c->bit_rate = 400000;/* resolution must be a multiple of two */c->width = 352;c->height = 288;/* time base: this is the fundamental unit of time (in seconds) in termsof which frame timestamps are represented. for fixed-fps content,timebase should be 1/framerate and timestamp increments should beidentically 1. */c->time_base.den = STREAM_FRAME_RATE;c->time_base.num = 1;c->gop_size = 12; /* emit one intra frame every twelve frames at most */c->pix_fmt = STREAM_PIX_FMT;if (c->codec_id == CODEC_ID_MPEG2VIDEO) {/* just for testing, we also add B frames */c->max_b_frames = 2;}if (c->codec_id == CODEC_ID_MPEG1VIDEO){/* needed to avoid using macroblocks in which some coeffs overflowthis doesnt happen with normal video, it just happens here as themotion of the chroma plane doesnt match the luma plane */c->mb_decision=2;}// some formats want stream headers to be separateif(!strcmp(oc->oformat->name, "mp4") || !strcmp(oc->oformat->name, "mov") || !strcmp(oc->oformat->name, "3gp"))c->flags |= CODEC_FLAG_GLOBAL_HEADER;return st;

}

[编辑]alloc_picture

static AVFrame *alloc_picture(int pix_fmt, int width, int height) {

   AVFrame *picture;uint8_t *picture_buf;int size;picture = avcodec_alloc_frame();if (!picture)return NULL;size = avpicture_get_size(pix_fmt, width, height);picture_buf = av_malloc(size);if (!picture_buf) {av_free(picture);return NULL;}avpicture_fill((AVPicture *)picture, picture_buf,pix_fmt, width, height);return picture;

}

[编辑]open_video

static void open_video(AVFormatContext *oc, AVStream *st) {

   AVCodec *codec;AVCodecContext *c;c = st->codec;/* find the video encoder */codec = avcodec_find_encoder(c->codec_id);if (!codec) {fprintf(stderr, "codec not found\n");exit(1);}/* open the codec */if (avcodec_open(c, codec) < 0) {fprintf(stderr, "could not open codec\n");exit(1);}video_outbuf = NULL;if (!(oc->oformat->flags & AVFMT_RAWPICTURE)) {/* allocate output buffer *//* XXX: API change will be done *//* buffers passed into lav* can be allocated any way you prefer,as long as they're aligned enough for the architecture, andthey're freed appropriately (such as using av_free for buffersallocated with av_malloc) */video_outbuf_size = 200000;video_outbuf = av_malloc(video_outbuf_size);}/* allocate the encoded raw picture */picture = alloc_picture(c->pix_fmt, c->width, c->height);if (!picture) {fprintf(stderr, "Could not allocate picture\n");exit(1);}/* if the output format is not YUV420P, then a temporary YUV420Ppicture is needed too. It is then converted to the requiredoutput format */tmp_picture = NULL;if (c->pix_fmt != PIX_FMT_YUV420P) {tmp_picture = alloc_picture(PIX_FMT_YUV420P, c->width, c->height);if (!tmp_picture) {fprintf(stderr, "Could not allocate temporary picture\n");exit(1);}}

}

[编辑]fill_yuv_image

/* prepare a dummy image */ static void fill_yuv_image(AVFrame *pict, int frame_index, int width, int height) {

   int x, y, i;i = frame_index;/* Y */for(y=0;y<height;y++) {for(x=0;x<width;x++) {pict->data[0][y * pict->linesize[0] + x] = x + y + i * 3;}}/* Cb and Cr */for(y=0;y<height/2;y++) {for(x=0;x<width/2;x++) {pict->data[1][y * pict->linesize[1] + x] = 128 + y + i * 2;pict->data[2][y * pict->linesize[2] + x] = 64 + x + i * 5;}}

}

[编辑]write_video_frame

static void write_video_frame(AVFormatContext *oc, AVStream *st) {

   int out_size, ret;AVCodecContext *c;static struct SwsContext *img_convert_ctx;c = st->codec;if (frame_count >= STREAM_NB_FRAMES) {/* no more frame to compress. The codec has a latency of a fewframes if using B frames, so we get the last frames bypassing the same picture again */} else {if (c->pix_fmt != PIX_FMT_YUV420P) {/* as we only generate a YUV420P picture, we must convert itto the codec pixel format if needed */if (img_convert_ctx == NULL) {img_convert_ctx = sws_getContext(c->width, c->height,PIX_FMT_YUV420P,c->width, c->height,c->pix_fmt,sws_flags, NULL, NULL, NULL);if (img_convert_ctx == NULL) {fprintf(stderr, "Cannot initialize the conversion context\n");exit(1);}}fill_yuv_image(tmp_picture, frame_count, c->width, c->height);sws_scale(img_convert_ctx, tmp_picture->data, tmp_picture->linesize,0, c->height, picture->data, picture->linesize);} else {fill_yuv_image(picture, frame_count, c->width, c->height);}}if (oc->oformat->flags & AVFMT_RAWPICTURE) {/* raw video case. The API will change slightly in the nearfutur for that */AVPacket pkt;av_init_packet(&pkt);pkt.flags |= PKT_FLAG_KEY;pkt.stream_index= st->index;pkt.data= (uint8_t *)picture;pkt.size= sizeof(AVPicture);ret = av_write_frame(oc, &pkt);} else {/* encode the image */out_size = avcodec_encode_video(c, video_outbuf, video_outbuf_size, picture);/* if zero size, it means the image was buffered */if (out_size > 0) {AVPacket pkt;av_init_packet(&pkt);pkt.pts= av_rescale_q(c->coded_frame->pts, c->time_base, st->time_base);if(c->coded_frame->key_frame)pkt.flags |= PKT_FLAG_KEY;pkt.stream_index= st->index;pkt.data= video_outbuf;pkt.size= out_size;/* write the compressed frame in the media file */ret = av_write_frame(oc, &pkt);} else {ret = 0;}}if (ret != 0) {fprintf(stderr, "Error while writing video frame\n");exit(1);}frame_count++;

}

[编辑]close_video

static void close_video(AVFormatContext *oc, AVStream *st) {

   avcodec_close(st->codec);av_free(picture->data[0]);av_free(picture);if (tmp_picture) {av_free(tmp_picture->data[0]);av_free(tmp_picture);}av_free(video_outbuf);

}

[编辑]媒体文件输出-main()

/**************************************************************/ /* media file output */

int main(int argc, char **argv) {

   const char *filename;AVOutputFormat *fmt;AVFormatContext *oc;AVStream *audio_st, *video_st;double audio_pts, video_pts;int i;/* initialize libavcodec, and register all codecs and formats */av_register_all();if (argc != 2) {printf("usage: %s output_file\n""API example program to output a media file with libavformat.\n""The output format is automatically guessed according to the file extension.\n""Raw images can also be output by using '%%d' in the filename\n""\n", argv[0]);exit(1);}filename = argv[1];/* auto detect the output format from the name. default ismpeg. */fmt = guess_format(NULL, filename, NULL);if (!fmt) {printf("Could not deduce output format from file extension: using MPEG.\n");fmt = guess_format("mpeg", NULL, NULL);}if (!fmt) {fprintf(stderr, "Could not find suitable output format\n");exit(1);}/* allocate the output media context */oc = av_alloc_format_context();if (!oc) {fprintf(stderr, "Memory error\n");exit(1);}oc->oformat = fmt;snprintf(oc->filename, sizeof(oc->filename), "%s", filename);/* add the audio and video streams using the default format codecsand initialize the codecs */video_st = NULL;audio_st = NULL;if (fmt->video_codec != CODEC_ID_NONE) {video_st = add_video_stream(oc, fmt->video_codec);}if (fmt->audio_codec != CODEC_ID_NONE) {audio_st = add_audio_stream(oc, fmt->audio_codec);}/* set the output parameters (must be done even if noparameters). */if (av_set_parameters(oc, NULL) < 0) {fprintf(stderr, "Invalid output format parameters\n");exit(1);}dump_format(oc, 0, filename, 1);/* now that all the parameters are set, we can open the audio andvideo codecs and allocate the necessary encode buffers */if (video_st)open_video(oc, video_st);if (audio_st)open_audio(oc, audio_st);/* open the output file, if needed */if (!(fmt->flags & AVFMT_NOFILE)) {if (url_fopen(&oc->pb, filename, URL_WRONLY) < 0) {fprintf(stderr, "Could not open '%s'\n", filename);exit(1);}}/* write the stream header, if any */av_write_header(oc);for(;;) {/* compute current audio and video time */if (audio_st)audio_pts = (double)audio_st->pts.val * audio_st->time_base.num / audio_st->time_base.den;elseaudio_pts = 0.0;if (video_st)video_pts = (double)video_st->pts.val * video_st->time_base.num / video_st->time_base.den;elsevideo_pts = 0.0;if ((!audio_st || audio_pts >= STREAM_DURATION) &&(!video_st || video_pts >= STREAM_DURATION))break;/* write interleaved audio and video frames */if (!video_st || (video_st && audio_st && audio_pts < video_pts)) {write_audio_frame(oc, audio_st);} else {write_video_frame(oc, video_st);}}/* close each codec */if (video_st)close_video(oc, video_st);if (audio_st)close_audio(oc, audio_st);/* write the trailer, if any */av_write_trailer(oc);/* free the streams */for(i = 0; i < oc->nb_streams; i++) {av_freep(&oc->streams[i]->codec);av_freep(&oc->streams[i]);}if (!(fmt->flags & AVFMT_NOFILE)) {/* close the output file */url_fclose(&oc->pb);}/* free the stream */av_free(oc);return 0;

}