编码为H264裸流并写文件一

为什么说是裸流呢，因为这篇只讲到把视频编码后写入文件，得到的是一个后缀应该为“.h264”的文件，只能用PotPlayer等功能强大的播放器才能打开，因为它没有容器信息，需要装在MP4、MKV之类的容器中，才是一个真正的视频文件。但这一篇只讲编码后直接写入文件，以生成一个H264裸流文件。

代码是从FFMPEG的2.8版本的官方示例中搬运的：https://ffmpeg.org/doxygen/2.8/decoding_encoding_8c-example.html

 

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static void video_encode_example(const char *filename, int codec_id) {     AVCodec *codec;     AVCodecContext *c= NULL;     int i, ret, x, y, got_output;     FILE *f;     AVFrame *frame;     AVPacket pkt;     uint8_t endcode[] = { 0, 0, 1, 0xb7 };     printf("Encode video file %s\n", filename);     /* find the mpeg1 video encoder */     codec = avcodec_find_encoder(codec_id);     if (!codec) {         fprintf(stderr, "Codec not found\n");         exit(1);     }     c = avcodec_alloc_context3(codec);     if (!c) {         fprintf(stderr, "Could not allocate video codec context\n");         exit(1);     }     /* put sample parameters */     c->bit_rate = 400000;     /* resolution must be a multiple of two */     c->width = 352;     c->height = 288;     /* frames per second */     c->time_base = (AVRational){1,25};     /* emit one intra frame every ten frames      * check frame pict_type before passing frame      * to encoder, if frame->pict_type is AV_PICTURE_TYPE_I      * then gop_size is ignored and the output of encoder      * will always be I frame irrespective to gop_size      */     c->gop_size = 10;     c->max_b_frames = 1;     c->pix_fmt = AV_PIX_FMT_YUV420P;     if (codec_id == AV_CODEC_ID_H264)         av_opt_set(c->priv_data, "preset", "slow", 0);     /* open it */     if (avcodec_open2(c, codec, NULL) < 0) {         fprintf(stderr, "Could not open codec\n");         exit(1);     }     f = fopen(filename, "wb");     if (!f) {         fprintf(stderr, "Could not open %s\n", filename);         exit(1);     }     frame = av_frame_alloc();     if (!frame) {         fprintf(stderr, "Could not allocate video frame\n");         exit(1);     }     frame->format = c->pix_fmt;     frame->width  = c->width;     frame->height = c->height;     /* the image can be allocated by any means and av_image_alloc() is      * just the most convenient way if av_malloc() is to be used */     ret = av_image_alloc(frame->data, frame->linesize, c->width, c->height,                          c->pix_fmt, 32);     if (ret < 0) {         fprintf(stderr, "Could not allocate raw picture buffer\n");         exit(1);     }     /* encode 1 second of video */     for (i = 0; i < 25; i++) {         av_init_packet(&pkt);         pkt.data = NULL;    // packet data will be allocated by the encoder         pkt.size = 0;         fflush(stdout);         /* prepare a dummy image */         /* Y */         for (y = 0; y < c->height; y++) {             for (x = 0; x < c->width; x++) {                 frame->data[0][y * frame->linesize[0] + x] = x + y + i * 3;             }         }         /* Cb and Cr */         for (y = 0; y < c->height/2; y++) {             for (x = 0; x < c->width/2; x++) {                 frame->data[1][y * frame->linesize[1] + x] = 128 + y + i * 2;                 frame->data[2][y * frame->linesize[2] + x] = 64 + x + i * 5;             }         }         frame->pts = i;         /* encode the image */         ret = avcodec_encode_video2(c, &pkt, frame, &got_output);         if (ret < 0) {             fprintf(stderr, "Error encoding frame\n");             exit(1);         }         if (got_output) {             printf("Write frame %3d (size=%5d)\n", i, pkt.size);             fwrite(pkt.data, 1, pkt.size, f);             av_free_packet(&pkt);         }     }     /* get the delayed frames */     for (got_output = 1; got_output; i++) {         fflush(stdout);         ret = avcodec_encode_video2(c, &pkt, NULL, &got_output);         if (ret < 0) {             fprintf(stderr, "Error encoding frame\n");             exit(1);         }         if (got_output) {             printf("Write frame %3d (size=%5d)\n", i, pkt.size);             fwrite(pkt.data, 1, pkt.size, f);             av_free_packet(&pkt);         }     }     /* add sequence end code to have a real mpeg file */     fwrite(endcode, 1, sizeof(endcode), f);     fclose(f);     avcodec_close(c);     av_free(c);     av_freep(&frame->data[0]);     av_frame_free(&frame);     printf("\n"); }

 

将上述代码中的AVFrame改为从摄像头获取的AVFrame，就实现了摄像头数据编码为H264裸流了，但有几点要注意。

1. 代码中的av_opt_set(c->priv_data, "preset", "slow", 0)声明了编码速度，值有ultrafast、superfast、veryfast、faster、fast、medium、slow、slower、veryslow、placebo，越快视频质量则越差，但如果想实现实时编码（即编码速度不慢于输入速度），还要加一句av_opt_set(pOutputVideoCodecCtx->priv_data, "tune", "zerolatency", 0)。
2. 每个packet的pts非常重要，当然也比较简单，设置好帧率（time_base）之后，pts累加即可。
3. 最后的“get the delayed frames”也叫作“flush encoder”，意思是编码器中一般会缓存一些数据，最后要取出来。第1点中提到的编码速度越快，编码器中缓存的Packet也会越少，反之则越多。