ffmpeg 编码（视屏）

分析ffmpeg_3.3.2 muxing

1:分析主函数，代码如下：

  1 int main(int argc, char **argv)
  2 {
  3     OutputStream video_st = { 0 }, audio_st = { 0 };
  4     const char *filename;
  5     AVOutputFormat *fmt;
  6     AVFormatContext *oc;
  7     AVCodec *audio_codec, *video_codec;
  8     int ret;
  9     int have_video = 0, have_audio = 0;
 10     int encode_video = 0, encode_audio = 0;
 11     AVDictionary *opt = NULL;
 12     int i;
 13 
 14     /* Initialize libavcodec, and register all codecs and formats. */
 15     av_register_all();
 16 
 17     if (argc < 2) {
 18         printf("usage: %s output_file
"
 19                "API example program to output a media file with libavformat.
"
 20                "This program generates a synthetic audio and video stream, encodes and
"
 21                "muxes them into a file named output_file.
"
 22                "The output format is automatically guessed according to the file extension.
"
 23                "Raw images can also be output by using '%%d' in the filename.
"
 24                "
", argv[0]);
 25         return 1;
 26     }
 27 
 28     filename = argv[1];
 29     for (i = 2; i+1 < argc; i+=2) {
 30         if (!strcmp(argv[i], "-flags") || !strcmp(argv[i], "-fflags"))
 31             av_dict_set(&opt, argv[i]+1, argv[i+1], 0);
 32     }
 33 
 34     /* allocate the output media context */
 35     avformat_alloc_output_context2(&oc, NULL, NULL, filename);
 36     if (!oc) {
 37         printf("Could not deduce output format from file extension: using MPEG.
");
 38         avformat_alloc_output_context2(&oc, NULL, "mpeg", filename);
 39     }
 40     if (!oc)
 41         return 1;
 42 
 43     fmt = oc->oformat;
 44 
 45     /* Add the audio and video streams using the default format codecs
 46      * and initialize the codecs. */
 47     if (fmt->video_codec != AV_CODEC_ID_NONE) {
 48         add_stream(&video_st, oc, &video_codec, fmt->video_codec);
 49         have_video = 1;
 50         encode_video = 1;
 51     }
 52     if (fmt->audio_codec != AV_CODEC_ID_NONE) {
 53         add_stream(&audio_st, oc, &audio_codec, fmt->audio_codec);
 54         have_audio = 1;
 55         encode_audio = 1;
 56     }
 57 
 58     /* Now that all the parameters are set, we can open the audio and
 59      * video codecs and allocate the necessary encode buffers. */
 60     if (have_video)
 61         open_video(oc, video_codec, &video_st, opt);
 62 
 63     if (have_audio)
 64         open_audio(oc, audio_codec, &audio_st, opt);
 65 
 66     av_dump_format(oc, 0, filename, 1);
 67 
 68     /* open the output file, if needed */
 69     if (!(fmt->flags & AVFMT_NOFILE)) {
 70         ret = avio_open(&oc->pb, filename, AVIO_FLAG_WRITE);
 71         if (ret < 0) {
 72             fprintf(stderr, "Could not open '%s': %s
", filename,
 73                     av_err2str(ret));
 74             return 1;
 75         }
 76     }
 77 
 78     /* Write the stream header, if any. */
 79     ret = avformat_write_header(oc, &opt);
 80     if (ret < 0) {
 81         fprintf(stderr, "Error occurred when opening output file: %s
",
 82                 av_err2str(ret));
 83         return 1;
 84     }
 85 
 86     while (encode_video || encode_audio) {
 87         /* select the stream to encode */
 88         if (encode_video &&
 89             (!encode_audio || av_compare_ts(video_st.next_pts, video_st.enc->time_base,
 90                                             audio_st.next_pts, audio_st.enc->time_base) <= 0)) {
 91             encode_video = !write_video_frame(oc, &video_st);
 92         } else {
 93             encode_audio = !write_audio_frame(oc, &audio_st);
 94         }
 95     }
 96 
 97     /* Write the trailer, if any. The trailer must be written before you
 98      * close the CodecContexts open when you wrote the header; otherwise
 99      * av_write_trailer() may try to use memory that was freed on
100      * av_codec_close(). */
101     av_write_trailer(oc);
102 
103     /* Close each codec. */
104     if (have_video)
105         close_stream(oc, &video_st);
106     if (have_audio)
107         close_stream(oc, &audio_st);
108 
109     if (!(fmt->flags & AVFMT_NOFILE))
110         /* Close the output file. */
111         avio_closep(&oc->pb);
112 
113     /* free the stream */
114     avformat_free_context(oc);
115 
116     return 0;
117 }

2：首先，定义了一个自定义结构体OutputStream，分别来代表一个音频，视频流。

 1 // a wrapper around a single output AVStream
 2 typedef struct OutputStream {
 3     AVStream *st;
 4     AVCodecContext *enc;
 5 
 6     /* pts of the next frame that will be generated */
 7     int64_t next_pts;
 8     int samples_count;
 9 
10     AVFrame *frame;
11     AVFrame *tmp_frame;
12 
13     float t, tincr, tincr2;
14 
15     struct SwsContext *sws_ctx;
16     struct SwrContext *swr_ctx;
17 } OutputStream;

　　st是创建的视频，音频流，enc为编码器上下文等等。

3：主函数中首先使用av_register_all()注册所有编码器和格式.

　　创建一个输出媒体上下文：avformat_alloc_output_context2(&oc,NULL,NULL,filename);

　　创建一个视屏轨道和音频轨道：add_stream(&video_st, oc, &video_codec, fmt->video_codec);

4：分析add_stream函数：

 1 static void add_stream(OutputStream *ost, AVFormatContext *oc,
 2                        AVCodec **codec,
 3                        enum AVCodecID codec_id)
 4 {
 5     AVCodecContext *c;
 6     int i;
 7 
 8     /* find the encoder */
 9     *codec = avcodec_find_encoder(codec_id);
10     if (!(*codec)) {
11         fprintf(stderr, "Could not find encoder for '%s'
",
12                 avcodec_get_name(codec_id));
13         exit(1);
14     }
15 
16     ost->st = avformat_new_stream(oc, NULL);
17     if (!ost->st) {
18         fprintf(stderr, "Could not allocate stream
");
19         exit(1);
20     }
21     ost->st->id = oc->nb_streams-1;
22     c = avcodec_alloc_context3(*codec);
23     if (!c) {
24         fprintf(stderr, "Could not alloc an encoding context
");
25         exit(1);
26     }
27     ost->enc = c;
28 
29     switch ((*codec)->type) {
30     case AVMEDIA_TYPE_AUDIO:
31         c->sample_fmt  = (*codec)->sample_fmts ?
32             (*codec)->sample_fmts[0] : AV_SAMPLE_FMT_FLTP;
33         c->bit_rate    = 64000;
34         c->sample_rate = 44100;
35         if ((*codec)->supported_samplerates) {
36             c->sample_rate = (*codec)->supported_samplerates[0];
37             for (i = 0; (*codec)->supported_samplerates[i]; i++) {
38                 if ((*codec)->supported_samplerates[i] == 44100)
39                     c->sample_rate = 44100;
40             }
41         }
42         c->channels        = av_get_channel_layout_nb_channels(c->channel_layout);
43         c->channel_layout = AV_CH_LAYOUT_STEREO;
44         if ((*codec)->channel_layouts) {
45             c->channel_layout = (*codec)->channel_layouts[0];
46             for (i = 0; (*codec)->channel_layouts[i]; i++) {
47                 if ((*codec)->channel_layouts[i] == AV_CH_LAYOUT_STEREO)
48                     c->channel_layout = AV_CH_LAYOUT_STEREO;
49             }
50         }
51         c->channels        = av_get_channel_layout_nb_channels(c->channel_layout);
52         ost->st->time_base = (AVRational){ 1, c->sample_rate };
53         break;
54 
55     case AVMEDIA_TYPE_VIDEO:
56         c->codec_id = codec_id;
57 
58         c->bit_rate = 400000;
59         /* Resolution must be a multiple of two. */
60         c->width    = 352;
61         c->height   = 288;
62         /* timebase: This is the fundamental unit of time (in seconds) in terms
63          * of which frame timestamps are represented. For fixed-fps content,
64          * timebase should be 1/framerate and timestamp increments should be
65          * identical to 1. */
66         ost->st->time_base = (AVRational){ 1, STREAM_FRAME_RATE };
67         c->time_base       = ost->st->time_base;
68 
69         c->gop_size      = 12; /* emit one intra frame every twelve frames at most */
70         c->pix_fmt       = STREAM_PIX_FMT;
71         if (c->codec_id == AV_CODEC_ID_MPEG2VIDEO) {
72             /* just for testing, we also add B-frames */
73             c->max_b_frames = 2;
74         }
75         if (c->codec_id == AV_CODEC_ID_MPEG1VIDEO) {
76             /* Needed to avoid using macroblocks in which some coeffs overflow.
77              * This does not happen with normal video, it just happens here as
78              * the motion of the chroma plane does not match the luma plane. */
79             c->mb_decision = 2;
80         }
81     break;
82 
83     default:
84         break;
85     }
86 
87     /* Some formats want stream headers to be separate. */
88     if (oc->oformat->flags & AVFMT_GLOBALHEADER)
89         c->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
90 }

5：创建完音频，视屏轨道后，则打开视频，音频编码器，准备编码数据 open_video(oc, video_codec, &video_st, opt);

 1 static void open_video(AVFormatContext *oc, AVCodec *codec, OutputStream *ost, AVDictionary *opt_arg)
 2 {
 3     int ret;
 4     AVCodecContext *c = ost->enc;
 5     AVDictionary *opt = NULL;
 6 
 7     av_dict_copy(&opt, opt_arg, 0);
 8 
 9     /* open the codec */
10     ret = avcodec_open2(c, codec, &opt);
11     av_dict_free(&opt);
12     if (ret < 0) {
13         fprintf(stderr, "Could not open video codec: %s
", av_err2str(ret));
14         exit(1);
15     }
16 
17     /* allocate and init a re-usable frame */
18     ost->frame = alloc_picture(c->pix_fmt, c->width, c->height);
19     if (!ost->frame) {
20         fprintf(stderr, "Could not allocate video frame
");
21         exit(1);
22     }
23 
24     /* If the output format is not YUV420P, then a temporary YUV420P
25      * picture is needed too. It is then converted to the required
26      * output format. */
27     ost->tmp_frame = NULL;
28     if (c->pix_fmt != AV_PIX_FMT_YUV420P) {
29         ost->tmp_frame = alloc_picture(AV_PIX_FMT_YUV420P, c->width, c->height);
30         if (!ost->tmp_frame) {
31             fprintf(stderr, "Could not allocate temporary picture
");
32             exit(1);
33         }
34     }
35 
36     /* copy the stream parameters to the muxer */
37     ret = avcodec_parameters_from_context(ost->st->codecpar, c);
38     if (ret < 0) {
39         fprintf(stderr, "Could not copy the stream parameters
");
40         exit(1);
41     }
42 }

注意：static AVFrame *alloc_picture(enum AVPixelFormat pix_fmt, int width, int height)，创建一个指定格式，宽和高的帧。

　　tmp_frame:如果格式不是YUV420P，则该帧用于转换格式。否则直接使用frame则可。

 1 static AVFrame *alloc_picture(enum AVPixelFormat pix_fmt, int width, int height)
 2 {
 3     AVFrame *picture;
 4     int ret;
 5 
 6     picture = av_frame_alloc();
 7     if (!picture)
 8         return NULL;
 9 
10     picture->format = pix_fmt;
11     picture->width  = width;
12     picture->height = height;
13 
14     /* allocate the buffers for the frame data */
15     ret = av_frame_get_buffer(picture, 32);
16     if (ret < 0) {
17         fprintf(stderr, "Could not allocate frame data.
");
18         exit(1);
19     }
20 
21     return picture;
22 }

6：打开输出文件：

1 if (!(fmt->flags & AVFMT_NOFILE)) {
2         ret = avio_open(&oc->pb, filename, AVIO_FLAG_WRITE);
3         if (ret < 0) {
4             fprintf(stderr, "Could not open '%s': %s
", filename,
5                     av_err2str(ret));
6             return 1;
7         }
8     }

7：写入文件头

1 ret = avformat_write_header(oc, &opt);
2     if (ret < 0) {
3         fprintf(stderr, "Error occurred when opening output file: %s
",
4                 av_err2str(ret));
5         return 1;
6     }

8：循环编码写入数据

 1 while (encode_video || encode_audio) {
 2         /* select the stream to encode */
 3         if (encode_video &&
 4             (!encode_audio || av_compare_ts(video_st.next_pts, video_st.enc->time_base,
 5                                             audio_st.next_pts, audio_st.enc->time_base) <= 0)) {
 6             encode_video = !write_video_frame(oc, &video_st);
 7         } else {
 8             encode_audio = !write_audio_frame(oc, &audio_st);
 9         }
10     }

　　分析static int write_video_frame(AVFormatContext *oc, OutputStream *ost)，编码一帧视屏，并且送入muxer（复合器），编码完成返回1，否则返回0

static int write_video_frame(AVFormatContext *oc, OutputStream *ost)
{
    int ret;
    AVCodecContext *c;
    AVFrame *frame;
    int got_packet = 0;
    AVPacket pkt = { 0 };

    c = ost->enc;

    frame = get_video_frame(ost);

    av_init_packet(&pkt);

    /* encode the image */
    ret = avcodec_encode_video2(c, &pkt, frame, &got_packet);
    if (ret < 0) {
        fprintf(stderr, "Error encoding video frame: %s
", av_err2str(ret));
        exit(1);
    }

    if (got_packet) {
        ret = write_frame(oc, &c->time_base, ost->st, &pkt);
    } else {
        ret = 0;
    }

    if (ret < 0) {
        fprintf(stderr, "Error while writing video frame: %s
", av_err2str(ret));
        exit(1);
    }

    return (frame || got_packet) ? 0 : 1;
}

　　创建一帧数据：frame = get_video_frame(ost);

 1 static AVFrame *get_video_frame(OutputStream *ost)
 2 {
 3     AVCodecContext *c = ost->enc;
 4 
 5     /* check if we want to generate more frames */
 6     if (av_compare_ts(ost->next_pts, c->time_base,
 7                       STREAM_DURATION, (AVRational){ 1, 1 }) >= 0)
 8         return NULL;
 9 
10     /* when we pass a frame to the encoder, it may keep a reference to it
11      * internally; make sure we do not overwrite it here */
12     if (av_frame_make_writable(ost->frame) < 0)
13         exit(1);
14 
15     if (c->pix_fmt != AV_PIX_FMT_YUV420P) {
16         /* as we only generate a YUV420P picture, we must convert it
17          * to the codec pixel format if needed */
18         if (!ost->sws_ctx) {
19             ost->sws_ctx = sws_getContext(c->width, c->height,
20                                           AV_PIX_FMT_YUV420P,
21                                           c->width, c->height,
22                                           c->pix_fmt,
23                                           SCALE_FLAGS, NULL, NULL, NULL);
24             if (!ost->sws_ctx) {
25                 fprintf(stderr,
26                         "Could not initialize the conversion context
");
27                 exit(1);
28             }
29         }
30         fill_yuv_image(ost->tmp_frame, ost->next_pts, c->width, c->height);
31         sws_scale(ost->sws_ctx,
32                   (const uint8_t * const *)ost->tmp_frame->data, ost->tmp_frame->linesize,
33                   0, c->height, ost->frame->data, ost->frame->linesize);
34     } else {
35         fill_yuv_image(ost->frame, ost->next_pts, c->width, c->height);
36     }
37 
38     ost->frame->pts = ost->next_pts++;
39 
40     return ost->frame;
41 }

　　使用static void fill_yuv_image(AVFrame *pict, int frame_index, int width, int height)填充一帧数据。

 1 static void fill_yuv_image(AVFrame *pict, int frame_index,
 2                            int width, int height)
 3 {
 4     int x, y, i;
 5 
 6     i = frame_index;
 7 
 8     /* Y */
 9     for (y = 0; y < height; y++)
10         for (x = 0; x < width; x++)
11             pict->data[0][y * pict->linesize[0] + x] = x + y + i * 3;
12 
13     /* Cb and Cr */
14     for (y = 0; y < height / 2; y++) {
15         for (x = 0; x < width / 2; x++) {
16             pict->data[1][y * pict->linesize[1] + x] = 128 + y + i * 2;
17             pict->data[2][y * pict->linesize[2] + x] = 64 + x + i * 5;
18         }
19     }
20 }

　　当编码成功后，使用自定义函数 write_frame(oc, &c->time_base, ost->st, &pkt)，将一帧数据包写入上下文。

 1 static int write_frame(AVFormatContext *fmt_ctx, const AVRational *time_base, AVStream *st, AVPacket *pkt)
 2 {
 3     /* rescale output packet timestamp values from codec to stream timebase */
 4     av_packet_rescale_ts(pkt, *time_base, st->time_base);
 5     pkt->stream_index = st->index;
 6 
 7     /* Write the compressed frame to the media file. */
 8     log_packet(fmt_ctx, pkt);
 9     return av_interleaved_write_frame(fmt_ctx, pkt);
10 }

9：写尾部，尾部必须在关闭编码器上下文之前写入：

1 av_write_trailer(oc);

10：最后编码每个编码器和输出文件，释放上下文

 1 /* Close each codec. */
 2     if (have_video)
 3         close_stream(oc, &video_st);
 4     if (have_audio)
 5         close_stream(oc, &audio_st);
 6 
 7     if (!(fmt->flags & AVFMT_NOFILE))
 8         /* Close the output file. */
 9         avio_closep(&oc->pb);
10 
11     /* free the stream */
12     avformat_free_context(oc);

11：close_stream的实现如下：

1 static void close_stream(AVFormatContext *oc, OutputStream *ost)
2 {
3     avcodec_free_context(&ost->enc);
4     av_frame_free(&ost->frame);
5     av_frame_free(&ost->tmp_frame);
6     sws_freeContext(ost->sws_ctx);
7     swr_free(&ost->swr_ctx);
8 }

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