FFMPEG解码学习笔记(一)

FFMPEG的基本处理流程

音频视频同步资料

av_frame_get_pkt_duration(frame)

获取当前帧的持续时间

av_frame_get_pkt_pos(frame)

从最后一个AVPacket进入解码重新排序的偏移量

av_frame_get_best_effort_timestamp(frame)

已流中的时间为基础预估的时间戳

1. /**
2. * When decoding, this signals how much the picture must be delayed.
3. * extra_delay = repeat_pict / (2*fps)
4. */
5. int repeat_pict;

当解码时，这个信号告诉你这张图片需要要延迟多少久。

需要求出扩展延时：

extra_delay = repeat_pict / (2*fps)

1. /**
2. * This is the fundamental unit of time (in seconds) in terms
3. * of which frame timestamps are represented. For fixed-fps content,
4. * timebase should be 1/framerate and timestamp increments should be
5. * identically 1.
6. * This often, but not always is the inverse of the frame rate or field rate
7. * for video. 1/time_base is not the average frame rate if the frame rate is not
8. * constant.
9. *
10. * Like containers, elementary streams also can store timestamps, 1/time_base
11. * is the unit in which these timestamps are specified.
12. * As example of such codec time base see ISO/IEC 14496-2:2001(E)
13. * vop_time_increment_resolution and fixed_vop_rate
14. * (fixed_vop_rate == 0 implies that it is different from the framerate)
15. *
16. * - encoding: MUST be set by user.
17. * - decoding: the use of this field for decoding is deprecated.
18. * Use framerate instead.
19. */
20. AVRational time_base;

当前帧需要耗时多少秒，每一帧显示的时间

由以上两个定义可以推导：

extra_delay = repeat_pict / (2*fps)

fps=1/time_base

那么

extra_delay= repeat_pict*time_base*0.5

AVCodecContent

1. /**
2. * Allocate an AVCodecContext and set its fields to default values. The
3. * resulting struct should be freed with avcodec_free_context().
4. *
5. * @param codec if non-NULL, allocate private data and initialize defaults
6. * for the given codec. It is illegal to then call avcodec_open2()
7. * with a different codec.
8. * If NULL, then the codec-specific defaults won't be initialized,
9. * which may result in suboptimal default settings (this is
10. * important mainly for encoders, e.g. libx264).
11. *
12. * @return An AVCodecContext filled with default values or NULL on failure.
13. */
14. AVCodecContext *avcodec_alloc_context3(const AVCodec *codec);

分配一个AVCodecContent并为其分配默认值。（可理解为一个AVCodecContent分配内存空间）

释放它的资源使用avcodec_free_context()。

int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);

1. /**
2. * Initialize the AVCodecContext to use the given AVCodec. Prior to using this
3. * function the context has to be allocated with avcodec_alloc_context3().
4. *
5. * The functions avcodec_find_decoder_by_name(), avcodec_find_encoder_by_name(),
6. * avcodec_find_decoder() and avcodec_find_encoder() provide an easy way for
7. * retrieving a codec.
8. *
9. * @warning This function is not thread safe!
10. *
11. * @note Always call this function before using decoding routines (such as
12. * @ref avcodec_receive_frame()).
13. *
14. * @code
15. * avcodec_register_all();
16. * av_dict_set(&opts, "b", "2.5M", 0);
17. * codec = avcodec_find_decoder(AV_CODEC_ID_H264);
18. * if (!codec)
19. * exit(1);
20. *
21. * context = avcodec_alloc_context3(codec);
22. *
23. * if (avcodec_open2(context, codec, opts) < 0)
24. * exit(1);
25. * @endcode
26. *
27. * @param avctx The context to initialize.
28. * @param codec The codec to open this context for. If a non-NULL codec has been
29. * previously passed to avcodec_alloc_context3() or
30. * for this context, then this parameter MUST be either NULL or
31. * equal to the previously passed codec.
32. * @param options A dictionary filled with AVCodecContext and codec-private options.
33. * On return this object will be filled with options that were not found.
34. *
35. * @return zero on success, a negative value on error
36. * @see avcodec_alloc_context3(), avcodec_find_decoder(), avcodec_find_encoder(),
37. * av_dict_set(), av_opt_find().
38. */
39. int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);

使用AVCodec初始化AVCodecContext。在初始化之前需要为其分配内存。

1. /**
2. * Copy the settings of the source AVCodecContext into the destination
3. * AVCodecContext. The resulting destination codec context will be
4. * unopened, i.e. you are required to call avcodec_open2() before you
5. * can use this AVCodecContext to decode/encode video/audio data.
6. *
7. * @param dest target codec context, should be initialized with
8. * avcodec_alloc_context3(NULL), but otherwise uninitialized
9. * @param src source codec context
10. * @return AVERROR() on error (e.g. memory allocation error), 0 on success
11. *
12. * @deprecated The semantics of this function are ill-defined and it should not
13. * be used. If you need to transfer the stream parameters from one codec context
14. * to another, use an intermediate AVCodecParameters instance and the
15. * avcodec_parameters_from_context() / avcodec_parameters_to_context()
16. * functions.
17. */
18. attribute_deprecated
19. int avcodec_copy_context(AVCodecContext *dest, const AVCodecContext *src);

该函数已不建议使用。它可以将原AVCodecContext的设置拷贝到目标AVCodecContent。

如果在使用这个数据之前调用avcodec_open2()，会导致无法打开。

建议使用avcodec_parameters_from_context() / avcodec_parameters_to_context()传递参数。

1. /**
2. * Fill the parameters struct based on the values from the supplied codec
3. * context. Any allocated fields in par are freed and replaced with duplicates
4. * of the corresponding fields in codec.
5. *
6. * @return >= 0 on success, a negative AVERROR code on failure
7. */
8. int avcodec_parameters_from_context(AVCodecParameters *par,
9. const AVCodecContext *codec);

从一个AVCodecContent中拷贝出AVCodecParameters

1. /**
2. * Fill the codec context based on the values from the supplied codec
3. * parameters. Any allocated fields in codec that have a corresponding field in
4. * par are freed and replaced with duplicates of the corresponding field in par.
5. * Fields in codec that do not have a counterpart in par are not touched.
6. *
7. * @return >= 0 on success, a negative AVERROR code on failure.
8. */
9. int avcodec_parameters_to_context(AVCodecContext *codec,
10. const AVCodecParameters *par);

向一个AVCodecContent拷贝一个AVCodecParameters

1. /**
2. * Return the next frame of a stream.
3. * This function returns what is stored in the file, and does not validate
4. * that what is there are valid frames for the decoder. It will split what is
5. * stored in the file into frames and return one for each call. It will not
6. * omit invalid data between valid frames so as to give the decoder the maximum
7. * information possible for decoding.
8. *
9. * If pkt->buf is NULL, then the packet is valid until the next
10. * av_read_frame() or until avformat_close_input(). Otherwise the packet
11. * is valid indefinitely. In both cases the packet must be freed with
12. * av_packet_unref when it is no longer needed. For video, the packet contains
13. * exactly one frame. For audio, it contains an integer number of frames if each
14. * frame has a known fixed size (e.g. PCM or ADPCM data). If the audio frames
15. * have a variable size (e.g. MPEG audio), then it contains one frame.
16. *
17. * pkt->pts, pkt->dts and pkt->duration are always set to correct
18. * values in AVStream.time_base units (and guessed if the format cannot
19. * provide them). pkt->pts can be AV_NOPTS_VALUE if the video format
20. * has B-frames, so it is better to rely on pkt->dts if you do not
21. * decompress the payload.
22. *
23. * @return 0 if OK, < 0 on error or end of file
24. */
25. int av_read_frame(AVFormatContext *s, AVPacket *pkt);

返回下一帧的流