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本文基于FFmpeg 4.1版本。

struct AVFrame定义于<libavutil/frame.h>

  1. struct AVFrame frame;

AVFrame中存储的是经过解码后的原始数据。在解码中，AVFrame是解码器的输出；在编码中，AVFrame是编码器的输入。下图中，“decoded frames”的数据类型就是AVFrame：


  2. _______ ______________
  3. | | | |
  4. | input | demuxer | encoded data | decoder
  5. | file | ---------> | packets | -----+
  6. |_______| |______________| |
  7. v
  8. _________
  9. | |
  10. | decoded |
  11. | frames |
  12. |_________|
  13. ________ ______________ |
  14. | | | | |
  15. | output | <-------- | encoded data | <----+
  16. | file | muxer | packets | encoder
  17. |________| |______________|

AVFrame数据结构非常重要，它的成员非常多，导致数据结构定义篇幅很长。下面引用的数据结构定义中省略冗长的注释以及大部分成员，先总体说明AVFrame的用法，然后再将一些重要成员摘录出来单独进行说明：

  1. /**
  2.  * This structure describes decoded (raw) audio or video data.
  3.  *
  4.  * AVFrame must be allocated using av_frame_alloc(). Note that this only
  5.  * allocates the AVFrame itself, the buffers for the data must be managed
  6.  * through other means (see below).
  7.  * AVFrame must be freed with av_frame_free().
  8.  *
  9.  * AVFrame is typically allocated once and then reused multiple times to hold
  10.  * different data (e.g. a single AVFrame to hold frames received from a
  11.  * decoder). In such a case, av_frame_unref() will free any references held by
  12.  * the frame and reset it to its original clean state before it
  13.  * is reused again.
  14.  *
  15.  * The data described by an AVFrame is usually reference counted through the
  16.  * AVBuffer API. The underlying buffer references are stored in AVFrame.buf /
  17.  * AVFrame.extended_buf. An AVFrame is considered to be reference counted if at
  18.  * least one reference is set, i.e. if AVFrame.buf[0] != NULL. In such a case,
  19.  * every single data plane must be contained in one of the buffers in
  20.  * AVFrame.buf or AVFrame.extended_buf.
  21.  * There may be a single buffer for all the data, or one separate buffer for
  22.  * each plane, or anything in between.
  23.  *
  24.  * sizeof(AVFrame) is not a part of the public ABI, so new fields may be added
  25.  * to the end with a minor bump.
  26.  *
  27.  * Fields can be accessed through AVOptions, the name string used, matches the
  28.  * C structure field name for fields accessible through AVOptions. The AVClass
  29.  * for AVFrame can be obtained from avcodec_get_frame_class()
  30.  */
  31. typedef struct AVFrame {
  32.     uint8_t *data[AV_NUM_DATA_POINTERS];
  33.     int linesize[AV_NUM_DATA_POINTERS];
  34.     uint8_t **extended_data;
  35.     int width, height;
  36.     int nb_samples;
  37.     int format;
  38.     int key_frame;
  39.     enum AVPictureType pict_type;
  40.     AVRational sample_aspect_ratio;
  41.     int64_t pts;
  42.     ......
  43. } AVFrame;

AVFrame的用法：

  1. AVFrame对象必须调用av_frame_alloc()在堆上分配，注意此处指的是AVFrame对象本身，AVFrame对象必须调用av_frame_free()进行销毁。
  2. AVFrame中包含的数据缓冲区是
  3. AVFrame通常只需分配一次，然后可以多次重用，每次重用前应调用av_frame_unref()将frame复位到原始的干净可用的状态。

下面将一些重要的成员摘录出来进行说明：
data

  1.     /**
  2.      * pointer to the picture/channel planes.
  3.      * This might be different from the first allocated byte
  4.      *
  5.      * Some decoders access areas outside 0,0 - width,height, please
  6.      * see avcodec_align_dimensions2(). Some filters and swscale can read
  7.      * up to 16 bytes beyond the planes, if these filters are to be used,
  8.      * then 16 extra bytes must be allocated.
  9.      *
  10.      * NOTE: Except for hwaccel formats, pointers not needed by the format
  11.      * MUST be set to NULL.
  12.      */
  13.     uint8_t *data[AV_NUM_DATA_POINTERS];

存储原始帧数据(未编码的原始图像或音频格式，作为解码器的输出或编码器的输入)。
data是一个指针数组，数组的每一个元素是一个指针，指向视频中图像的某一plane或音频中某一声道的plane。
关于图像plane的详细说明参考“色彩空间与像素格式”，音频plane的详细说明参数“原始音频格式”。下面简单说明：
对于packet格式，一幅YUV图像的Y、U、V交织存储在一个plane中，形如YUVYUV…，data[0]指向这个plane；
一个双声道的音频帧其左声道L、右声道R交织存储在一个plane中，形如LRLRLR…，data[0]指向这个plane。
对于planar格式，一幅YUV图像有Y、U、V三个plane，data[0]指向Y plane，data[1]指向U plane，data[2]指向V plane；
一个双声道的音频帧有左声道L和右声道R两个plane，data[0]指向L plane，data[1]指向R plane。

linesize

  1.     /**
  2.      * For video, size in bytes of each picture line.
  3.      * For audio, size in bytes of each plane.
  4.      *
  5.      * For audio, only linesize[0] may be set. For planar audio, each channel
  6.      * plane must be the same size.
  7.      *
  8.      * For video the linesizes should be multiples of the CPUs alignment
  9.      * preference, this is 16 or 32 for modern desktop CPUs.
  10.      * Some code requires such alignment other code can be slower without
  11.      * correct alignment, for yet other it makes no difference.
  12.      *
  13.      * @note The linesize may be larger than the size of usable data -- there
  14.      * may be extra padding present for performance reasons.
  15.      */
  16.     int linesize[AV_NUM_DATA_POINTERS];

对于视频来说，linesize是每行图像的大小(字节数)。注意有对齐要求。
对于音频来说，linesize是每个plane的大小(字节数)。音频只使用linesize[0]。对于planar音频来说，每个plane的大小必须一样。
linesize可能会因性能上的考虑而填充一些额外的数据，因此linesize可能比实际对应的音视频数据尺寸要大。

extended_data

  1.     /**
  2.      * pointers to the data planes/channels.
  3.      *
  4.      * For video, this should simply point to data[].
  5.      *
  6.      * For planar audio, each channel has a separate data pointer, and
  7.      * linesize[0] contains the size of each channel buffer.
  8.      * For packed audio, there is just one data pointer, and linesize[0]
  9.      * contains the total size of the buffer for all channels.
  10.      *
  11.      * Note: Both data and extended_data should always be set in a valid frame,
  12.      * but for planar audio with more channels that can fit in data,
  13.      * extended_data must be used in order to access all channels.
  14.      */
  15.     uint8_t **extended_data;

????extended_data是干啥的????
对于视频来说，直接指向data[]成员。
对于音频来说，packet格式音频只有一个plane，一个音频帧中各个声道的采样点交织存储在此plane中；planar格式音频每个声道一个plane。在多声道planar格式音频中，必须使用extended_data才能访问所有声道，什么意思？
在有效的视频/音频frame中，data和extended_data两个成员都必须设置有效值。

width, height

  1.     /**
  2.      * @name Video dimensions
  3.      * Video frames only. The coded dimensions (in pixels) of the video frame,
  4.      * i.e. the size of the rectangle that contains some well-defined values.
  5.      *
  6.      * @note The part of the frame intended for display/presentation is further
  7.      * restricted by the @ref cropping "Cropping rectangle".
  8.      * @{
  9.      */
  10.     int width, height;

视频帧宽和高(像素)。

nb_samples

  1.     /**
  2.      * number of audio samples (per channel) described by this frame
  3.      */
  4.     int nb_samples;

音频帧中单个声道中包含的采样点数。

format

  1.     /**
  2.      * format of the frame, -1 if unknown or unset
  3.      * Values correspond to enum AVPixelFormat for video frames,
  4.      * enum AVSampleFormat for audio)
  5.      */
  6.     int format;

帧格式。如果是未知格式或未设置，则值为-1。
对于视频帧，此值对应于“enum AVPixelFormat”结构：

  1. enum AVPixelFormat {
  2.     AV_PIX_FMT_NONE = -1,
  3.     AV_PIX_FMT_YUV420P,   ///< planar YUV 4:2:0, 12bpp, (1 Cr & Cb sample per 2x2 Y samples)
  4.     AV_PIX_FMT_YUYV422,   ///< packed YUV 4:2:2, 16bpp, Y0 Cb Y1 Cr
  5.     AV_PIX_FMT_RGB24,     ///< packed RGB 8:8:8, 24bpp, RGBRGB...
  6.     AV_PIX_FMT_BGR24,     ///< packed RGB 8:8:8, 24bpp, BGRBGR...
  7.     ......
  8. }

对于音频帧，此值对应于“enum AVSampleFormat”格式：

  1. enum AVSampleFormat {
  2.     AV_SAMPLE_FMT_NONE = -1,
  3.     AV_SAMPLE_FMT_U8,          ///< unsigned 8 bits
  4.     AV_SAMPLE_FMT_S16,         ///< signed 16 bits
  5.     AV_SAMPLE_FMT_S32,         ///< signed 32 bits
  6.     AV_SAMPLE_FMT_FLT,         ///< float
  7.     AV_SAMPLE_FMT_DBL,         ///< double
  9.     AV_SAMPLE_FMT_U8P,         ///< unsigned 8 bits, planar
  10.     AV_SAMPLE_FMT_S16P,        ///< signed 16 bits, planar
  11.     AV_SAMPLE_FMT_S32P,        ///< signed 32 bits, planar
  12.     AV_SAMPLE_FMT_FLTP,        ///< float, planar
  13.     AV_SAMPLE_FMT_DBLP,        ///< double, planar
  14.     AV_SAMPLE_FMT_S64,         ///< signed 64 bits
  15.     AV_SAMPLE_FMT_S64P,        ///< signed 64 bits, planar
  17.     AV_SAMPLE_FMT_NB           ///< Number of sample formats. DO NOT USE if linking dynamically
  18. };

key_frame

  1.     /**
  2.      * 1 -> keyframe, 0-> not
  3.      */
  4.     int key_frame;

视频帧是否是关键帧的标识，1->关键帧，0->非关键帧。

pict_type

  1.     /**
  2.      * Picture type of the frame.
  3.      */
  4.     enum AVPictureType pict_type;

视频帧类型(I、B、P等)。如下：

  1. /**
  2.  * @}
  3.  * @}
  4.  * @defgroup lavu_picture Image related
  5.  *
  6.  * AVPicture types, pixel formats and basic image planes manipulation.
  7.  *
  8.  * @{
  9.  */
  11. enum AVPictureType {
  12.     AV_PICTURE_TYPE_NONE = 0, ///< Undefined
  13.     AV_PICTURE_TYPE_I,     ///< Intra
  14.     AV_PICTURE_TYPE_P,     ///< Predicted
  15.     AV_PICTURE_TYPE_B,     ///< Bi-dir predicted
  16.     AV_PICTURE_TYPE_S,     ///< S(GMC)-VOP MPEG-4
  17.     AV_PICTURE_TYPE_SI,    ///< Switching Intra
  18.     AV_PICTURE_TYPE_SP,    ///< Switching Predicted
  19.     AV_PICTURE_TYPE_BI,    ///< BI type
  20. };

sample_aspect_ratio

  1.     /**
  2.      * Sample aspect ratio for the video frame, 0/1 if unknown/unspecified.
  3.      */
  4.     AVRational sample_aspect_ratio;

视频帧的宽高比。

pts

  1.     /**
  2.      * Presentation timestamp in time_base units (time when frame should be shown to user).
  3.      */
  4.     int64_t pts;

显示时间戳。单位是time_base。

pkt_pts

  1. #if FF_API_PKT_PTS
  2.     /**
  3.      * PTS copied from the AVPacket that was decoded to produce this frame.
  4.      * @deprecated use the pts field instead
  5.      */
  6.     attribute_deprecated
  7.     int64_t pkt_pts;
  8. #endif

此frame对应的packet中的显示时间戳。是从对应packet(解码生成此frame)中拷贝PTS得到此值。

pkt_dts

  1.     /**
  2.      * DTS copied from the AVPacket that triggered returning this frame. (if frame threading isn't used)
  3.      * This is also the Presentation time of this AVFrame calculated from
  4.      * only AVPacket.dts values without pts values.
  5.      */
  6.     int64_t pkt_dts;

此frame对应的packet中的解码时间戳。是从对应packet(解码生成此frame)中拷贝DTS得到此值。
如果对应的packet中只有dts而未设置pts，则此值也是此frame的pts。

coded_picture_number

  1.     /**
  2.      * picture number in bitstream order
  3.      */
  4.     int coded_picture_number;

在编码流中当前图像的序号。

display_picture_number

  1.     /**
  2.      * picture number in display order
  3.      */
  4.     int display_picture_number;

在显示序列中当前图像的序号。

interlaced_frame

  1.     /**
  2.      * The content of the picture is interlaced.
  3.      */
  4.     int interlaced_frame;

图像逐行/隔行模式标识。

sample_rate

  1.     /**
  2.      * Sample rate of the audio data.
  3.      */
  4.     int sample_rate;

音频采样率。

channel_layout

  1.     /**
  2.      * Channel layout of the audio data.
  3.      */
  4.     uint64_t channel_layout;

音频声道布局。每bit代表一个特定的声道，参考channel_layout.h中的定义，一目了然：

  1. /**
  2.  * @defgroup channel_masks Audio channel masks
  3.  *
  4.  * A channel layout is a 64-bits integer with a bit set for every channel.
  5.  * The number of bits set must be equal to the number of channels.
  6.  * The value 0 means that the channel layout is not known.
  7.  * @note this data structure is not powerful enough to handle channels
  8.  * combinations that have the same channel multiple times, such as
  9.  * dual-mono.
  10.  *
  11.  * @{
  12.  */
  13. #define AV_CH_FRONT_LEFT             0x00000001
  14. #define AV_CH_FRONT_RIGHT            0x00000002
  15. #define AV_CH_FRONT_CENTER           0x00000004
  16. #define AV_CH_LOW_FREQUENCY          0x00000008
  17. ......
  19. /**
  20.  * @}
  21.  * @defgroup channel_mask_c Audio channel layouts
  22.  * @{
  23.  * */
  24. #define AV_CH_LAYOUT_MONO              (AV_CH_FRONT_CENTER)
  25. #define AV_CH_LAYOUT_STEREO            (AV_CH_FRONT_LEFT|AV_CH_FRONT_RIGHT)
  26. #define AV_CH_LAYOUT_2POINT1           (AV_CH_LAYOUT_STEREO|AV_CH_LOW_FREQUENCY)

buf

  1.     /**
  2.      * AVBuffer references backing the data for this frame. If all elements of
  3.      * this array are NULL, then this frame is not reference counted. This array
  4.      * must be filled contiguously -- if buf[i] is non-NULL then buf[j] must
  5.      * also be non-NULL for all j < i.
  6.      *
  7.      * There may be at most one AVBuffer per data plane, so for video this array
  8.      * always contains all the references. For planar audio with more than
  9.      * AV_NUM_DATA_POINTERS channels, there may be more buffers than can fit in
  10.      * this array. Then the extra AVBufferRef pointers are stored in the
  11.      * extended_buf array.
  12.      */
  13.     AVBufferRef *buf[AV_NUM_DATA_POINTERS];

此帧的数据可以由AVBufferRef管理，AVBufferRef提供AVBuffer引用机制。这里涉及到缓冲区引用计数概念：
AVBuffer是FFmpeg中很常用的一种缓冲区，缓冲区使用引用计数(reference-counted)机制。
AVBufferRef则对AVBuffer缓冲区提供了一层封装，最主要的是作引用计数处理，实现了一种安全机制。用户不应直接访问AVBuffer，应通过AVBufferRef来访问AVBuffer，以保证安全。
FFmpeg中很多基础的数据结构都包含了AVBufferRef成员，来间接使用AVBuffer缓冲区。
相关内容参考“FFmpeg数据结构AVBuffer”
????帧的数据缓冲区AVBuffer就是前面的data成员，用户不应直接使用data成员，应通过buf成员间接使用data成员。那extended_data又是做什么的呢????

如果buf[]的所有元素都为NULL，则此帧不会被引用计数。必须连续填充buf[] – 如果buf[i]为非NULL，则对于所有j<i，buf[j]也必须为非NULL。
每个plane最多可以有一个AVBuffer，一个AVBufferRef指针指向一个AVBuffer，一个AVBuffer引用指的就是一个AVBufferRef指针。
对于视频来说，buf[]包含所有AVBufferRef指针。对于具有多于AV_NUM_DATA_POINTERS个声道的planar音频来说，可能buf[]存不下所有的AVBbufferRef指针，多出的AVBufferRef指针存储在extended_buf数组中。

extended_buf&nb_extended_buf

  1.     /**
  2.      * For planar audio which requires more than AV_NUM_DATA_POINTERS
  3.      * AVBufferRef pointers, this array will hold all the references which
  4.      * cannot fit into AVFrame.buf.
  5.      *
  6.      * Note that this is different from AVFrame.extended_data, which always
  7.      * contains all the pointers. This array only contains the extra pointers,
  8.      * which cannot fit into AVFrame.buf.
  9.      *
  10.      * This array is always allocated using av_malloc() by whoever constructs
  11.      * the frame. It is freed in av_frame_unref().
  12.      */
  13.     AVBufferRef **extended_buf;
  14.     /**
  15.      * Number of elements in extended_buf.
  16.      */
  17.     int        nb_extended_buf;

对于具有多于AV_NUM_DATA_POINTERS个声道的planar音频来说，可能buf[]存不下所有的AVBbufferRef指针，多出的AVBufferRef指针存储在extended_buf数组中。
注意此处的extended_buf和AVFrame.extended_data的不同，AVFrame.extended_data包含所有指向各plane的指针，而extended_buf只包含AVFrame.buf中装不下的指针。
extended_buf是构造frame时av_frame_alloc()中自动调用av_malloc()来分配空间的。调用av_frame_unref会释放掉extended_buf。
nb_extended_buf是extended_buf中的元素数目。

best_effort_timestamp

  1.     /**
  2.      * frame timestamp estimated using various heuristics, in stream time base
  3.      * - encoding: unused
  4.      * - decoding: set by libavcodec, read by user.
  5.      */
  6.     int64_t best_effort_timestamp;

????

pkt_pos

  1.     /**
  2.      * reordered pos from the last AVPacket that has been input into the decoder
  3.      * - encoding: unused
  4.      * - decoding: Read by user.
  5.      */
  6.     int64_t pkt_pos;

记录最后一个扔进解码器的packet在输入文件中的位置偏移量。

pkt_duration

  1.     /**
  2.      * duration of the corresponding packet, expressed in
  3.      * AVStream->time_base units, 0 if unknown.
  4.      * - encoding: unused
  5.      * - decoding: Read by user.
  6.      */
  7.     int64_t pkt_duration;

对应packet的时长，单位是AVStream->time_base。

channels

  1.     /**
  2.      * number of audio channels, only used for audio.
  3.      * - encoding: unused
  4.      * - decoding: Read by user.
  5.      */
  6.     int channels;

音频声道数量。

pkt_size

  1.     /**
  2.      * size of the corresponding packet containing the compressed
  3.      * frame.
  4.      * It is set to a negative value if unknown.
  5.      * - encoding: unused
  6.      * - decoding: set by libavcodec, read by user.
  7.      */
  8.     int pkt_size;

对应packet的大小。

crop_

  1.     /**
  2.      * @anchor cropping
  3.      * @name Cropping
  4.      * Video frames only. The number of pixels to discard from the the
  5.      * top/bottom/left/right border of the frame to obtain the sub-rectangle of
  6.      * the frame intended for presentation.
  7.      * @{
  8.      */
  9.     size_t crop_top;
  10.     size_t crop_bottom;
  11.     size_t crop_left;
  12.     size_t crop_right;
  13.     /**
  14.      * @}
  15.      */

用于视频帧图像裁切。四个值分别为从frame的上/下/左/右边界裁切的像素数。

  1. /**
  2.  * Allocate an AVFrame and set its fields to default values.  The resulting
  3.  * struct must be freed using av_frame_free().
  4.  *
  5.  * @return An AVFrame filled with default values or NULL on failure.
  6.  *
  7.  * @note this only allocates the AVFrame itself, not the data buffers. Those
  8.  * must be allocated through other means, e.g. with av_frame_get_buffer() or
  9.  * manually.
  10.  */
  11. AVFrame *av_frame_alloc(void);

构造一个frame，对象各成员被设为默认值。
此函数只分配AVFrame对象本身，而不分配AVFrame中的数据缓冲区。

  1. /**
  2.  * Free the frame and any dynamically allocated objects in it,
  3.  * e.g. extended_data. If the frame is reference counted, it will be
  4.  * unreferenced first.
  5.  *
  6.  * @param frame frame to be freed. The pointer will be set to NULL.
  7.  */
  8. void av_frame_free(AVFrame **frame);

释放一个frame。

  1. /**
  2.  * Set up a new reference to the data described by the source frame.
  3.  *
  4.  * Copy frame properties from src to dst and create a new reference for each
  5.  * AVBufferRef from src.
  6.  *
  7.  * If src is not reference counted, new buffers are allocated and the data is
  8.  * copied.
  9.  *
  10.  * @warning: dst MUST have been either unreferenced with av_frame_unref(dst),
  11.  *           or newly allocated with av_frame_alloc() before calling this
  12.  *           function, or undefined behavior will occur.
  13.  *
  14.  * @return 0 on success, a negative AVERROR on error
  15.  */
  16. int av_frame_ref(AVFrame *dst, const AVFrame *src);

为src中的数据建立一个新的引用。
将src中帧的各属性拷到dst中，并且为src中每个AVBufferRef创建一个新的引用。
如果src未使用引用计数，则dst中会分配新的数据缓冲区，将将src中缓冲区的数据拷贝到dst中的缓冲区。

  1. /**
  2.  * Create a new frame that references the same data as src.
  3.  *
  4.  * This is a shortcut for av_frame_alloc()+av_frame_ref().
  5.  *
  6.  * @return newly created AVFrame on success, NULL on error.
  7.  */
  8. AVFrame *av_frame_clone(const AVFrame *src);

创建一个新的frame，新的frame和src使用同一数据缓冲区，缓冲区管理使用引用计数机制。
本函数相当于av_frame_alloc()+av_frame_ref()

  1. /**
  2.  * Unreference all the buffers referenced by frame and reset the frame fields.
  3.  */
  4. void av_frame_unref(AVFrame *frame);

解除本frame对本frame中所有缓冲区的引用，并复位frame中各成员。

  1. /**
  2.  * Move everything contained in src to dst and reset src.
  3.  *
  4.  * @warning: dst is not unreferenced, but directly overwritten without reading
  5.  *           or deallocating its contents. Call av_frame_unref(dst) manually
  6.  *           before calling this function to ensure that no memory is leaked.
  7.  */
  8. void av_frame_move_ref(AVFrame *dst, AVFrame *src);

将src中所有数据拷贝到dst中，并复位src。
为避免内存泄漏，在调用av_frame_move_ref(dst, src)之前应先调用av_frame_unref(dst) 。

  1. /**
  2.  * Allocate new buffer(s) for audio or video data.
  3.  *
  4.  * The following fields must be set on frame before calling this function:
  5.  * - format (pixel format for video, sample format for audio)
  6.  * - width and height for video
  7.  * - nb_samples and channel_layout for audio
  8.  *
  9.  * This function will fill AVFrame.data and AVFrame.buf arrays and, if
  10.  * necessary, allocate and fill AVFrame.extended_data and AVFrame.extended_buf.
  11.  * For planar formats, one buffer will be allocated for each plane.
  12.  *
  13.  * @warning: if frame already has been allocated, calling this function will
  14.  *           leak memory. In addition, undefined behavior can occur in certain
  15.  *           cases.
  16.  *
  17.  * @param frame frame in which to store the new buffers.
  18.  * @param align Required buffer size alignment. If equal to 0, alignment will be
  19.  *              chosen automatically for the current CPU. It is highly
  20.  *              recommended to pass 0 here unless you know what you are doing.
  21.  *
  22.  * @return 0 on success, a negative AVERROR on error.
  23.  */
  24. int av_frame_get_buffer(AVFrame *frame, int align);

为音频或视频数据分配新的缓冲区。
调用本函数前，帧中的如下成员必须先设置好：

  • format (视频像素格式或音频采样格式)
  • width、height(视频画面和宽和高)
  • nb_samples、channel_layout(音频单个声道中的采样点数目和声道布局)

本函数会填充AVFrame.data和AVFrame.buf数组，如果有需要，还会分配和填充AVFrame.extended_data和AVFrame.extended_buf。
对于planar格式，会为每个plane分配一个缓冲区。

  1. /**
  2.  * Copy the frame data from src to dst.
  3.  *
  4.  * This function does not allocate anything, dst must be already initialized and
  5.  * allocated with the same parameters as src.
  6.  *
  7.  * This function only copies the frame data (i.e. the contents of the data /
  8.  * extended data arrays), not any other properties.
  9.  *
  10.  * @return >= 0 on success, a negative AVERROR on error.
  11.  */
  12. int av_frame_copy(AVFrame *dst, const AVFrame *src);

将src中的帧数据拷贝到dst中。
本函数并不会有任何分配缓冲区的动作，调用此函数前dst必须已经使用了和src同样的参数完成了初始化。
本函数只拷贝帧中的数据缓冲区的内容(data/extended_data数组中的内容)，而不涉及帧中任何其他的属性。

[1] FFMPEG结构体分析：AVFrame, https://blog.csdn.net/leixiaohua1020/article/details/14214577

2019-01-13 V1.0 初稿

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