本文为作者原创，转载请注明出处：https://www.cnblogs.com/leisure_chn/p/10404502.html
本文基于FFmpeg 4.1版本。
1. 数据结构定义
struct AVFrame定义于<libavutil/frame.h>
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：
_______ ______________
| | | |
| input | demuxer | encoded data | decoder
| file | ---------> | packets | -----+
|_______| |______________| |
v
_________
| |
| decoded |
| frames |
|_________|
________ ______________ |
| | | | |
| output | <-------- | encoded data | <----+
| file | muxer | packets | encoder
|________| |______________|
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：
/**
* This structure describes decoded (raw) audio or video data.
*
* AVFrame must be allocated using av_frame_alloc(). Note that this only
* allocates the AVFrame itself, the buffers for the data must be managed
* through other means (see below).
* AVFrame must be freed with av_frame_free().
*
* AVFrame is typically allocated once and then reused multiple times to hold
* different data (e.g. a single AVFrame to hold frames received from a
* decoder). In such a case, av_frame_unref() will free any references held by
* the frame and reset it to its original clean state before it
* is reused again.
*
* The data described by an AVFrame is usually reference counted through the
* AVBuffer API. The underlying buffer references are stored in AVFrame.buf /
* AVFrame.extended_buf. An AVFrame is considered to be reference counted if at
* least one reference is set, i.e. if AVFrame.buf[0] != NULL. In such a case,
* every single data plane must be contained in one of the buffers in
* AVFrame.buf or AVFrame.extended_buf.
* There may be a single buffer for all the data, or one separate buffer for
* each plane, or anything in between.
*
* sizeof(AVFrame) is not a part of the public ABI, so new fields may be added
* to the end with a minor bump.
*
* Fields can be accessed through AVOptions, the name string used, matches the
* C structure field name for fields accessible through AVOptions. The AVClass
* for AVFrame can be obtained from avcodec_get_frame_class()
*/
typedef struct AVFrame {
uint8_t *data[AV_NUM_DATA_POINTERS];
int linesize[AV_NUM_DATA_POINTERS];
uint8_t **extended_data;
int width, height;
int nb_samples;
int format;
int key_frame;
enum AVPictureType pict_type;
AVRational sample_aspect_ratio;
int64_t pts;
......
} AVFrame;AVFrame的用法：
- 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。
- AVFrame中包含的数据缓冲区是
- 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
/**
* pointer to the picture/channel planes.
* This might be different from the first allocated byte
*
* Some decoders access areas outside 0,0 - width,height, please
* see avcodec_align_dimensions2(). Some filters and swscale can read
* up to 16 bytes beyond the planes, if these filters are to be used,
* then 16 extra bytes must be allocated.
*
* NOTE: Except for hwaccel formats, pointers not needed by the format
* MUST be set to NULL.
*/
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“ffplay源码解析6-音频重采样 6.1.1节”。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
/**
* For video, size in bytes of each picture line.
* For audio, size in bytes of each plane.
*
* For audio, only linesize[0] may be set. For planar audio, each channel
* plane must be the same size.
*
* For video the linesizes should be multiples of the CPUs alignment
* preference, this is 16 or 32 for modern desktop CPUs.
* Some code requires such alignment other code can be slower without
* correct alignment, for yet other it makes no difference.
*
* @note The linesize may be larger than the size of usable data -- there
* may be extra padding present for performance reasons.
*/
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
/**
* pointers to the data planes/channels.
*
* For video, this should simply point to data[].
*
* For planar audio, each channel has a separate data pointer, and
* linesize[0] contains the size of each channel buffer.
* For packed audio, there is just one data pointer, and linesize[0]
* contains the total size of the buffer for all channels.
*
* Note: Both data and extended_data should always be set in a valid frame,
* but for planar audio with more channels that can fit in data,
* extended_data must be used in order to access all channels.
*/
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
/**
* @name Video dimensions
* Video frames only. The coded dimensions (in pixels) of the video frame,
* i.e. the size of the rectangle that contains some well-defined values.
*
* @note The part of the frame intended for display/presentation is further
* restricted by the @ref cropping "Cropping rectangle".
* @{
*/
int width, height;视频帧宽和高(像素)。
nb_samples
/**
* number of audio samples (per channel) described by this frame
*/
int nb_samples;音频帧中单个声道中包含的采样点数。
format
/**
* format of the frame, -1 if unknown or unset
* Values correspond to enum AVPixelFormat for video frames,
* enum AVSampleFormat for audio)
*/
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：
enum AVPixelFormat {
AV_PIX_FMT_NONE = -1,
AV_PIX_FMT_YUV420P, ///< planar YUV 4:2:0, 12bpp, (1 Cr & Cb sample per 2x2 Y samples)
AV_PIX_FMT_YUYV422, ///< packed YUV 4:2:2, 16bpp, Y0 Cb Y1 Cr
AV_PIX_FMT_RGB24, ///< packed RGB 8:8:8, 24bpp, RGBRGB...
AV_PIX_FMT_BGR24, ///< packed RGB 8:8:8, 24bpp, BGRBGR...
......
}对于音频帧，此值对应于“enum AVSampleFormat”格式：
enum AVSampleFormat {
AV_SAMPLE_FMT_NONE = -1,
AV_SAMPLE_FMT_U8, ///< unsigned 8 bits
AV_SAMPLE_FMT_S16, ///< signed 16 bits
AV_SAMPLE_FMT_S32, ///< signed 32 bits
AV_SAMPLE_FMT_FLT, ///< float
AV_SAMPLE_FMT_DBL, ///< double
AV_SAMPLE_FMT_U8P, ///< unsigned 8 bits, planar
AV_SAMPLE_FMT_S16P, ///< signed 16 bits, planar
AV_SAMPLE_FMT_S32P, ///< signed 32 bits, planar
AV_SAMPLE_FMT_FLTP, ///< float, planar
AV_SAMPLE_FMT_DBLP, ///< double, planar
AV_SAMPLE_FMT_S64, ///< signed 64 bits
AV_SAMPLE_FMT_S64P, ///< signed 64 bits, planar
AV_SAMPLE_FMT_NB ///< Number of sample formats. DO NOT USE if linking dynamically
};key_frame
/**
* 1 -> keyframe, 0-> not
*/
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
/**
* Picture type of the frame.
*/
enum AVPictureType pict_type;视频帧类型(I、B、P等)。如下：
/**
* @}
* @}
* @defgroup lavu_picture Image related
*
* AVPicture types, pixel formats and basic image planes manipulation.
*
* @{
*/
enum AVPictureType {
AV_PICTURE_TYPE_NONE = 0, ///< Undefined
AV_PICTURE_TYPE_I, ///< Intra
AV_PICTURE_TYPE_P, ///< Predicted
AV_PICTURE_TYPE_B, ///< Bi-dir predicted
AV_PICTURE_TYPE_S, ///< S(GMC)-VOP MPEG-4
AV_PICTURE_TYPE_SI, ///< Switching Intra
AV_PICTURE_TYPE_SP, ///< Switching Predicted
AV_PICTURE_TYPE_BI, ///< BI type
};sample_aspect_ratio
/**
* Sample aspect ratio for the video frame, 0/1 if unknown/unspecified.
*/
AVRational sample_aspect_ratio;视频帧的宽高比。
pts
/**
* Presentation timestamp in time_base units (time when frame should be shown to user).
*/
int64_t pts;显示时间戳。单位是time_base。
pkt_pts
#if FF_API_PKT_PTS
/**
* PTS copied from the AVPacket that was decoded to produce this frame.
* @deprecated use the pts field instead
*/
attribute_deprecated
int64_t pkt_pts;
#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
/**
* DTS copied from the AVPacket that triggered returning this frame. (if frame threading isn't used)
* This is also the Presentation time of this AVFrame calculated from
* only AVPacket.dts values without pts values.
*/
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
/**
* picture number in bitstream order
*/
int coded_picture_number;在编码流中当前图像的序号。
display_picture_number
/**
* picture number in display order
*/
int display_picture_number;在显示序列中当前图像的序号。
interlaced_frame
/**
* The content of the picture is interlaced.
*/
int interlaced_frame;图像逐行/隔行模式标识。
sample_rate
/**
* Sample rate of the audio data.
*/
int sample_rate;音频采样率。
channel_layout
/**
* Channel layout of the audio data.
*/
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：
/**
* @defgroup channel_masks Audio channel masks
*
* A channel layout is a 64-bits integer with a bit set for every channel.
* The number of bits set must be equal to the number of channels.
* The value 0 means that the channel layout is not known.
* @note this data structure is not powerful enough to handle channels
* combinations that have the same channel multiple times, such as
* dual-mono.
*
* @{
*/
#define AV_CH_FRONT_LEFT 0x00000001
#define AV_CH_FRONT_RIGHT 0x00000002
#define AV_CH_FRONT_CENTER 0x00000004
#define AV_CH_LOW_FREQUENCY 0x00000008
......
/**
* @}
* @defgroup channel_mask_c Audio channel layouts
* @{
* */
#define AV_CH_LAYOUT_MONO (AV_CH_FRONT_CENTER)
#define AV_CH_LAYOUT_STEREO (AV_CH_FRONT_LEFT|AV_CH_FRONT_RIGHT)
#define AV_CH_LAYOUT_2POINT1 (AV_CH_LAYOUT_STEREO|AV_CH_LOW_FREQUENCY)buf
/**
* AVBuffer references backing the data for this frame. If all elements of
* this array are NULL, then this frame is not reference counted. This array
* must be filled contiguously -- if buf[i] is non-NULL then buf[j] must
* also be non-NULL for all j < i.
*
* There may be at most one AVBuffer per data plane, so for video this array
* always contains all the references. For planar audio with more than
* AV_NUM_DATA_POINTERS channels, there may be more buffers than can fit in
* this array. Then the extra AVBufferRef pointers are stored in the
* extended_buf array.
*/
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
/**
* For planar audio which requires more than AV_NUM_DATA_POINTERS
* AVBufferRef pointers, this array will hold all the references which
* cannot fit into AVFrame.buf.
*
* Note that this is different from AVFrame.extended_data, which always
* contains all the pointers. This array only contains the extra pointers,
* which cannot fit into AVFrame.buf.
*
* This array is always allocated using av_malloc() by whoever constructs
* the frame. It is freed in av_frame_unref().
*/
AVBufferRef **extended_buf;
/**
* Number of elements in extended_buf.
*/
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
/**
* frame timestamp estimated using various heuristics, in stream time base
* - encoding: unused
* - decoding: set by libavcodec, read by user.
*/
int64_t best_effort_timestamp;????
pkt_pos
/**
* reordered pos from the last AVPacket that has been input into the decoder
* - encoding: unused
* - decoding: Read by user.
*/
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
/**
* duration of the corresponding packet, expressed in
* AVStream->time_base units, 0 if unknown.
* - encoding: unused
* - decoding: Read by user.
*/
int64_t pkt_duration;对应packet的时长，单位是AVStream->time_base。
channels
/**
* number of audio channels, only used for audio.
* - encoding: unused
* - decoding: Read by user.
*/
int channels;音频声道数量。
pkt_size
/**
* size of the corresponding packet containing the compressed
* frame.
* It is set to a negative value if unknown.
* - encoding: unused
* - decoding: set by libavcodec, read by user.
*/
int pkt_size;对应packet的大小。
crop_
/**
* @anchor cropping
* @name Cropping
* Video frames only. The number of pixels to discard from the the
* top/bottom/left/right border of the frame to obtain the sub-rectangle of
* the frame intended for presentation.
* @{
*/
size_t crop_top;
size_t crop_bottom;
size_t crop_left;
size_t crop_right;
/**
* @}
*/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。
2. 相关函数使用说明
2.1 av_frame_alloc()
/**
* Allocate an AVFrame and set its fields to default values. The resulting
* struct must be freed using av_frame_free().
*
* @return An AVFrame filled with default values or NULL on failure.
*
* @note this only allocates the AVFrame itself, not the data buffers. Those
* must be allocated through other means, e.g. with av_frame_get_buffer() or
* manually.
*/
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。
2.2 av_frame_free()
/**
* Free the frame and any dynamically allocated objects in it,
* e.g. extended_data. If the frame is reference counted, it will be
* unreferenced first.
*
* @param frame frame to be freed. The pointer will be set to NULL.
*/
void av_frame_free(AVFrame **frame);释放一个frame。
2.3 av_frame_ref()
/**
* Set up a new reference to the data described by the source frame.
*
* Copy frame properties from src to dst and create a new reference for each
* AVBufferRef from src.
*
* If src is not reference counted, new buffers are allocated and the data is
* copied.
*
* @warning: dst MUST have been either unreferenced with av_frame_unref(dst),
* or newly allocated with av_frame_alloc() before calling this
* function, or undefined behavior will occur.
*
* @return 0 on success, a negative AVERROR on error
*/
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。
2.4 av_frame_clone()
/**
* Create a new frame that references the same data as src.
*
* This is a shortcut for av_frame_alloc()+av_frame_ref().
*
* @return newly created AVFrame on success, NULL on error.
*/
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()
2.5 av_frame_unref()
/**
* Unreference all the buffers referenced by frame and reset the frame fields.
*/
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。
2.6 av_frame_move_ref()
/**
* Move everything contained in src to dst and reset src.
*
* @warning: dst is not unreferenced, but directly overwritten without reading
* or deallocating its contents. Call av_frame_unref(dst) manually
* before calling this function to ensure that no memory is leaked.
*/
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) 。
2.7 av_frame_get_buffer()
/**
* Allocate new buffer(s) for audio or video data.
*
* The following fields must be set on frame before calling this function:
* - format (pixel format for video, sample format for audio)
* - width and height for video
* - nb_samples and channel_layout for audio
*
* This function will fill AVFrame.data and AVFrame.buf arrays and, if
* necessary, allocate and fill AVFrame.extended_data and AVFrame.extended_buf.
* For planar formats, one buffer will be allocated for each plane.
*
* @warning: if frame already has been allocated, calling this function will
* leak memory. In addition, undefined behavior can occur in certain
* cases.
*
* @param frame frame in which to store the new buffers.
* @param align Required buffer size alignment. If equal to 0, alignment will be
* chosen automatically for the current CPU. It is highly
* recommended to pass 0 here unless you know what you are doing.
*
* @return 0 on success, a negative AVERROR on error.
*/
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。
2.8 av_frame_copy()
/**
* Copy the frame data from src to dst.
*
* This function does not allocate anything, dst must be already initialized and
* allocated with the same parameters as src.
*
* This function only copies the frame data (i.e. the contents of the data /
* extended data arrays), not any other properties.
*
* @return >= 0 on success, a negative AVERROR on error.
*/
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。
3. 参考资料
[1] FFMPEG结构体分析：AVFrame, https://blog.csdn.net/leixiaohua1020/article/details/14214577
4. 修改记录
2019-01-13 V1.0 初稿