FFMPeg代码分析：AVFrame结构体及其相关的函数

AVFrame结构体保存的是解码后和原始的音视频信息。AVFrame通过函数av_frame_alloc()初始化，该函数仅仅分配AVFrame实例本身，而没有分配其内部的缓存。AVFrame实例由av_frame_free()释放；AVFrame实例通常分配一次，重复使用，如分配一个AVFrame实例来保留解码器中输出的视频帧（此时应在恰当的时候使用av_frame_unref()清理参考帧并将AVFrame归零）。该类所描述的数据通常由AVBuffer的API来保存一个引用计数，并保存于AVFrame.buf
/AVFrame.extended_buf，在至少存在一个参考的时候（如AVFrame.buf[0] != NULL），则该对象被标记为“被引用”。在此情况下，AVFrame所包含的每一组数据必须包含于AVFrame的缓存中。

AVFrame的定义如下：

typedef struct AVFrame {
#define AV_NUM_DATA_POINTERS 8
/**
* 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.
*/
uint8_t *data[AV_NUM_DATA_POINTERS];//指向实际音视频数据的指针

/**
* 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 multiplies 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];

/**
* 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;

/**
* width and height of the video frame
*/
int width, height;//视频的分辨率

/**
* number of audio samples (per channel) described by this frame
*/
int nb_samples;//音频采样频率

/**
* format of the frame, -1 if unknown or unset
* Values correspond to enum AVPixelFormat for video frames,
* enum AVSampleFormat for audio)
*/
int format;//解码后的视频帧的格式，为AVPixelFormat 枚举类型（YUV420，YUV422，RGB24等）

/**
* 1 -> keyframe, 0-> not
*/
int key_frame;//是否关键帧，1是0否

/**
* Picture type of the frame.
*/
enum AVPictureType pict_type;//帧类型，IPB

#if FF_API_AVFRAME_LAVC
attribute_deprecated
uint8_t *base[AV_NUM_DATA_POINTERS];
#endif

/**
* Sample aspect ratio for the video frame, 0/1 if unknown/unspecified.
*/
AVRational sample_aspect_ratio;//视频的宽高比

/**
* Presentation timestamp in time_base units (time when frame should be shown to user).
*/
int64_t pts;//当前帧的显示时间戳

/**
* PTS copied from the AVPacket that was decoded to produce this frame.
*/
int64_t pkt_pts;

/**
* DTS copied from the AVPacket that triggered returning this frame. (if frame threading isnt used)
* This is also the Presentation time of this AVFrame calculated from
* only AVPacket.dts values without pts values.
*/
int64_t pkt_dts;

/**
* picture number in bitstream order
*/
int coded_picture_number;//当前帧以解码顺序的编号
/**
* picture number in display order
*/
int display_picture_number;//当前帧以输出顺序的编号

/**
* quality (between 1 (good) and FF_LAMBDA_MAX (bad))
*/
int quality;

#if FF_API_AVFRAME_LAVC
attribute_deprecated
int reference;

/**
* QP table
*/
attribute_deprecated
int8_t *qscale_table;//量化参数表
/**
* QP store stride
*/
attribute_deprecated
int qstride;

attribute_deprecated
int qscale_type;

/**
* mbskip_table[mb]>=1 if MB didn't change
* stride= mb_width = (width+15)>>4
*/
attribute_deprecated
uint8_t *mbskip_table;

/**
* motion vector table
* @code
* example:
* int mv_sample_log2= 4 - motion_subsample_log2;
* int mb_width= (width+15)>>4;
* int mv_stride= (mb_width << mv_sample_log2) + 1;
* motion_val[direction][x + y*mv_stride][0->mv_x, 1->mv_y];
* @endcode
*/
attribute_deprecated
int16_t (*motion_val[2])[2];//运动矢量表

/**
* macroblock type table
* mb_type_base + mb_width + 2
*/
attribute_deprecated
uint32_t *mb_type;//宏块类型

/**
* DCT coefficients
*/
attribute_deprecated
short *dct_coeff;//变换系数

/**
* motion reference frame index
* the order in which these are stored can depend on the codec.
*/
attribute_deprecated
int8_t *ref_index[2];//参考帧索引
#endif

/**
* for some private data of the user
*/
void *opaque;

/**
* error
*/
uint64_t error[AV_NUM_DATA_POINTERS];

#if FF_API_AVFRAME_LAVC
attribute_deprecated
int type;
#endif

/**
* When decoding, this signals how much the picture must be delayed.
* extra_delay = repeat_pict / (2*fps)
*/
int repeat_pict;//表示有多少帧被延迟显示，延迟即重复。

/**
* The content of the picture is interlaced.
*/
int interlaced_frame;//隔行视频标识

/**
* If the content is interlaced, is top field displayed first.
*/
int top_field_first;

/**
* Tell user application that palette has changed from previous frame.
*/
int palette_has_changed;

#if FF_API_AVFRAME_LAVC
attribute_deprecated
int buffer_hints;

/**
* Pan scan.
*/
attribute_deprecated
struct AVPanScan *pan_scan;
#endif

/**
* reordered opaque 64bit (generally an integer or a double precision float
* PTS but can be anything).
* The user sets AVCodecContext.reordered_opaque to represent the input at
* that time,
* the decoder reorders values as needed and sets AVFrame.reordered_opaque
* to exactly one of the values provided by the user through AVCodecContext.reordered_opaque
* @deprecated in favor of pkt_pts
*/
int64_t reordered_opaque;

#if FF_API_AVFRAME_LAVC
/**
* @deprecated this field is unused
*/
attribute_deprecated void *hwaccel_picture_private;

attribute_deprecated
struct AVCodecContext *owner;
attribute_deprecated
void *thread_opaque;

/**
* log2 of the size of the block which a single vector in motion_val represents:
* (4->16x16, 3->8x8, 2-> 4x4, 1-> 2x2)
*/
attribute_deprecated
uint8_t motion_subsample_log2;
#endif

/**
* Sample rate of the audio data.
*/
int sample_rate;

/**
* Channel layout of the audio data.
*/
uint64_t channel_layout;

/**
* AVBuffer references backing the data for this frame. If all elements of
* this array are NULL, then this frame is not reference counted.
*
* 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];

/**
* 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;

AVFrameSideData **side_data;
int            nb_side_data;

/**
* frame timestamp estimated using various heuristics, in stream time base
* Code outside libavcodec should access this field using:
* av_frame_get_best_effort_timestamp(frame)
* - encoding: unused
* - decoding: set by libavcodec, read by user.
*/
int64_t best_effort_timestamp;

/**
* reordered pos from the last AVPacket that has been input into the decoder
* Code outside libavcodec should access this field using:
* av_frame_get_pkt_pos(frame)
* - encoding: unused
* - decoding: Read by user.
*/
int64_t pkt_pos;

/**
* duration of the corresponding packet, expressed in
* AVStream->time_base units, 0 if unknown.
* Code outside libavcodec should access this field using:
* av_frame_get_pkt_duration(frame)
* - encoding: unused
* - decoding: Read by user.
*/
int64_t pkt_duration;

/**
* metadata.
* Code outside libavcodec should access this field using:
* av_frame_get_metadata(frame)
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
AVDictionary *metadata;

/**
* decode error flags of the frame, set to a combination of
* FF_DECODE_ERROR_xxx flags if the decoder produced a frame, but there
* were errors during the decoding.
* Code outside libavcodec should access this field using:
* av_frame_get_decode_error_flags(frame)
* - encoding: unused
* - decoding: set by libavcodec, read by user.
*/
int decode_error_flags;
#define FF_DECODE_ERROR_INVALID_BITSTREAM   1
#define FF_DECODE_ERROR_MISSING_REFERENCE   2

/**
* number of audio channels, only used for audio.
* Code outside libavcodec should access this field using:
* av_frame_get_channels(frame)
* - encoding: unused
* - decoding: Read by user.
*/
int channels;

/**
* size of the corresponding packet containing the compressed
* frame. It must be accessed using av_frame_get_pkt_size() and
* av_frame_set_pkt_size().
* It is set to a negative value if unknown.
* - encoding: unused
* - decoding: set by libavcodec, read by user.
*/
int pkt_size;

/**
* YUV colorspace type.
* It must be accessed using av_frame_get_colorspace() and
* av_frame_set_colorspace().
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
enum AVColorSpace colorspace;

/**
* MPEG vs JPEG YUV range.
* It must be accessed using av_frame_get_color_range() and
* av_frame_set_color_range().
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
enum AVColorRange color_range;

/**
* Not to be accessed directly from outside libavutil
*/
AVBufferRef *qp_table_buf;
} AVFrame;

通过对其内部的成员变量的研究可以看出，除了基本的储存像素的缓存之外，其成员大多还是与编解码相关的一些参数，如分辨率、帧类型、量化参数表、参考帧列表等。所以从大体上看，AVFrame就是一个容器类，同AVPacket不同的是所容纳的是原始的像素数据，相同点是都有很多编解码相关的参数作为补充数据。