AVFrame
AVFrame是包含码流参数较多的结构体。本文将会详细分析一下该结构体里主要变量的含义和作用。
首先看一下结构体的定义（位于avcodec.h）：
/**
* Audio Video Frame.
* New fields can be added to the end of AVFRAME with minor version
* bumps. Similarly fields that are marked as to be only accessed by
* av_opt_ptr() can be reordered. This allows 2 forks to add fields
* without breaking compatibility with each other.
* Removal, reordering and changes in the remaining cases require
* a major version bump.
* sizeof(AVFrame) must not be used outside libavcodec.
*/
typedef struct AVFrame {
#define AV_NUM_DATA_POINTERS 8
/**图像数据
* pointer to the picture/channel planes.
* This might be different from the first allocated byte
* - encoding: Set by user
* - decoding: set by AVCodecContext.get_buffer()
*/
uint8_t *data[AV_NUM_DATA_POINTERS];
/**
* Size, in bytes, of the data for each picture/channel plane.
*
* For audio, only linesize[0] may be set. For planar audio, each channel
* plane must be the same size.
*
* - encoding: Set by user
* - decoding: set by AVCodecContext.get_buffer()
*/
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 will always be set by get_buffer(),
* but for planar audio with more channels that can fit in data,
* extended_data must be used by the decoder in order to access all
* channels.
*
* encoding: unused
* decoding: set by AVCodecContext.get_buffer()
*/
uint8_t **extended_data;
/**宽高
* width and height of the video frame
* - encoding: unused
* - decoding: Read by user.
*/
int width, height;
/**
* number of audio samples (per channel) described by this frame
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
int nb_samples;
/**
* format of the frame, -1 if unknown or unset
* Values correspond to enum AVPixelFormat for video frames,
* enum AVSampleFormat for audio)
* - encoding: unused
* - decoding: Read by user.
*/
int format;
/**是否是关键帧
* 1 -> keyframe, 0-> not
* - encoding: Set by libavcodec.
* - decoding: Set by libavcodec.
*/
int key_frame;
/**帧类型（I,B,P）
* Picture type of the frame, see ?_TYPE below.
* - encoding: Set by libavcodec. for coded_picture (and set by user for input).
* - decoding: Set by libavcodec.
*/
enum AVPictureType pict_type;
/**
* pointer to the first allocated byte of the picture. Can be used in get_buffer/release_buffer.
* This isn't used by libavcodec unless the default get/release_buffer() is used.
* - encoding:
* - decoding:
*/
uint8_t *base[AV_NUM_DATA_POINTERS];
/**
* sample aspect ratio for the video frame, 0/1 if unknown/unspecified
* - encoding: unused
* - decoding: Read by user.
*/
AVRational sample_aspect_ratio;
/**
* presentation timestamp in time_base units (time when frame should be shown to user)
* If AV_NOPTS_VALUE then frame_rate = 1/time_base will be assumed.
* - encoding: MUST be set by user.
* - decoding: Set by libavcodec.
*/
int64_t pts;
/**
* reordered pts from the last AVPacket that has been input into the decoder
* - encoding: unused
* - decoding: Read by user.
*/
int64_t pkt_pts;
/**
* dts from the last AVPacket that has been input into the decoder
* - encoding: unused
* - decoding: Read by user.
*/
int64_t pkt_dts;
/**
* picture number in bitstream order
* - encoding: set by
* - decoding: Set by libavcodec.
*/
int coded_picture_number;
/**
* picture number in display order
* - encoding: set by
* - decoding: Set by libavcodec.
*/
int display_picture_number;
/**
* quality (between 1 (good) and FF_LAMBDA_MAX (bad))
* - encoding: Set by libavcodec. for coded_picture (and set by user for input).
* - decoding: Set by libavcodec.
*/
int quality;
/**
* is this picture used as reference
* The values for this are the same as the MpegEncContext.picture_structure
* variable, that is 1->top field, 2->bottom field, 3->frame/both fields.
* Set to 4 for delayed, non-reference frames.
* - encoding: unused
* - decoding: Set by libavcodec. (before get_buffer() call)).
*/
int reference;
/**QP表
* QP table
* - encoding: unused
* - decoding: Set by libavcodec.
*/
int8_t *qscale_table;
/**
* QP store stride
* - encoding: unused
* - decoding: Set by libavcodec.
*/
int qstride;
/**
*
*/
int qscale_type;
/**跳过宏块表
* mbskip_table[mb]>=1 if MB didn't change
* stride= mb_width = (width+15)>>4
* - encoding: unused
* - decoding: Set by libavcodec.
*/
uint8_t *mbskip_table;
/**运动矢量表
* 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
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
int16_t (*motion_val[2])[2];
/**宏块类型表
* macroblock type table
* mb_type_base + mb_width + 2
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
uint32_t *mb_type;
/**DCT系数
* DCT coefficients
* - encoding: unused
* - decoding: Set by libavcodec.
*/
short *dct_coeff;
/**参考帧列表
* motion reference frame index
* the order in which these are stored can depend on the codec.
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
int8_t *ref_index[2];
/**
* for some private data of the user
* - encoding: unused
* - decoding: Set by user.
*/
void *opaque;
/**
* error
* - encoding: Set by libavcodec. if flags&CODEC_FLAG_PSNR.
* - decoding: unused
*/
uint64_t error[AV_NUM_DATA_POINTERS];
/**
* type of the buffer (to keep track of who has to deallocate data[*])
* - encoding: Set by the one who allocates it.
* - decoding: Set by the one who allocates it.
* Note: User allocated (direct rendering) & internal buffers cannot coexist currently.
*/
int type;
/**
* When decoding, this signals how much the picture must be delayed.
* extra_delay = repeat_pict / (2*fps)
* - encoding: unused
* - decoding: Set by libavcodec.
*/
int repeat_pict;
/**
* The content of the picture is interlaced.
* - encoding: Set by user.
* - decoding: Set by libavcodec. (default 0)
*/
int interlaced_frame;
/**
* If the content is interlaced, is top field displayed first.
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
int top_field_first;
/**
* Tell user application that palette has changed from previous frame.
* - encoding: ??? (no palette-enabled encoder yet)
* - decoding: Set by libavcodec. (default 0).
*/
int palette_has_changed;
/**
* codec suggestion on buffer type if != 0
* - encoding: unused
* - decoding: Set by libavcodec. (before get_buffer() call)).
*/
int buffer_hints;
/**
* Pan scan.
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
AVPanScan *pan_scan;
/**
* 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
* - encoding: unused
* - decoding: Read by user.
*/
int64_t reordered_opaque;
/**
* hardware accelerator private data (FFmpeg-allocated)
* - encoding: unused
* - decoding: Set by libavcodec
*/
void *hwaccel_picture_private;
/**
* the AVCodecContext which ff_thread_get_buffer() was last called on
* - encoding: Set by libavcodec.
* - decoding: Set by libavcodec.
*/
struct AVCodecContext *owner;
/**
* used by multithreading to store frame-specific info
* - encoding: Set by libavcodec.
* - decoding: Set by libavcodec.
*/
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)
* - encoding: unused
* - decoding: Set by libavcodec.
*/
uint8_t motion_subsample_log2;
/**（音频）采样率
* Sample rate of the audio data.
*
* - encoding: unused
* - decoding: read by user
*/
int sample_rate;
/**
* Channel layout of the audio data.
*
* - encoding: unused
* - decoding: read by user.
*/
uint64_t channel_layout;
/**
* 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.
*/
int64_t channels;
} AVFrame;
AVFrame结构体一般用于存储原始数据（即非压缩数据，例如对视频来说是YUV，RGB，对音频来说是PCM），此外还包含了一些相关的信息。比如说，解码的时候存储了宏块类型表，QP表，运动矢量表等数据。编码的时候也存储了相关的数据。因此在使用FFMPEG进行码流分析的时候，AVFrame是一个很重要的结构体。
下面看几个主要变量的作用（在这里考虑解码的情况）：
**uint8_t *data[AV_NUM_DATA_POINTERS]：解码后原始数据（对视频来说是YUV，RGB，对音频来说是PCM）
int linesize[AV_NUM_DATA_POINTERS]：data中“一行”数据的大小。注意：未必等于图像的宽，一般大于图像的宽。
int width, height：视频帧宽和高（1920x1080,1280x720...）
int nb_samples：音频的一个AVFrame中可能包含多个音频帧，在此标记包含了几个
int format：解码后原始数据类型（YUV420，YUV422，RGB24...）
int key_frame：是否是关键帧
enum AVPictureType pict_type：帧类型（I,B,P...）
AVRational sample_aspect_ratio：宽高比（16:9，4:3...）
int64_t pts：显示时间戳
int coded_picture_number：编码帧序号
int display_picture_number：显示帧序号
int8_t *qscale_table：QP表
uint8_t mbskip_table：跳过宏块表
int16_t (motion_val[2])[2]：运动矢量表
uint32_t *mb_type：宏块类型表
short dct_coeff：DCT系数，这个没有提取过
int8_t ref_index[2]：运动估计参考帧列表（貌似H.264这种比较新的标准才会涉及到多参考帧）
int interlaced_frame：是否是隔行扫描
uint8_t motion_subsample_log2：一个宏块中的运动矢量采样个数，取log的
其他的变量不再一一列举，源代码中都有详细的说明。在这里重点分析一下几个需要一定的理解的变量：
1.data[]
对于packed格式的数据（例如RGB24），会存到data[0]里面。
对于planar格式的数据（例如YUV420P），则会分开成data[0]，data[1]，data[2]...（YUV420P中data[0]存Y，data[1]存U，data[2]存V）
具体参见：FFMPEG 实现 YUV，RGB各种图像原始数据之间的转换（swscale）
2.pict_type
包含以下类型：
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 MPEG4
AV_PICTURE_TYPE_SI, ///< Switching Intra
AV_PICTURE_TYPE_SP, ///< Switching Predicted
AV_PICTURE_TYPE_BI, ///< BI type
};
3.sample_aspect_ratio
宽高比是一个分数，FFMPEG中用AVRational表达分数：
/**
* rational number numerator/denominator
*/
typedef struct AVRational{
int num; ///< numerator
int den; ///< denominator
} AVRational;
4.qscale_table
QP表指向一块内存，里面存储的是每个宏块的QP值。宏块的标号是从左往右，一行一行的来的。每个宏块对应1个QP。
qscale_table[0]就是第1行第1列宏块的QP值；qscale_table[1]就是第1行第2列宏块的QP值；qscale_table[2]就是第1行第3列宏块的QP值。以此类推...
宏块的个数用下式计算：
注：宏块大小是16x16的。
每行宏块数：
int mb_stride = pCodecCtx->width/16+1
宏块的总数：
int mb_sum = ((pCodecCtx->height+15)>>4)*(pCodecCtx->width/16+1)
5.motion_subsample_log2
1个运动矢量所能代表的画面大小（用宽或者高表示，单位是像素），注意，这里取了log2。
代码注释中给出以下数据：
4->16x16, 3->8x8, 2-> 4x4, 1-> 2x2
即1个运动矢量代表16x16的画面的时候，该值取4；1个运动矢量代表8x8的画面的时候，该值取3...以此类推
6.motion_val
运动矢量表存储了一帧视频中的所有运动矢量。
该值的存储方式比较特别：
int16_t (*motion_val[2])[2];
注释中给了一段代码：
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];
该数据的结构：
1.首先分为两个列表L0和L1
2.每个列表（L0或L1）存储了一系列的MV（每个MV对应一个画面，大小由motion_subsample_log2决定）
3.每个MV分为横坐标和纵坐标（x,y）
注意，在FFMPEG中MV和MB在存储的结构上是没有什么关联的，第1个MV是屏幕上左上角画面的MV（画面的大小取决于motion_subsample_log2），第2个MV是屏幕上第1行第2列的画面的MV，以此类推。因此在一个宏块（16x16）的运动矢量很有可能如下图所示（line代表一行运动矢量的个数）：
//例如8x8划分的运动矢量与宏块的关系：
//-------------------------
//| | |
//|mv[x] |mv[x+1] |
//-------------------------
//| | |
//|mv[x+line]|mv[x+line+1]|
//-------------------------
7.mb_type
宏块类型表存储了一帧视频中的所有宏块的类型。其存储方式和QP表差不多。只不过其是uint32类型的，而QP表是uint8类型的。每个宏块对应一个宏块类型变量。
宏块类型如下定义所示：
//The following defines may change, don't expect compatibility if you use them.
#define MB_TYPE_INTRA4x4 0x0001
#define MB_TYPE_INTRA16x16 0x0002 //FIXME H.264-specific
#define MB_TYPE_INTRA_PCM 0x0004 //FIXME H.264-specific
#define MB_TYPE_16x16 0x0008
#define MB_TYPE_16x8 0x0010
#define MB_TYPE_8x16 0x0020
#define MB_TYPE_8x8 0x0040
#define MB_TYPE_INTERLACED 0x0080
#define MB_TYPE_DIRECT2 0x0100 //FIXME
#define MB_TYPE_ACPRED 0x0200
#define MB_TYPE_GMC 0x0400
#define MB_TYPE_SKIP 0x0800
#define MB_TYPE_P0L0 0x1000
#define MB_TYPE_P1L0 0x2000
#define MB_TYPE_P0L1 0x4000
#define MB_TYPE_P1L1 0x8000
#define MB_TYPE_L0 (MB_TYPE_P0L0 | MB_TYPE_P1L0)
#define MB_TYPE_L1 (MB_TYPE_P0L1 | MB_TYPE_P1L1)
#define MB_TYPE_L0L1 (MB_TYPE_L0 | MB_TYPE_L1)
#define MB_TYPE_QUANT 0x00010000
#define MB_TYPE_CBP 0x00020000
//Note bits 24-31 are reserved for codec specific use (h264 ref0, mpeg1 0mv, ...)
一个宏块如果包含上述定义中的一种或两种类型，则其对应的宏块变量的对应位会被置1。
注：一个宏块可以包含好几种类型，但是有些类型是不能重复包含的，比如说一个宏块不可能既是16x16又是8x8。
8.ref_index
运动估计参考帧列表存储了一帧视频中所有宏块的参考帧索引。这个列表其实在比较早的压缩编码标准中是没有什么用的。只有像H.264这样的编码标准才有多参考帧的概念。但是这个字段目前我还没有研究透。只是知道每个宏块包含有4个该值，该值反映的是参考帧的索引。