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。