注：写了一系列的结构体的分析的文章，在这里列一个列表：
FFMPEG结构体分析：AVFrame
FFMPEG结构体分析：AVFormatContext
FFMPEG结构体分析：AVCodecContext
FFMPEG结构体分析：AVIOContext
FFMPEG结构体分析：AVCodec
FFMPEG结构体分析：AVStream
FFMPEG结构体分析：AVPacket
FFMPEG有几个最重要的结构体，包含了解协议，解封装，解码操作，此前已经进行过分析：
在此不再详述，其中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）：
/* *雷霄骅 *[email protected] *中国传媒大学/数字电视技术 */ /** * 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的大小
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];
为了弄清楚该值究竟是怎么存的，花了我好一阵子功夫...
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];
大概知道了该数据的结构：
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。以后有机会再进行细研究吧。
在这里展示一下自己做的码流分析软件的运行结果。将上文介绍的几个列表图像化显示了出来（在这里是使用MFC的绘图函数画出来的）
视频帧：
QP参数提取的结果：
美化过的（加上了颜色）：
宏块类型参数提取的结果：
美化过的（加上了颜色，更清晰一些，s代表skip宏块）：
运动矢量参数提取的结果（在这里是List0）：
运动估计参考帧参数提取的结果：
