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537 lines
15 KiB
537 lines
15 KiB
AVFrame是包含码流参数较多的结构体。下面我们来分析一下该结构体里重要变量的含义和作用。
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# 一、源码整理
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首先我们先看一下结构体AVFrame的定义的结构体源码(位于libavcodec/avcodec.h)：
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```
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/*
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*雷霄骅
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*leixiaohua1020@126.com
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*中国传媒大学/数字电视技术
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*/
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/**
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* Audio Video Frame.
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* New fields can be added to the end of AVFRAME with minor version
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* bumps. Similarly fields that are marked as to be only accessed by
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* av_opt_ptr() can be reordered. This allows 2 forks to add fields
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* without breaking compatibility with each other.
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* Removal, reordering and changes in the remaining cases require
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* a major version bump.
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* sizeof(AVFrame) must not be used outside libavcodec.
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*/
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typedef struct AVFrame {
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#define AV_NUM_DATA_POINTERS 8
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/**图像数据
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* pointer to the picture/channel planes.
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* This might be different from the first allocated byte
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* - encoding: Set by user
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* - decoding: set by AVCodecContext.get_buffer()
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*/
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uint8_t *data[AV_NUM_DATA_POINTERS];
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/**
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* Size, in bytes, of the data for each picture/channel plane.
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*
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* For audio, only linesize[0] may be set. For planar audio, each channel
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* plane must be the same size.
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*
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* - encoding: Set by user
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* - decoding: set by AVCodecContext.get_buffer()
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*/
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int linesize[AV_NUM_DATA_POINTERS];
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/**
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* pointers to the data planes/channels.
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*
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* For video, this should simply point to data[].
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*
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* For planar audio, each channel has a separate data pointer, and
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* linesize[0] contains the size of each channel buffer.
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* For packed audio, there is just one data pointer, and linesize[0]
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* contains the total size of the buffer for all channels.
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*
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* Note: Both data and extended_data will always be set by get_buffer(),
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* but for planar audio with more channels that can fit in data,
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* extended_data must be used by the decoder in order to access all
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* channels.
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*
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* encoding: unused
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* decoding: set by AVCodecContext.get_buffer()
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*/
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uint8_t **extended_data;
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/**宽高
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* width and height of the video frame
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* - encoding: unused
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* - decoding: Read by user.
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*/
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int width, height;
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/**
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* number of audio samples (per channel) described by this frame
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* - encoding: Set by user
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* - decoding: Set by libavcodec
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*/
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int nb_samples;
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/**
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* format of the frame, -1 if unknown or unset
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* Values correspond to enum AVPixelFormat for video frames,
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* enum AVSampleFormat for audio)
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* - encoding: unused
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* - decoding: Read by user.
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*/
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int format;
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/**是否是关键帧
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* 1 -> keyframe, 0-> not
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* - encoding: Set by libavcodec.
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* - decoding: Set by libavcodec.
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*/
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int key_frame;
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/**帧类型（I,B,P）
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* Picture type of the frame, see ?_TYPE below.
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* - encoding: Set by libavcodec. for coded_picture (and set by user for input).
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* - decoding: Set by libavcodec.
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*/
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enum AVPictureType pict_type;
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/**
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* pointer to the first allocated byte of the picture. Can be used in get_buffer/release_buffer.
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* This isn't used by libavcodec unless the default get/release_buffer() is used.
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* - encoding:
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* - decoding:
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*/
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uint8_t *base[AV_NUM_DATA_POINTERS];
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/**
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* sample aspect ratio for the video frame, 0/1 if unknown/unspecified
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* - encoding: unused
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* - decoding: Read by user.
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*/
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AVRational sample_aspect_ratio;
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/**
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* presentation timestamp in time_base units (time when frame should be shown to user)
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* If AV_NOPTS_VALUE then frame_rate = 1/time_base will be assumed.
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* - encoding: MUST be set by user.
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* - decoding: Set by libavcodec.
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*/
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int64_t pts;
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/**
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* reordered pts from the last AVPacket that has been input into the decoder
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* - encoding: unused
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* - decoding: Read by user.
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*/
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int64_t pkt_pts;
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/**
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* dts from the last AVPacket that has been input into the decoder
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* - encoding: unused
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* - decoding: Read by user.
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*/
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int64_t pkt_dts;
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/**
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* picture number in bitstream order
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* - encoding: set by
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* - decoding: Set by libavcodec.
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*/
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int coded_picture_number;
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/**
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* picture number in display order
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* - encoding: set by
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* - decoding: Set by libavcodec.
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*/
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int display_picture_number;
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/**
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* quality (between 1 (good) and FF_LAMBDA_MAX (bad))
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* - encoding: Set by libavcodec. for coded_picture (and set by user for input).
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* - decoding: Set by libavcodec.
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*/
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int quality;
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/**
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* is this picture used as reference
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* The values for this are the same as the MpegEncContext.picture_structure
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* variable, that is 1->top field, 2->bottom field, 3->frame/both fields.
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* Set to 4 for delayed, non-reference frames.
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* - encoding: unused
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* - decoding: Set by libavcodec. (before get_buffer() call)).
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*/
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int reference;
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/**QP表
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* QP table
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* - encoding: unused
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* - decoding: Set by libavcodec.
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*/
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int8_t *qscale_table;
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/**
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* QP store stride
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* - encoding: unused
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* - decoding: Set by libavcodec.
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*/
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int qstride;
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/**
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*
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*/
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int qscale_type;
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/**跳过宏块表
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* mbskip_table[mb]>=1 if MB didn't change
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* stride= mb_width = (width+15)>>4
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* - encoding: unused
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* - decoding: Set by libavcodec.
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*/
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uint8_t *mbskip_table;
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/**运动矢量表
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* motion vector table
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* @code
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* example:
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* int mv_sample_log2= 4 - motion_subsample_log2;
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* int mb_width= (width+15)>>4;
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* int mv_stride= (mb_width << mv_sample_log2) + 1;
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* motion_val[direction][x + y*mv_stride][0->mv_x, 1->mv_y];
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* @endcode
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* - encoding: Set by user.
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* - decoding: Set by libavcodec.
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*/
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int16_t (*motion_val[2])[2];
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/**宏块类型表
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* macroblock type table
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* mb_type_base + mb_width + 2
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* - encoding: Set by user.
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* - decoding: Set by libavcodec.
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*/
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uint32_t *mb_type;
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/**DCT系数
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* DCT coefficients
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* - encoding: unused
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* - decoding: Set by libavcodec.
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*/
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short *dct_coeff;
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/**参考帧列表
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* motion reference frame index
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* the order in which these are stored can depend on the codec.
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* - encoding: Set by user.
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* - decoding: Set by libavcodec.
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*/
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int8_t *ref_index[2];
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/**
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* for some private data of the user
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* - encoding: unused
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* - decoding: Set by user.
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*/
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void *opaque;
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/**
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* error
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* - encoding: Set by libavcodec. if flags&CODEC_FLAG_PSNR.
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* - decoding: unused
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*/
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uint64_t error[AV_NUM_DATA_POINTERS];
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/**
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* type of the buffer (to keep track of who has to deallocate data[*])
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* - encoding: Set by the one who allocates it.
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* - decoding: Set by the one who allocates it.
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* Note: User allocated (direct rendering) & internal buffers cannot coexist currently.
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*/
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int type;
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/**
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* When decoding, this signals how much the picture must be delayed.
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* extra_delay = repeat_pict / (2*fps)
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* - encoding: unused
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* - decoding: Set by libavcodec.
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*/
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int repeat_pict;
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/**
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* The content of the picture is interlaced.
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* - encoding: Set by user.
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* - decoding: Set by libavcodec. (default 0)
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*/
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int interlaced_frame;
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/**
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* If the content is interlaced, is top field displayed first.
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* - encoding: Set by user.
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* - decoding: Set by libavcodec.
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*/
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int top_field_first;
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/**
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* Tell user application that palette has changed from previous frame.
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* - encoding: ??? (no palette-enabled encoder yet)
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* - decoding: Set by libavcodec. (default 0).
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*/
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int palette_has_changed;
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/**
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* codec suggestion on buffer type if != 0
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* - encoding: unused
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* - decoding: Set by libavcodec. (before get_buffer() call)).
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*/
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int buffer_hints;
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/**
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* Pan scan.
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* - encoding: Set by user.
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* - decoding: Set by libavcodec.
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*/
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AVPanScan *pan_scan;
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/**
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* reordered opaque 64bit (generally an integer or a double precision float
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* PTS but can be anything).
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* The user sets AVCodecContext.reordered_opaque to represent the input at
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* that time,
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* the decoder reorders values as needed and sets AVFrame.reordered_opaque
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* to exactly one of the values provided by the user through AVCodecContext.reordered_opaque
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* @deprecated in favor of pkt_pts
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* - encoding: unused
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* - decoding: Read by user.
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*/
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int64_t reordered_opaque;
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/**
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* hardware accelerator private data (FFmpeg-allocated)
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* - encoding: unused
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* - decoding: Set by libavcodec
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*/
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void *hwaccel_picture_private;
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/**
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* the AVCodecContext which ff_thread_get_buffer() was last called on
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* - encoding: Set by libavcodec.
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* - decoding: Set by libavcodec.
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*/
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struct AVCodecContext *owner;
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/**
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* used by multithreading to store frame-specific info
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* - encoding: Set by libavcodec.
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* - decoding: Set by libavcodec.
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*/
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void *thread_opaque;
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/**
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* log2 of the size of the block which a single vector in motion_val represents:
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* (4->16x16, 3->8x8, 2-> 4x4, 1-> 2x2)
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* - encoding: unused
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* - decoding: Set by libavcodec.
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*/
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uint8_t motion_subsample_log2;
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/**（音频）采样率
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* Sample rate of the audio data.
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*
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* - encoding: unused
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* - decoding: read by user
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*/
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int sample_rate;
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/**
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* Channel layout of the audio data.
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*
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* - encoding: unused
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* - decoding: read by user.
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*/
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uint64_t channel_layout;
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/**
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* frame timestamp estimated using various heuristics, in stream time base
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* Code outside libavcodec should access this field using:
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* av_frame_get_best_effort_timestamp(frame)
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* - encoding: unused
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* - decoding: set by libavcodec, read by user.
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*/
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int64_t best_effort_timestamp;
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/**
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* reordered pos from the last AVPacket that has been input into the decoder
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* Code outside libavcodec should access this field using:
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* av_frame_get_pkt_pos(frame)
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* - encoding: unused
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* - decoding: Read by user.
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*/
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int64_t pkt_pos;
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/**
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* duration of the corresponding packet, expressed in
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* AVStream->time_base units, 0 if unknown.
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* Code outside libavcodec should access this field using:
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* av_frame_get_pkt_duration(frame)
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* - encoding: unused
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* - decoding: Read by user.
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*/
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int64_t pkt_duration;
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/**
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* metadata.
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* Code outside libavcodec should access this field using:
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* av_frame_get_metadata(frame)
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* - encoding: Set by user.
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* - decoding: Set by libavcodec.
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*/
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AVDictionary *metadata;
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/**
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* decode error flags of the frame, set to a combination of
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* FF_DECODE_ERROR_xxx flags if the decoder produced a frame, but there
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* were errors during the decoding.
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* Code outside libavcodec should access this field using:
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* av_frame_get_decode_error_flags(frame)
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* - encoding: unused
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* - decoding: set by libavcodec, read by user.
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*/
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int decode_error_flags;
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#define FF_DECODE_ERROR_INVALID_BITSTREAM 1
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#define FF_DECODE_ERROR_MISSING_REFERENCE 2
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/**
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* number of audio channels, only used for audio.
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* Code outside libavcodec should access this field using:
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* av_frame_get_channels(frame)
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* - encoding: unused
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* - decoding: Read by user.
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*/
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int64_t channels;
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} AVFrame;
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```
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# 二、AVFrame 重点字段
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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。
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下面看几个主要变量的作用（在这里考虑解码的情况）：
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```
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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）
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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。
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int width, height：视频帧宽和高（1920x1080,1280x720...）
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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
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int format：解码后原始数据类型（YUV420，YUV422，RGB24...）
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int key_frame：是否是关键帧
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enum AVPictureType pict_type：帧类型（I,B,P...）
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AVRational sample_aspect_ratio：宽高比（16:9，4:3...）
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int64_t pts：显示时间戳
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int coded_picture_number：编码帧序号
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int display_picture_number：显示帧序号
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int8_t *qscale_table：QP表
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uint8_t *mbskip_table：跳过宏块表
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int16_t (*motion_val[2])[2]：运动矢量表
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uint32_t *mb_type：宏块类型表
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short *dct_coeff：DCT系数，这个没有提取过
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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）
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int interlaced_frame：是否是隔行扫描
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uint8_t motion_subsample_log2：一个宏块中的运动矢量采样个数，取log的
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```
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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：
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### 1.data[]
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对于packed格式的数据（例如RGB24），会存到data[0]里面。
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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）
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### 2.pict_type
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包含以下类型：
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```
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enum AVPictureType {
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AV_PICTURE_TYPE_NONE = 0, ///< Undefined
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AV_PICTURE_TYPE_I, ///< Intra
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AV_PICTURE_TYPE_P, ///< Predicted
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AV_PICTURE_TYPE_B, ///< Bi-dir predicted
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AV_PICTURE_TYPE_S, ///< S(GMC)-VOP MPEG4
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AV_PICTURE_TYPE_SI, ///< Switching Intra
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AV_PICTURE_TYPE_SP, ///< Switching Predicted
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AV_PICTURE_TYPE_BI, ///< BI type
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};
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```
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### 3.sample_aspect_ratio
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宽高比是一个分数，FFMPEG中用AVRational表达分数：
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```
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/**
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* rational number numerator/denominator
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*/
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typedef struct AVRational{
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int num; ///< numerator
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int den; ///< denominator
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} AVRational;
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```
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### 4.qscale_table
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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。
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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...
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宏块的个数用下式计算：
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注：宏块大小是16x16的。
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|
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)
|
|
```
|
|
|
|
|