ffmpeg 基本数据结构和对象（一）： AVPacket、AVPicture、AVFrame

一、AVPacket

/**
 * AVPacket 作为解码器的输入 或 编码器的输出。
 * 当作为解码器的输入时，它由demuxer生成，然后传递给解码器
 * 当作为编码器的输出时，由编码器生成，然后传递给muxer
 * 在视频中，AVPacket 只能包含不大于1帧的内容，而视频的1帧可能要包含在多个AVPacket中，AVPacket < AVFrame
 * 
 *
 * AVPacket 是ffmpeg中少数的几个公共ABI，它只能被libavcodec和libformat在栈上分配
 *
 * The side data is always allocated with av_malloc() and is freed in
 * av_free_packet().
 */
typedef struct AVPacket {
    /**
     * packet的内存空间来自于一个叫做“引用计数缓冲区”的地方，这个指针就指向一块引用计数缓冲区
     */
    AVBufferRef *buf;
    /**
     * 显示时间戳 单位是 AVStream->time_base units
     */
    int64_t pts;
    /**
     * 解压时间戳，在这个时刻该包需要被解码
     */
    int64_t dts;
    uint8_t *data;
    int   size;
    int   stream_index;
    /**
     * A combination of AV_PKT_FLAG values
     */
    int   flags;
    /**
     * 存放额外的包信息
     */
    struct {
        uint8_t *data;
        int      size;
        enum AVPacketSideDataType type;
    } *side_data;
    int side_data_elems;

    /**
     * 这个包的时间长度 in AVStream->time_base units, 设置0 表示未知.
     * duration = next_pts - this_pts i.
     */
    int   duration;

    int64_t pos;                            ///< 在数据流中的字节偏移量, -1 if unknown

    int64_t convergence_duration;
} AVPacket;

二、AVPicture

typedef struct AVPicture {
    uint8_t *data[AV_NUM_DATA_POINTERS];    ///< pointers to the image data planes
    int linesize[AV_NUM_DATA_POINTERS];     ///< number of bytes per line
}

三、AVFrame

/**
 * AVFrame表示解码过后的一个数据帧
 *
 * AVFrame 通过使用 av_frame_alloc()来创建. 这个函数只是创建了AVFrame结构本身，在结构
 * 中定义的指向其他内存块的缓冲区指针要用其他方法来分配
 * 使用 av_frame_free()来释放AVFrame.
 *
 */
typedef struct AVFrame {
#define AV_NUM_DATA_POINTERS 8
    /**
     * pointer to the picture/channel planes.
     */
    uint8_t *data[AV_NUM_DATA_POINTERS];

    /**
     * For video, size in bytes of each picture line.
     * For audio, size in bytes of each plane.
     */
    int linesize[AV_NUM_DATA_POINTERS];

    /**
     * pointers to the data planes/channels.
     */
    uint8_t **extended_data;

    /**
     * width and height of the video frame
     */
    int width, height;

    /**
     * number of audio samples (per channel) described by this frame
     */
    int nb_samples;

    /**
     * format of the frame, -1 if unknown or unset
     */
    int format;

    /**
     * 1 -> keyframe, 0-> not
     */
    int key_frame;

    /**
     * Picture type of the frame.
     */
    enum AVPictureType pict_type;

    /**
     * Sample aspect ratio for the video frame, 0/1 if unknown/unspecified.
     */
    AVRational sample_aspect_ratio;

    /**
     * Presentation timestamp in time_base units (time when frame should be shown to user).
     */
    int64_t pts;

    /**
     * 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;
    /**
     * picture number in display order
     */
    int display_picture_number;

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

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

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

    /**
     * When decoding, this signals how much the picture must be delayed.
     * extra_delay = repeat_pict / (2*fps)
     */
    int repeat_pict;

    /**
     * The content of the picture is interlaced.
     */
    int interlaced_frame;

    /**
     * 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;

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

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

    /**
     * 指向这个帧要用到的AVBuffer中的数据缓冲区.
     *
     * 一般每个图像位面（就时data[0],data[1] data[2]）只有一个指向AVBuffer的缓冲区, 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.
     */
    AVBufferRef **extended_buf;
    /**
     * Number of elements in extended_buf.
     */
    int        nb_extended_buf;

    AVFrameSideData **side_data;
    int            nb_side_data;

/**
 * 可能因为解码错误，数据帧Frame会成为无效的帧，下面的结构就是当数据帧无效时使用的
 */
#define AV_FRAME_FLAG_CORRUPT       (1 << 0)

    /**
     * Frame flags, a combination of AV_FRAME_FLAG_*
     */
    int flags;

    
    int64_t best_effort_timestamp;

   
    int64_t pkt_pos;

    
    int64_t pkt_duration;

   
    AVDictionary *metadata;

    
    int decode_error_flags;
#define FF_DECODE_ERROR_INVALID_BITSTREAM   1
#define FF_DECODE_ERROR_MISSING_REFERENCE   2

   
    int channels;

    
    int pkt_size;

    
    enum AVColorSpace colorspace;

   
    enum AVColorRange color_range;


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