FFmpeg数据结构AVFrame (4)

对于具有多于AV_NUM_DATA_POINTERS个声道的planar音频来说，可能buf[]存不下所有的AVBbufferRef指针，多出的AVBufferRef指针存储在extended_buf数组中。
注意此处的extended_buf和AVFrame.extended_data的不同，AVFrame.extended_data包含所有指向各plane的指针，而extended_buf只包含AVFrame.buf中装不下的指针。
extended_buf是构造frame时av_frame_alloc()中自动调用av_malloc()来分配空间的。调用av_frame_unref会释放掉extended_buf。
nb_extended_buf是extended_buf中的元素数目。

best_effort_timestamp

/** * frame timestamp estimated using various heuristics, in stream time base * - encoding: unused * - decoding: set by libavcodec, read by user. */ int64_t best_effort_timestamp;

????

pkt_pos

/** * reordered pos from the last AVPacket that has been input into the decoder * - encoding: unused * - decoding: Read by user. */ int64_t pkt_pos;

记录最后一个扔进解码器的packet在输入文件中的位置偏移量。

pkt_duration

/** * duration of the corresponding packet, expressed in * AVStream->time_base units, 0 if unknown. * - encoding: unused * - decoding: Read by user. */ int64_t pkt_duration;

对应packet的时长，单位是AVStream->time_base。

channels

/** * number of audio channels, only used for audio. * - encoding: unused * - decoding: Read by user. */ int channels;

音频声道数量。

pkt_size

/** * size of the corresponding packet containing the compressed * frame. * It is set to a negative value if unknown. * - encoding: unused * - decoding: set by libavcodec, read by user. */ int pkt_size;

对应packet的大小。

crop_

/** * @anchor cropping * @name Cropping * Video frames only. The number of pixels to discard from the the * top/bottom/left/right border of the frame to obtain the sub-rectangle of * the frame intended for presentation. * @{ */ size_t crop_top; size_t crop_bottom; size_t crop_left; size_t crop_right; /** * @} */

用于视频帧图像裁切。四个值分别为从frame的上/下/左/右边界裁切的像素数。

2. 相关函数使用说明 2.1 av_frame_alloc() /** * Allocate an AVFrame and set its fields to default values. The resulting * struct must be freed using av_frame_free(). * * @return An AVFrame filled with default values or NULL on failure. * * @note this only allocates the AVFrame itself, not the data buffers. Those * must be allocated through other means, e.g. with av_frame_get_buffer() or * manually. */ AVFrame *av_frame_alloc(void);

构造一个frame，对象各成员被设为默认值。
此函数只分配AVFrame对象本身，而不分配AVFrame中的数据缓冲区。

2.2 av_frame_free() /** * Free the frame and any dynamically allocated objects in it, * e.g. extended_data. If the frame is reference counted, it will be * unreferenced first. * * @param frame frame to be freed. The pointer will be set to NULL. */ void av_frame_free(AVFrame **frame);

释放一个frame。

2.3 av_frame_ref() /** * Set up a new reference to the data described by the source frame. * * Copy frame properties from src to dst and create a new reference for each * AVBufferRef from src. * * If src is not reference counted, new buffers are allocated and the data is * copied. * * @warning: dst MUST have been either unreferenced with av_frame_unref(dst), * or newly allocated with av_frame_alloc() before calling this * function, or undefined behavior will occur. * * @return 0 on success, a negative AVERROR on error */ int av_frame_ref(AVFrame *dst, const AVFrame *src);

为src中的数据建立一个新的引用。
将src中帧的各属性拷到dst中，并且为src中每个AVBufferRef创建一个新的引用。
如果src未使用引用计数，则dst中会分配新的数据缓冲区，将将src中缓冲区的数据拷贝到dst中的缓冲区。

2.4 av_frame_clone() /** * Create a new frame that references the same data as src. * * This is a shortcut for av_frame_alloc()+av_frame_ref(). * * @return newly created AVFrame on success, NULL on error. */ AVFrame *av_frame_clone(const AVFrame *src);

创建一个新的frame，新的frame和src使用同一数据缓冲区，缓冲区管理使用引用计数机制。
本函数相当于av_frame_alloc()+av_frame_ref()

2.5 av_frame_unref() /** * Unreference all the buffers referenced by frame and reset the frame fields. */ void av_frame_unref(AVFrame *frame);

解除本frame对本frame中所有缓冲区的引用，并复位frame中各成员。

2.6 av_frame_move_ref() /** * Move everything contained in src to dst and reset src. * * @warning: dst is not unreferenced, but directly overwritten without reading * or deallocating its contents. Call av_frame_unref(dst) manually * before calling this function to ensure that no memory is leaked. */ void av_frame_move_ref(AVFrame *dst, AVFrame *src);