1.描述
AVFrame用来存储解码后的（或原始）音频或视频数据，位于avcodec.h文件中。
AVFrame必须由av_frame_alloc()分配内存，同时必须由av_frame_free()释放。
AVFrame分配内存后能够被多次用来存储不同的数据（例如：decoder解码后的帧）。av_frame_unref释放任何持帧的引用，并结构体还原到未被使用的状态。
2.常见变量及其作用
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];//在视频中，表示图片一行数据的大小。
uint8_t **extended_data;//指向数据平面/通道。
int width, height;//一个视频帧的宽度和高度。
int nb_samples;//这个AVFrame中的每个音频通道中包含的音频帧个数。
int format;//表示解码后的数据类型或格式，-1表示未被设置或不能识别的类型。
int key_frame;//是否为关键帧。
enum AVPictureType pict_type;//帧的类型。
AVRational sample_aspect_ratio;//视频帧的宽高比，0表示未知。
int64_t pts;//显示时间戳，表示该什么时候被显示。
int64_t pkt_dts;//从AVPacket中拷贝的值。
int coded_picture_number;//编码帧序号。
int display_picture_number;//显示帧需要。
void *opaque;//用户私有信息。
int repeat_pict;//解码时，每帧图片延迟的时间，extra_delay = repeat_pict / (2*fps)。
int interlaced_frame;//是否是隔行扫描
int sample_rate;//音频的采样率。
uint64_t channel_layout;//音频的布局方式。
/**
* MPEG vs JPEG YUV range.
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
enum AVColorRange color_range;
enum AVColorPrimaries color_primaries;
enum AVColorTransferCharacteristic color_trc;
/**
* YUV colorspace type.
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
enum AVColorSpace colorspace;
enum AVChromaLocation chroma_location;
/**
* frame timestamp estimated using various heuristics, in stream time base
* - 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
* - encoding: unused
* - decoding: Read by user.
*/
int64_t pkt_pos;
/**
* duration of the corresponding packet, expressed in
* AVStream->time_base units, 0 if unknown.
* - encoding: unused
* - decoding: Read by user.
*/
int64_t pkt_duration;
/**
* metadata.
* - 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.
* - encoding: unused
* - decoding: set by libavcodec, read by user.
*/
int decode_error_flags;
/**
* number of audio channels, only used for audio.
* - encoding: unused
* - decoding: Read by user.
*/
int channels;//音频通道个数
/**
* 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;
int8_t *qscale_table;
int qstride;
int qscale_type;
AVBufferRef *qp_table_buf;
/**
* For hwaccel-format frames, this should be a reference to the
* AVHWFramesContext describing the frame.
*/
AVBufferRef *hw_frames_ctx;
/**
* AVBufferRef for free use by the API user. FFmpeg will never check the
* contents of the buffer ref. FFmpeg calls av_buffer_unref() on it when
* the frame is unreferenced. av_frame_copy_props() calls create a new
* reference with av_buffer_ref() for the target frame's opaque_ref field.
*
* This is unrelated to the opaque field, although it serves a similar
* purpose.
*/
AVBufferRef *opaque_ref;
