AVFrame:( This structure describes decoded (raw) audio or video data. AVFrame must be allocated using av_frame_alloc(). Note that this only allocates the AVFrame itself, the buffers for the data must be managed through other means (see below). AVFrame must be freed with av_frame_free().)
typedef struct AVFrame {
#define AV_NUM_DATA_POINTERS 8
/**
* pointer to the picture/channel planes.
* This might be different from the first allocated byte
*
* Some decoders access areas outside 0,0 - width,height, please
* see avcodec_align_dimensions2(). Some filters and swscale can read
* up to 16 bytes beyond the planes, if these filters are to be used,
* then 16 extra bytes must be allocated.
*/
uint8_t *data[AV_NUM_DATA_POINTERS];
/**
* For video, size in bytes of each picture line.
* For audio, size in bytes of each plane.
*
* For audio, only linesize[0] may be set. For planar audio, each channel
* plane must be the same size.
*
* For video the linesizes should be multiplies of the CPUs alignment
* preference, this is 16 or 32 for modern desktop CPUs.
* Some code requires such alignment other code can be slower without
* correct alignment, for yet other it makes no difference.
*
* @note The linesize may be larger than the size of usable data -- there
* may be extra padding present for performance reasons.
*/
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 should always be set in a valid frame,
* but for planar audio with more channels that can fit in data,
* extended_data must be used in order to access all 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
* Values correspond to enum AVPixelFormat for video frames,
* enum AVSampleFormat for audio)
*/
int format;
/**
* 1 -> keyframe, 0-> not
*/
int key_frame;
/**
* Picture type of the frame.
*/
enum AVPictureType pict_type;
#if FF_API_AVFRAME_LAVC
attribute_deprecated
uint8_t *base[AV_NUM_DATA_POINTERS];
#endif
/**
* 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 isn't 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;
#if FF_API_AVFRAME_LAVC
attribute_deprecated
int reference;
/**
* QP table
*/
attribute_deprecated
int8_t *qscale_table;
/**
* QP store stride
*/
attribute_deprecated
int qstride;
attribute_deprecated
int qscale_type;
/**
* mbskip_table[mb]>=1 if MB didn't change
* stride= mb_width = (width+15)>>4
*/
attribute_deprecated
uint8_t *mbskip_table;
/**
* 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
*/
int16_t (*motion_val[2])[2];
/**
* macroblock type table
* mb_type_base + mb_width + 2
*/
attribute_deprecated
uint32_t *mb_type;
/**
* DCT coefficients
*/
attribute_deprecated
short *dct_coeff;
/**
* motion reference frame index
* the order in which these are stored can depend on the codec.
*/
attribute_deprecated
int8_t *ref_index[2];
#endif
/**
* for some private data of the user
*/
void *opaque;
/**
* error
*/
uint64_t error[AV_NUM_DATA_POINTERS];
#if FF_API_AVFRAME_LAVC
attribute_deprecated
int type;
#endif
/**
* 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;
#if FF_API_AVFRAME_LAVC
attribute_deprecated
int buffer_hints;
/**
* Pan scan.
*/
attribute_deprecated
struct AVPanScan *pan_scan;
#endif
/**
* 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
*/
int64_t reordered_opaque;
#if FF_API_AVFRAME_LAVC
/**
* @deprecated this field is unused
*/
attribute_deprecated void *hwaccel_picture_private;
attribute_deprecated
struct AVCodecContext *owner;
attribute_deprecated
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)
*/
uint8_t motion_subsample_log2;
#endif
/**
* Sample rate of the audio data.
*/
int sample_rate;
/**
* Channel layout of the audio data.
*/
uint64_t channel_layout;
/**
* AVBuffer references backing the data for this frame. If all elements of
* this array are NULL, then this frame is not reference counted.
*
* There may be at most one AVBuffer per data plane, 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.
*
* Note that this is different from AVFrame.extended_data, which always
* contains all the pointers. This array only contains the extra pointers,
* which cannot fit into AVFrame.buf.
*
* This array is always allocated using av_malloc() by whoever constructs
* the frame. It is freed in av_frame_unref().
*/
AVBufferRef **extended_buf;
/**
* Number of elements in extended_buf.
*/
int nb_extended_buf;
AVFrameSideData **side_data;
int nb_side_data;
/**
* @defgroup lavu_frame_flags AV_FRAME_FLAGS
* Flags describing additional frame properties.
*
* @{
*/
/**
* The frame data may be corrupted, e.g. due to decoding errors.
*/
#define AV_FRAME_FLAG_CORRUPT (1 << 0)
/**
* @}
*/
/**
* Frame flags, a combination of @ref lavu_frame_flags
*/
int flags;
#if FF_API_AVFRAME_COLORSPACE
/**
* MPEG vs JPEG YUV range.
* It must be accessed using av_frame_get_color_range() and
* av_frame_set_color_range().
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
enum AVColorRange color_range;
enum AVColorPrimaries color_primaries;
enum AVColorTransferCharacteristic color_trc;
/**
* YUV colorspace type.
* It must be accessed using av_frame_get_colorspace() and
* av_frame_set_colorspace().
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
enum AVColorSpace colorspace;
enum AVChromaLocation chroma_location;
#endif
/**
* 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.
*/
int channels;
/**
* size of the corresponding packet containing the compressed
* frame. It must be accessed using av_frame_get_pkt_size() and
* av_frame_set_pkt_size().
* It is set to a negative value if unknown.
* - encoding: unused
* - decoding: set by libavcodec, read by user.
*/
int pkt_size;
/**
* Not to be accessed directly from outside libavutil
*/
AVBufferRef *qp_table_buf;
} AVFrame;
AVFrame 是用来描述 解码后的数据;即存储原始数据;(非压缩数据),对视频来说是YUV RGB,对音频来说是PCM;此外还包括一些相关的信息;,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,
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。
nt 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
1.data[] 2.pict_type
<strong>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
};</strong>
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)
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];
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];
解析:
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。
ffmpeg使用AVPacket来暂存解复用之后,解码之前的媒体数据(一个音/视频帧、一个字母包等)以及附加信息(解码时间、显示时间、时长等)
AVPacket 结构本身只是个容器,它使用data成员指向实际的数据缓冲区,这个缓冲区可以通过av_new_packet创建,可以通过 av_dup_packet 拷贝,也可以由FFMPEG的API产生(如av_read_frame),使用之后需要通过调用av_free_packet释放。
av_free_packet调用的是结构体本身的destruct函数,它的值有两种情况:(1)av_destruct_packet_nofree或 0;(2)av_destruct_packet,其中,前者仅仅是将data和size的值清0而已,后者才会真正地释放缓冲区。FFMPEG内部使用 AVPacket结构建立缓冲区装载数据,同时提供destruct函数,如果FFMPEG打算自己维护缓冲区,则将destruct设为 av_destruct_packet_nofree,用户调用av_free_packet清理缓冲区时并不能够将其释放;如果FFMPEG不会再使用 该缓冲区,则将destruct设为av_destruct_packet,表示它能够被释放。对于缓冲区不能够被释放的AVPackt,用户在使用之前 最好调用av_dup_packet进行缓冲区的克隆,将其转化为缓冲区能够被释放的AVPacket,以免对缓冲区的不当占用造成异常错误。而 av_dup_packet会为destruct指针为av_destruct_packet_nofree的AVPacket新建一个缓冲区,然后将原 缓冲区的数据拷贝至新缓冲
区,置data的值为新缓冲区的地址,同时设destruct指针为av_destruct_packet。
时间信息
时间信息用于实现多媒体同步。
同步的目的在于展示多媒体信息时,能够保持媒体对象之间固有的时间关系。同步有两类,一类是流内同步,其主要任务是保证单个媒体流内的时间关系,以满足感知 要求,如按照规定的帧率播放一段视频;另一类是流间同步,主要任务是保证不同媒体流之间的时间关系,如音频和视频之间的关系(lipsync)。
对于固定速率的媒体,如固定帧率的视频或固定比特率的音频,可以将时间信息(帧率或比特率)置于文件首部(header),如AVI的hdrl List、MP4的moov box,还有一种相对复杂的方案是将时间信息嵌入媒体流的内部,如MPEG TS和Real video,这种方案可以处理变速率的媒体,亦可有效避免同步过程中的时间漂移。
FFMPEG会为每一个数据包打上时间标 签,以更有效地支持上层应用的同步机制。时间标签有两种,一种是DTS,称为解码时间标签,另一种是PTS,称为显示时间标签。对于声音来说 ,这两个时间标签是相同的,但对于某些视频编码格式,由于采用了双向预测技术,会造成DTS和PTS的不一致。
时间信息的获取:
通过调用av_find_stream_info,多媒体应用可以从AVFormatContext对象中拿到媒体文件的时间信息:主要是总时间长度和开始时间,此外还有与时间信息相关的比特率和文件大小。其中时间信息的单位是AV_TIME_BASE:微秒。
/**
* This structure stores compressed data. It is typically exported by demuxers
* and then passed as input to decoders, or received as output from encoders and
* then passed to muxers.
*
* For video, it should typically contain one compressed frame. For audio it may
* contain several compressed frames.
*
* AVPacket is one of the few structs in FFmpeg, whose size is a part of public
* ABI. Thus it may be allocated on stack and no new fields can be added to it
* without libavcodec and libavformat major bump.
*
* The semantics of data ownership depends on the buf or destruct (deprecated)
* fields. If either is set, the packet data is dynamically allocated and is
* valid indefinitely until av_free_packet() is called (which in turn calls
* av_buffer_unref()/the destruct callback to free the data). If neither is set,
* the packet data is typically backed by some static buffer somewhere and is
* only valid for a limited time (e.g. until the next read call when demuxing).
*
* The side data is always allocated with av_malloc() and is freed in
* av_free_packet().
*/
typedef struct AVPacket {
/**
* A reference to the reference-counted buffer where the packet data is
* stored.
* May be NULL, then the packet data is not reference-counted.
*/
AVBufferRef *buf;
/**
* Presentation timestamp in AVStream->time_base units; the time at which
* the decompressed packet will be presented to the user.
* Can be AV_NOPTS_VALUE if it is not stored in the file.
* pts MUST be larger or equal to dts as presentation cannot happen before
* decompression, unless one wants to view hex dumps. Some formats misuse
* the terms dts and pts/cts to mean something different. Such timestamps
* must be converted to true pts/dts before they are stored in AVPacket.
*/
int64_t pts;
/**
* Decompression timestamp in AVStream->time_base units; the time at which
* the packet is decompressed.
* Can be AV_NOPTS_VALUE if it is not stored in the file.
*/
int64_t dts;
uint8_t *data;
int size;
int stream_index;
/**
* A combination of AV_PKT_FLAG values
*/
int flags;
/**
* Additional packet data that can be provided by the container.
* Packet can contain several types of side information.
*/
AVPacketSideData *side_data;
int side_data_elems;
/**
* Duration of this packet in AVStream->time_base units, 0 if unknown.
* Equals next_pts - this_pts in presentation order.
*/
int duration;
#if FF_API_DESTRUCT_PACKET
attribute_deprecated
void (*destruct)(struct AVPacket *);
attribute_deprecated
void *priv;
#endif
int64_t pos; ///< byte position in stream, -1 if unknown
/**
* Time difference in AVStream->time_base units from the pts of this
* packet to the point at which the output from the decoder has converged
* independent from the availability of previous frames. That is, the
* frames are virtually identical no matter if decoding started from
* the very first frame or from this keyframe.
* Is AV_NOPTS_VALUE if unknown.
* This field is not the display duration of the current packet.
* This field has no meaning if the packet does not have AV_PKT_FLAG_KEY
* set.
*
* The purpose of this field is to allow seeking in streams that have no
* keyframes in the conventional sense. It corresponds to the
* recovery point SEI in H.264 and match_time_delta in NUT. It is also
* essential for some types of subtitle streams to ensure that all
* subtitles are correctly displayed after seeking.
*/
int64_t convergence_duration;
} AVPacket;
在AVPacket结构体中,重要的变量有以下几个:
uint8_t *data:压缩编码的数据。
例如对于H.264来说。1个AVPacket的data通常对应一个NAL。
注意:在这里只是对应,而不是一模一样。他们之间有微小的差别:使用FFMPEG类库分离出多媒体文件中的H.264码流
因此在使用FFMPEG进行视音频处理的时候,常常可以将得到的AVPacket的data数据直接写成文件,从而得到视音频的码流文件。
int size:data的大小
int64_t pts:显示时间戳
int64_t dts:解码时间戳
int stream_index:标识该AVPacket所属的视频/音频流。
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