FFmpeg源码分析: AVFrame与AVpacket
2021/11/26 9:39:48
本文主要是介绍FFmpeg源码分析: AVFrame与AVpacket,对大家解决编程问题具有一定的参考价值,需要的程序猿们随着小编来一起学习吧!
FFmpeg有两个存储帧数据的结构体,其中AVPacket是解封装后保存压缩数据包,AVFrame是解码后保存音视频帧。
AVPacket结构体以前放在avcodec.h头文件中,在FFmpeg4.4以后放在单独packet.h头文件。官方对AVPacket的说明如下:
/** * 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. Encoders are allowed to output empty * packets, with no compressed data, containing only side data * (e.g. to update some stream parameters at the end of encoding). * */
AVPacket结构体定义如下:
typedef struct AVPacket { AVBufferRef *buf; // 显示时间戳,单位为AVStream->time_base int64_t pts; // 解码时间戳,单位为AVStream->time_base int64_t dts; // 音视频数据 uint8_t *data; // 数据包大小 int size; // 码流索引下标 int stream_index; // 帧类型 int flags; // 额外数据 AVPacketSideData *side_data; int side_data_elems; // 帧显示时长,单位为AVStream->time_base int64_t duration; // 数据包所在码流的position int64_t pos; } AVPacket;
AVPacket的分配与释放有对应的API,需要注意的是释放所传的参数为AVPacket指针的地址。API与示例如下:
API: AVPacket *av_packet_alloc(void); void av_packet_free(AVPacket **pkt); demo: AVPacket *pkt = av_packet_alloc(); av_packet_free(&pkt);
AVFrame结构体位于frame.h头文件,用于存储解码后的音视频帧数据,使用av_frame_alloc进行分配,使用av_frame_free进行释放,AVFrame分配一次,多次复用,使用av_frame_unref可以去引用。官方关于AVFrame的描述如下:
/** * 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(). * * AVFrame is typically allocated once and then reused multiple times to hold * different data (e.g. a single AVFrame to hold frames received from a * decoder). In such a case, av_frame_unref() will free any references held by * the frame and reset it to its original clean state before it * is reused again. */
AVFrame是解码后用于存储音视频帧,包括:data数组、width、height、pts、pkt_dts、pkt_size、pkt_duration、pkt_pos等信息,我们在判断是否为关键帧时,用key_frame参数进行判断。结构体的定义如下:
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. * * 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 multiples 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. */ 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. */ uint8_t **extended_data; /** * @name Video dimensions */ 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; /** * Sample aspect ratio for the video frame, 0/1 if unknown/unspecified. */ AVRational sample_aspect_ratio; /** * Presentation timestamp in time_base units. */ int64_t pts; /** * DTS copied from the AVPacket that triggered returning this frame. * 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; /** * 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; /** * reordered opaque 64 bits. */ int64_t reordered_opaque; /** * 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. */ 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; /** * The frame data may be corrupted, e.g. due to decoding errors. */ #define AV_FRAME_FLAG_CORRUPT (1 << 0) /** * A flag to mark the frames which need to be decoded. */ #define AV_FRAME_FLAG_DISCARD (1 << 2) /** * Frame flags, a combination of @ref lavu_frame_flags */ int flags; /** * 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. */ int decode_error_flags; #define FF_DECODE_ERROR_INVALID_BITSTREAM 1 #define FF_DECODE_ERROR_MISSING_REFERENCE 2 #define FF_DECODE_ERROR_CONCEALMENT_ACTIVE 4 #define FF_DECODE_ERROR_DECODE_SLICES 8 // number of audio channels, only used for audio. int channels; // size of the corresponding packet containing the compressed frame. int pkt_size; /** * 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. */ AVBufferRef *opaque_ref; /** * 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; /** * AVBufferRef for internal use by a single libav* library. * Must not be used to transfer data between libraries. */ AVBufferRef *private_ref; } AVFrame;
其中,帧类型使用AVPictureType(位于libavutil/avutil.h)表示,枚举定义如下:
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 MPEG-4 AV_PICTURE_TYPE_SI, ///< Switching Intra AV_PICTURE_TYPE_SP, ///< Switching Predicted AV_PICTURE_TYPE_BI, ///< BI type };
AVFrame的分配与释放也有相应API,具体API以及使用实例如下:
API: AVFrame *av_frame_alloc(void); void av_frame_free(AVFrame **frame); demo: AVFrame *frame = av_frame_alloc(); av_frame_free(&frame);
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