diff --git a/FFmpeg 结构体学习(四)： AVFrame 分析.md b/FFmpeg 结构体学习(四)： AVFrame 分析.md new file mode 100644 index 0000000..c621f3a --- /dev/null +++ b/FFmpeg 结构体学习(四)： AVFrame 分析.md @@ -0,0 +1,537 @@ +AVFrame是包含码流参数较多的结构体。下面我们来分析一下该结构体里重要变量的含义和作用。 + +# 一、源码整理 + +首先我们先看一下结构体AVFrame的定义的结构体源码(位于libavcodec/avcodec.h)： + + + +``` +/* + *雷霄骅 + *leixiaohua1020@126.com + *中国传媒大学/数字电视技术 + */ +/** + * Audio Video Frame. + * New fields can be added to the end of AVFRAME with minor version + * bumps. Similarly fields that are marked as to be only accessed by + * av_opt_ptr() can be reordered. This allows 2 forks to add fields + * without breaking compatibility with each other. + * Removal, reordering and changes in the remaining cases require + * a major version bump. + * sizeof(AVFrame) must not be used outside libavcodec. + */ +typedef struct AVFrame { +#define AV_NUM_DATA_POINTERS 8 + /**图像数据 + * pointer to the picture/channel planes. + * This might be different from the first allocated byte + * - encoding: Set by user + * - decoding: set by AVCodecContext.get_buffer() + */ + uint8_t *data[AV_NUM_DATA_POINTERS]; + + /** + * Size, in bytes, of the data for each picture/channel plane. + * + * For audio, only linesize[0] may be set. For planar audio, each channel + * plane must be the same size. + * + * - encoding: Set by user + * - decoding: set by AVCodecContext.get_buffer() + */ + 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 will always be set by get_buffer(), + * but for planar audio with more channels that can fit in data, + * extended_data must be used by the decoder in order to access all + * channels. + * + * encoding: unused + * decoding: set by AVCodecContext.get_buffer() + */ + uint8_t **extended_data; + + /**宽高 + * width and height of the video frame + * - encoding: unused + * - decoding: Read by user. + */ + int width, height; + + /** + * number of audio samples (per channel) described by this frame + * - encoding: Set by user + * - decoding: Set by libavcodec + */ + int nb_samples; + + /** + * format of the frame, -1 if unknown or unset + * Values correspond to enum AVPixelFormat for video frames, + * enum AVSampleFormat for audio) + * - encoding: unused + * - decoding: Read by user. + */ + int format; + + /**是否是关键帧 + * 1 -> keyframe, 0-> not + * - encoding: Set by libavcodec. + * - decoding: Set by libavcodec. + */ + int key_frame; + + /**帧类型（I,B,P） + * Picture type of the frame, see ?_TYPE below. + * - encoding: Set by libavcodec. for coded_picture (and set by user for input). + * - decoding: Set by libavcodec. + */ + enum AVPictureType pict_type; + + /** + * pointer to the first allocated byte of the picture. Can be used in get_buffer/release_buffer. + * This isn't used by libavcodec unless the default get/release_buffer() is used. + * - encoding: + * - decoding: + */ + uint8_t *base[AV_NUM_DATA_POINTERS]; + + /** + * sample aspect ratio for the video frame, 0/1 if unknown/unspecified + * - encoding: unused + * - decoding: Read by user. + */ + AVRational sample_aspect_ratio; + + /** + * presentation timestamp in time_base units (time when frame should be shown to user) + * If AV_NOPTS_VALUE then frame_rate = 1/time_base will be assumed. + * - encoding: MUST be set by user. + * - decoding: Set by libavcodec. + */ + int64_t pts; + + /** + * reordered pts from the last AVPacket that has been input into the decoder + * - encoding: unused + * - decoding: Read by user. + */ + int64_t pkt_pts; + + /** + * dts from the last AVPacket that has been input into the decoder + * - encoding: unused + * - decoding: Read by user. + */ + int64_t pkt_dts; + + /** + * picture number in bitstream order + * - encoding: set by + * - decoding: Set by libavcodec. + */ + int coded_picture_number; + /** + * picture number in display order + * - encoding: set by + * - decoding: Set by libavcodec. + */ + int display_picture_number; + + /** + * quality (between 1 (good) and FF_LAMBDA_MAX (bad)) + * - encoding: Set by libavcodec. for coded_picture (and set by user for input). + * - decoding: Set by libavcodec. + */ + int quality; + + /** + * is this picture used as reference + * The values for this are the same as the MpegEncContext.picture_structure + * variable, that is 1->top field, 2->bottom field, 3->frame/both fields. + * Set to 4 for delayed, non-reference frames. + * - encoding: unused + * - decoding: Set by libavcodec. (before get_buffer() call)). + */ + int reference; + + /**QP表 + * QP table + * - encoding: unused + * - decoding: Set by libavcodec. + */ + int8_t *qscale_table; + /** + * QP store stride + * - encoding: unused + * - decoding: Set by libavcodec. + */ + int qstride; + + /** + * + */ + int qscale_type; + + /**跳过宏块表 + * mbskip_table[mb]>=1 if MB didn't change + * stride= mb_width = (width+15)>>4 + * - encoding: unused + * - decoding: Set by libavcodec. + */ + uint8_t *mbskip_table; + + /**运动矢量表 + * 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 + * - encoding: Set by user. + * - decoding: Set by libavcodec. + */ + int16_t (*motion_val[2])[2]; + + /**宏块类型表 + * macroblock type table + * mb_type_base + mb_width + 2 + * - encoding: Set by user. + * - decoding: Set by libavcodec. + */ + uint32_t *mb_type; + + /**DCT系数 + * DCT coefficients + * - encoding: unused + * - decoding: Set by libavcodec. + */ + short *dct_coeff; + + /**参考帧列表 + * motion reference frame index + * the order in which these are stored can depend on the codec. + * - encoding: Set by user. + * - decoding: Set by libavcodec. + */ + int8_t *ref_index[2]; + + /** + * for some private data of the user + * - encoding: unused + * - decoding: Set by user. + */ + void *opaque; + + /** + * error + * - encoding: Set by libavcodec. if flags&CODEC_FLAG_PSNR. + * - decoding: unused + */ + uint64_t error[AV_NUM_DATA_POINTERS]; + + /** + * type of the buffer (to keep track of who has to deallocate data[*]) + * - encoding: Set by the one who allocates it. + * - decoding: Set by the one who allocates it. + * Note: User allocated (direct rendering) & internal buffers cannot coexist currently. + */ + int type; + + /** + * When decoding, this signals how much the picture must be delayed. + * extra_delay = repeat_pict / (2*fps) + * - encoding: unused + * - decoding: Set by libavcodec. + */ + int repeat_pict; + + /** + * The content of the picture is interlaced. + * - encoding: Set by user. + * - decoding: Set by libavcodec. (default 0) + */ + int interlaced_frame; + + /** + * If the content is interlaced, is top field displayed first. + * - encoding: Set by user. + * - decoding: Set by libavcodec. + */ + int top_field_first; + + /** + * Tell user application that palette has changed from previous frame. + * - encoding: ??? (no palette-enabled encoder yet) + * - decoding: Set by libavcodec. (default 0). + */ + int palette_has_changed; + + /** + * codec suggestion on buffer type if != 0 + * - encoding: unused + * - decoding: Set by libavcodec. (before get_buffer() call)). + */ + int buffer_hints; + + /** + * Pan scan. + * - encoding: Set by user. + * - decoding: Set by libavcodec. + */ + AVPanScan *pan_scan; + + /** + * 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 + * - encoding: unused + * - decoding: Read by user. + */ + int64_t reordered_opaque; + + /** + * hardware accelerator private data (FFmpeg-allocated) + * - encoding: unused + * - decoding: Set by libavcodec + */ + void *hwaccel_picture_private; + + /** + * the AVCodecContext which ff_thread_get_buffer() was last called on + * - encoding: Set by libavcodec. + * - decoding: Set by libavcodec. + */ + struct AVCodecContext *owner; + + /** + * used by multithreading to store frame-specific info + * - encoding: Set by libavcodec. + * - decoding: Set by libavcodec. + */ + 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) + * - encoding: unused + * - decoding: Set by libavcodec. + */ + uint8_t motion_subsample_log2; + + /**（音频）采样率 + * Sample rate of the audio data. + * + * - encoding: unused + * - decoding: read by user + */ + int sample_rate; + + /** + * Channel layout of the audio data. + * + * - encoding: unused + * - decoding: read by user. + */ + uint64_t channel_layout; + + /** + * 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. + */ + int64_t channels; +} AVFrame; +``` + + + +# 二、AVFrame 重点字段 + +AVFrame结构体一般用于存储原始数据（即非压缩数据，例如对视频来说是YUV，RGB，对音频来说是PCM），此外还包含了一些相关的信息。比如说，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，AVFrame是一个很重要的结构体。 + +下面看几个主要变量的作用（在这里考虑解码的情况）： + + + +``` +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。 + +int 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 +``` + + + +其他的变量不再一一列举，源代码中都有详细的说明。在这里重点分析一下几个需要一定的理解的变量： + +### 1.data[] + +对于packed格式的数据（例如RGB24），会存到data[0]里面。 + +对于planar格式的数据（例如YUV420P），则会分开成data[0]，data[1]，data[2]...（YUV420P中data[0]存Y，data[1]存U，data[2]存V） + +### 2.pict_type + +包含以下类型： + + + +``` +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 +}; +``` + + + +### 3.sample_aspect_ratio + +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) +``` +