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				| @ -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) | ||||
| ``` | ||||
| 
 | ||||
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