x264重要结构体详细说明（4）： x264

本博客转载于：http://nkwavelet.blog.163.com/blog/static/227756038201431010539683/

x264_t是一个控制X264编码的全局性结构体，该结构体控制着视频一帧一帧的编码，包括中间参考帧管理、码率控制、全局参数等一些重要参数和结构体。该结构体定义在common/common.h中，接下来我们详细分析该结构体的元素：

struct x264_t

{

/* encoder parameters */

x264_param_t param; // 编码器编码参数，包括量化步长、编码级别等等一些参数

x264_t *thread[X264_THREAD_MAX+1];
x264_t *lookahead_thread[X264_LOOKAHEAD_THREAD_MAX];

int b_thread_active;

int i_thread_phase; /* which thread to use for the next frame */

int i_thread_idx; /* which thread this is */

int i_threadslice_start; /* first row in this thread slice */

int i_threadslice_end; /* row after the end of this thread slice */

int i_threadslice_pass; /* which pass of encoding we are on */

x264_threadpool_t *threadpool;

x264_threadpool_t *lookaheadpool;

x264_pthread_mutex_t mutex;

x264_pthread_cond_t cv;

/* bitstream output */

struct

{

int i_nal;

int i_nals_allocated;

int i_bitstream; /* size of p_bitstream */

uint8_t *p_bitstream; /* will hold data for all nal */

bs_t bs;
x264_nal_t *nal;

} out;

uint8_t *nal_buffer;

int nal_buffer_size;

int reconfig;
x264_t *reconfig_h;

/**** thread synchronization starts here ****/

/* frame number/poc */

int i_frame; // 编码帧号，用于计算POC（picture of count 标识视频帧的解码顺序）

int i_frame_num;

int i_thread_frames; // Number of different frames being encoded by threads;

// 1 when sliced-threads is on.

int i_nal_type; // Nal单元的类型，这些类型值定义在x264.h中，需要理解的类型有：

// 不分区（一帧作为一个片）非IDR图像的片；片分区A、片分区B、片分区C、

// IDR图像中的片、序列参数集、图像参数集

int i_nal_ref_idc; // Nal单元的优先级别,取值范围[0,1,2,3]，值越大表示优先级越高，此Nal单元就越重要

int64_t i_disp_fields; // Number of displayed fields (both coded and implied via pic_struct)
int i_disp_fields_last_frame;

int64_t i_prev_duration; // Duration of previous frame

int64_t i_coded_fields; // Number of coded fields (both coded and implied via pic_struct)

int64_t i_cpb_delay; // Equal to number of fields preceding this field since last buffering_period SEI

int64_t i_coded_fields_lookahead; // Use separate counters for lookahead

int64_t i_cpb_delay_lookahead;

int64_t i_cpb_delay_pir_offset;

int64_t i_cpb_delay_pir_offset_next;
int64_t i_last_idr_pts;

int b_queued_intra_refresh;

int i_idr_pic_id;

/* quantization matrix for decoding, [cqm][qp%6][coef] */

int (*dequant4_mf[4])[16]; /* [4][6][16] */

int (*dequant8_mf[4])[64]; /* [4][6][64] */

/* quantization matrix for trellis, [cqm][qp][coef] */

int (*unquant4_mf[4])[16]; /* [4][QP_MAX_SPEC][16] */

int (*unquant8_mf[4])[64]; /* [4][QP_MAX_SPEC][64] */

/* quantization matrix for deadzone */

udctcoef (*quant4_mf[4])[16]; /* [4][QP_MAX_SPEC][16] */

udctcoef (*quant8_mf[4])[64]; /* [4][QP_MAX_SPEC][64] */

udctcoef (*quant4_bias[4])[16]; /* [4][QP_MAX_SPEC][16] */

udctcoef (*quant8_bias[4])[64]; /* [4][QP_MAX_SPEC][64] */

udctcoef (*quant4_bias0[4])[16]; /* [4][QP_MAX_SPEC][16] */

udctcoef (*quant8_bias0[4])[64]; /* [4][QP_MAX_SPEC][64] */

udctcoef (*nr_offset_emergency)[4][64];

/* mv/ref cost arrays. */

uint16_t *cost_mv[QP_MAX+1];

uint16_t *cost_mv_fpel[QP_MAX+1][4];

/* includes both the nonlinear luma->chroma mapping and chroma_qp_offset */

const uint8_t *chroma_qp_table;

/* Slice header，条带头结构体定义在common/common.h中 */

x264_slice_header_t sh;

/* SPS / PPS，我们只是用一个sps和一个pps */

x264_sps_t sps[1];

x264_pps_t pps[1];

/* Slice header backup, for SEI_DEC_REF_PIC_MARKING */

int b_sh_backup;

x264_slice_header_t sh_backup;

/* cabac context */

x264_cabac_t cabac;

/* 指示和控制帧编码过程的结构 */

struct

{

/* Frames to be encoded (whose types have been decided) */
/* 这个结构体涉及到X264编码过程中的帧管理，理解这个结构体中的变量在编码标准的理论意义是非常重要的 */

x264_frame_t **current; // 已确定帧类型，待编码帧，每一个GOP在编码前，每一帧的类型在编码前已经确定。

// 当进行编码时，从这里取出一帧数据
/* Unused frames: 0 = fenc, 1 = fdec */

x264_frame_t **unused[2]; // 这个数组用于回收那些在编码中分配的frame空间，

// 当有新的需要时，直接拿过来用，不用重新分配新的空间，提高效率

/* Unused blank frames (for duplicates) */

x264_frame_t **blank_unused;

/* frames used for reference + sentinels */

x264_frame_t *reference[X264_REF_MAX+2]; // 参考帧队列，注意参考帧都是重建帧

int i_last_keyframe; // Frame number of the last keyframe，上一个关键字的帧序号

int i_last_idr; // Frame number of the last IDR (not RP)，上一个IDR帧的帧序号，

// 配合前面的i_frame，可以用来计算POC

int i_poc_last_open_gop; // Poc of the I frame of the last open-gop. The value is only assigned during the

// period between that I frame and the next P or I frame, else -1

int i_input; // Number of input frames already accepted，已经接收的输入帧数

// frames结构体中i_input指示当前输入的帧的（播放顺序）序号。

int i_max_dpb; // Number of frames allocated in the decoded picture buffer

int i_max_ref0; // 最大前向参考帧数量

int i_max_ref1; // 最大后向参考帧数量

int i_delay; // Number of frames buffered for B reordering；缓存的帧数，用于B帧重排序

int i_bframe_delay;

int64_t i_bframe_delay_time;

int64_t i_first_pts;

int64_t i_prev_reordered_pts[2];

int64_t i_largest_pts;

int64_t i_second_largest_pts;

int b_have_lowres; // Whether 1/2 resolution luma planes are being used，亮度是否使用半像素精度

int b_have_sub8x8_esa;

} frames; // 指示和控制帧编码过程的结构

/* current frame being encoded */

x264_frame_t *fenc; // 指向当前编码帧

/* frame being reconstructed */

x264_frame_t *fdec; // 指向当前重建帧，重建帧的帧号要比当前编码帧的帧号小1

/* references lists */

int i_ref[2]; // i_ref[0]和i_ref[1]分别表示前向和后向参考帧的数量

int b_ref_reorder[2];

x264_frame_t *fref[2][X264_REF_MAX+3]; // fref[0]和fref[1]分别表示存放前向和后向参考帧的数组（参考帧均是重建帧）

x264_frame_t *fref_nearest[2];

/* hrd */

int initial_cpb_removal_delay;

int initial_cpb_removal_delay_offset;

int64_t i_reordered_pts_delay;

/* Current MB DCT coeffs */

struct

{

ALIGNED_N( dctcoef luma16x16_dc[3][16] );

ALIGNED_16( dctcoef chroma_dc[2][8] );

// FIXME share memory?

ALIGNED_N( dctcoef luma8x8[12][64] );

ALIGNED_N( dctcoef luma4x4[16*3][16] );

} dct;

/* MB table and cache for current frame/mb */
struct
{

int i_mb_width;

int i_mb_height;

int i_mb_count; /* number of mbs in a frame */

/* Chroma subsampling */

int chroma_h_shift;

int chroma_v_shift;

/* Strides */

int i_mb_stride;

int i_b8_stride;

int i_b4_stride;

int left_b8[2];

int left_b4[2];

/* Current index */

int i_mb_x;

int i_mb_y;

int i_mb_xy;

int i_b8_xy;

int i_b4_xy;

/* Search parameters */

int i_me_method;

int i_subpel_refine;

int b_chroma_me;

int b_trellis;

int b_noise_reduction;

int b_dct_decimate;

int i_psy_rd; /* Psy RD strength--fixed point value*/

int i_psy_trellis; /* Psy trellis strength--fixed point value*/

int b_interlaced;

int b_adaptive_mbaff; /* MBAFF+subme 0 requires non-adaptive MBAFF i.e. all field mbs */

/* Allowed qpel MV range to stay within the picture + emulated edge pixels */

int mv_min[2];

int mv_max[2];

int mv_miny_row[3]; /* 0 == top progressive, 1 == bot progressive, 2 == interlaced */

int mv_maxy_row[3];

/* Subpel MV range for motion search. same mv_min/max but includes levels' i_mv_range. */

int mv_min_spel[2];

int mv_max_spel[2];

int mv_miny_spel_row[3];

int mv_maxy_spel_row[3];

/* Fullpel MV range for motion search */

ALIGNED_8( int16_t mv_limit_fpel[2][2] ); /* min_x, min_y, max_x, max_y */

int mv_miny_fpel_row[3];

int mv_maxy_fpel_row[3];

/* neighboring MBs */

unsigned int i_neighbour;

unsigned int i_neighbour8[4]; /* neighbours of each 8x8 or 4x4 block that are available */

unsigned int i_neighbour4[16]; /* at the time the block is coded */

unsigned int i_neighbour_intra; /* for constrained intra pred */

unsigned int i_neighbour_frame; /* ignoring slice boundaries */

int i_mb_type_top;

int i_mb_type_left[2];

int i_mb_type_topleft;

int i_mb_type_topright;

int i_mb_prev_xy;

int i_mb_left_xy[2];

int i_mb_top_xy;

int i_mb_topleft_xy;

int i_mb_topright_xy;

int i_mb_top_y;

int i_mb_topleft_y;

int i_mb_topright_y;

const x264_left_table_t *left_index_table;

int i_mb_top_mbpair_xy;

int topleft_partition;

int b_allow_skip;

int field_decoding_flag;

/**** thread synchronization ends here ****/

/* subsequent variables are either thread-local or constant,
* and won't be copied from one thread to another */

/* mb table */

uint8_t *base; /* base pointer for all malloced data in this mb */

int8_t *type; /* mb type */

uint8_t *partition; /* mb partition */

int8_t *qp; /* mb qp */

int16_t *cbp; // mb cbp:0x0?:luma, 0x?0:chroma, 0x100:luma dc,
// 0x0200 and 0x0400: chroma dc(all set for PCM)

int8_t (*intra4x4_pred_mode)[8]; /* intra4x4 pred mode. for non I4x4 set to I_PRED_4x4_DC(2) */

/* actually has only 7 entries; set to 8 for write-combining optimizations */

uint8_t (*non_zero_count)[16*3]; /* nzc. for I_PCM set to 16 */

int8_t *chroma_pred_mode; /* chroma_pred_mode. cabac only. for non intra I_PRED_CHROMA_DC(0) */

int16_t (*mv[2])[2]; /* mb mv. set to 0 for intra mb */

uint8_t (*mvd[2])[8][2]; // absolute value of mb mv difference with predict, clipped to [0,33].
// set to 0 if intra. cabac only

int8_t *ref[2]; /* mb ref. set to -1 if non used (intra or Lx only) */

int16_t (*mvr[2][X264_REF_MAX*2])[2]; /* 16x16 mv for each possible ref */

int8_t *skipbp; /* block pattern for SKIP or DIRECT (sub)mbs. B-frames + cabac only */

int8_t *mb_transform_size; /* transform_size_8x8_flag of each mb */

uint16_t *slice_table; /* sh->first_mb of the slice that the indexed mb is part of

* NOTE: this will fail on resolutions above 2^16 MBs... */

uint8_t *field;

/* buffer for weighted versions of the reference frames */

pixel *p_weight_buf[X264_REF_MAX];

}

待续！

};

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