[H264]x264_encoder_headers函数

##x264_encoder_headers
#####H264中我们所说的帧是不包含全部的视频的数据的，它是由网络数据NAL和视频数据VCL组成。这个函数主要用来生成一个帧的NAL数据层，这个函数主要是输出SPS、PPS、SEI等数据封装。
具体来看代码

int x264_encoder_headers( x264_t *h, x264_nal_t **pp_nal, int *pi_nal )
{int frame_size = 0;/* init bitstream context */h->out.i_nal = 0;bs_init( &h->out.bs, h->out.p_bitstream, h->out.i_bitstream );/* Write SEI, SPS and PPS. *//* generate sequence parameters */x264_nal_start( h, NAL_SPS, NAL_PRIORITY_HIGHEST );x264_sps_write( &h->out.bs, h->sps );if( x264_nal_end( h ) )return -1;/* generate picture parameters */x264_nal_start( h, NAL_PPS, NAL_PRIORITY_HIGHEST );x264_pps_write( &h->out.bs, h->sps, h->pps );if( x264_nal_end( h ) )return -1;/* identify ourselves */x264_nal_start( h, NAL_SEI, NAL_PRIORITY_DISPOSABLE );if( x264_sei_version_write( h, &h->out.bs ) )return -1;if( x264_nal_end( h ) )return -1;frame_size = x264_encoder_encapsulate_nals( h, 0 );if( frame_size < 0 )return -1;/* now set output*/*pi_nal = h->out.i_nal;*pp_nal = &h->out.nal[0];h->out.i_nal = 0;return frame_size;
}

进入这个函数后，bs_init（）先申请内存用来存放比特流数据的内存块，x264_nal_start接着生成SPS信息并存放在h->out.nal[h->out.inal]中，接着，将SPS信息写入h->sps[0]之中，如下

void x264_sps_write( bs_t *s, x264_sps_t *sps )
{bs_realign( s );//这个函数将8bit写入s->cur_bits中//profile的含义见https://blog.csdn.net/heanyu/article/details/6204414bs_write( s, 8, sps->i_profile_idc );//左移写入四bit数据，为0不用遵从附录A2.1的条件bs_write1( s, sps->b_constraint_set0 );bs_write1( s, sps->b_constraint_set1 );bs_write1( s, sps->b_constraint_set2 );bs_write1( s, sps->b_constraint_set3 );//保留的4位bs_write( s, 4, 0 );    /* reserved *///写入的水平，代表分辨率，同样见https://blog.csdn.net/heanyu/article/details/6204414bs_write( s, 8, sps->i_level_idc );//写入SPS的ID号码bs_write_ue( s, sps->i_id );if( sps->i_profile_idc >= PROFILE_HIGH ){bs_write_ue( s, sps->i_chroma_format_idc );if( sps->i_chroma_format_idc == CHROMA_444 )bs_write1( s, 0 ); // separate_colour_plane_flagbs_write_ue( s, BIT_DEPTH-8 ); // bit_depth_luma_minus8bs_write_ue( s, BIT_DEPTH-8 ); // bit_depth_chroma_minus8bs_write1( s, sps->b_qpprime_y_zero_transform_bypass );bs_write1( s, 0 ); // seq_scaling_matrix_present_flag}bs_write_ue( s, sps->i_log2_max_frame_num - 4 );bs_write_ue( s, sps->i_poc_type );if( sps->i_poc_type == 0 )bs_write_ue( s, sps->i_log2_max_poc_lsb - 4 );bs_write_ue( s, sps->i_num_ref_frames );bs_write1( s, sps->b_gaps_in_frame_num_value_allowed );bs_write_ue( s, sps->i_mb_width - 1 );bs_write_ue( s, (sps->i_mb_height >> !sps->b_frame_mbs_only) - 1);bs_write1( s, sps->b_frame_mbs_only );if( !sps->b_frame_mbs_only )bs_write1( s, sps->b_mb_adaptive_frame_field );bs_write1( s, sps->b_direct8x8_inference );bs_write1( s, sps->b_crop );if( sps->b_crop ){int h_shift = sps->i_chroma_format_idc == CHROMA_420 || sps->i_chroma_format_idc == CHROMA_422;int v_shift = sps->i_chroma_format_idc == CHROMA_420;bs_write_ue( s, sps->crop.i_left   >> h_shift );bs_write_ue( s, sps->crop.i_right  >> h_shift );bs_write_ue( s, sps->crop.i_top    >> v_shift );bs_write_ue( s, sps->crop.i_bottom >> v_shift );}bs_write1( s, sps->b_vui );if( sps->b_vui ){bs_write1( s, sps->vui.b_aspect_ratio_info_present );if( sps->vui.b_aspect_ratio_info_present ){int i;static const struct { uint8_t w, h, sar; } sar[] ={// aspect_ratio_idc = 0 -> unspecified{  1,  1, 1 }, { 12, 11, 2 }, { 10, 11, 3 }, { 16, 11, 4 },{ 40, 33, 5 }, { 24, 11, 6 }, { 20, 11, 7 }, { 32, 11, 8 },{ 80, 33, 9 }, { 18, 11, 10}, { 15, 11, 11}, { 64, 33, 12},{160, 99, 13}, {  4,  3, 14}, {  3,  2, 15}, {  2,  1, 16},// aspect_ratio_idc = [17..254] -> reserved{ 0, 0, 255 }};for( i = 0; sar[i].sar != 255; i++ ){if( sar[i].w == sps->vui.i_sar_width &&sar[i].h == sps->vui.i_sar_height )break;}bs_write( s, 8, sar[i].sar );if( sar[i].sar == 255 ) /* aspect_ratio_idc (extended) */{bs_write( s, 16, sps->vui.i_sar_width );bs_write( s, 16, sps->vui.i_sar_height );}}bs_write1( s, sps->vui.b_overscan_info_present );if( sps->vui.b_overscan_info_present )bs_write1( s, sps->vui.b_overscan_info );bs_write1( s, sps->vui.b_signal_type_present );if( sps->vui.b_signal_type_present ){bs_write( s, 3, sps->vui.i_vidformat );bs_write1( s, sps->vui.b_fullrange );bs_write1( s, sps->vui.b_color_description_present );if( sps->vui.b_color_description_present ){bs_write( s, 8, sps->vui.i_colorprim );bs_write( s, 8, sps->vui.i_transfer );bs_write( s, 8, sps->vui.i_colmatrix );}}bs_write1( s, sps->vui.b_chroma_loc_info_present );if( sps->vui.b_chroma_loc_info_present ){bs_write_ue( s, sps->vui.i_chroma_loc_top );bs_write_ue( s, sps->vui.i_chroma_loc_bottom );}bs_write1( s, sps->vui.b_timing_info_present );if( sps->vui.b_timing_info_present ){bs_write32( s, sps->vui.i_num_units_in_tick );bs_write32( s, sps->vui.i_time_scale );bs_write1( s, sps->vui.b_fixed_frame_rate );}bs_write1( s, sps->vui.b_nal_hrd_parameters_present );if( sps->vui.b_nal_hrd_parameters_present ){bs_write_ue( s, sps->vui.hrd.i_cpb_cnt - 1 );bs_write( s, 4, sps->vui.hrd.i_bit_rate_scale );bs_write( s, 4, sps->vui.hrd.i_cpb_size_scale );bs_write_ue( s, sps->vui.hrd.i_bit_rate_value - 1 );bs_write_ue( s, sps->vui.hrd.i_cpb_size_value - 1 );bs_write1( s, sps->vui.hrd.b_cbr_hrd );bs_write( s, 5, sps->vui.hrd.i_initial_cpb_removal_delay_length - 1 );bs_write( s, 5, sps->vui.hrd.i_cpb_removal_delay_length - 1 );bs_write( s, 5, sps->vui.hrd.i_dpb_output_delay_length - 1 );bs_write( s, 5, sps->vui.hrd.i_time_offset_length );}bs_write1( s, sps->vui.b_vcl_hrd_parameters_present );if( sps->vui.b_nal_hrd_parameters_present || sps->vui.b_vcl_hrd_parameters_present )bs_write1( s, 0 );   /* low_delay_hrd_flag */bs_write1( s, sps->vui.b_pic_struct_present );bs_write1( s, sps->vui.b_bitstream_restriction );if( sps->vui.b_bitstream_restriction ){bs_write1( s, sps->vui.b_motion_vectors_over_pic_boundaries );bs_write_ue( s, sps->vui.i_max_bytes_per_pic_denom );bs_write_ue( s, sps->vui.i_max_bits_per_mb_denom );bs_write_ue( s, sps->vui.i_log2_max_mv_length_horizontal );bs_write_ue( s, sps->vui.i_log2_max_mv_length_vertical );bs_write_ue( s, sps->vui.i_num_reorder_frames );bs_write_ue( s, sps->vui.i_max_dec_frame_buffering );}}bs_rbsp_trailing( s );bs_flush( s );
}