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源码分析
h264编码原理复杂，参数众多。为了方便使用无论x264还是其他编码的实现框架，都封装了几种现有的编码模型，只需要根据编码速度的要求和视频质量的要求选择模型，并修改部分视频参数即可编码。
模型的选择就是通过x264_param_default_preset（）实现。
int x264_param_default_preset( x264_param_t *param, const char *preset, const char *tune ) { x264_param_default( param ); if( preset && x264_param_apply_preset( param, preset ) < 0 ) return -1; if( tune && x264_param_apply_tune( param, tune )< 0 ) return -1; return 0; }
代码可见设置该函数分为三步处理。
-
第二部步根据preset值通过x264_param_apply_preset（x264_param_t *param, const char *preset )函数设置设置编码参数。
preset 接受取值为：
x264_preset_names[] = { "ultrafast", "superfast", "veryfast", "faster", "fast", "medium", "slow", "slower", "veryslow", "placebo", 0 };
此步骤通过速度设定某些参数。static int x264_param_apply_preset( x264_param_t *param, const char *preset ) { char *end; int i = strtol( preset, &end, 10 ); if( *end == 0 && i >= 0 && i < sizeof(x264_preset_names)/sizeof(*x264_preset_names)-1 ) preset = x264_preset_names[i]; if( !strcasecmp( preset, "ultrafast" ) ) { param->i_frame_reference = 1;//参考帧的最大帧数设为1 param->i_scenecut_threshold = 0; param->b_deblocking_filter = 0;//不使用去块滤波 param->b_cabac = 0;//关闭cabac param->i_bframe = 0;//关闭b帧 param->analyse.intra = 0; param->analyse.inter = 0; param->analyse.b_transform_8x8 = 0; param->analyse.i_me_method = X264_ME_DIA;;////运动估算法的选择 param->analyse.i_subpel_refine = 0; param->rc.i_aq_mode = 0; param->analyse.b_mixed_references = 0; param->analyse.i_trellis = 0; param->i_bframe_adaptive = X264_B_ADAPT_NONE; //关闭b帧判定选项 param->rc.b_mb_tree = 0; param->analyse.i_weighted_pred = X264_WEIGHTP_NONE; param->analyse.b_weighted_bipred = 0; param->rc.i_lookahead = 0; } else if( !strcasecmp( preset, "superfast" ) ) { param->analyse.inter = X264_ANALYSE_I8x8|X264_ANALYSE_I4x4; param->analyse.i_me_method = X264_ME_DIA;;//钻石模板 param->analyse.i_subpel_refine = 1;/亚像素运动估计质量为1 param->i_frame_reference = 1;//参考帧的最大帧数设为1 param->analyse.b_mixed_references = 0; param->analyse.i_trellis = 0; param->rc.b_mb_tree = 0; param->analyse.i_weighted_pred = X264_WEIGHTP_SIMPLE; param->rc.i_lookahead = 0; } else if( !strcasecmp( preset, "veryfast" ) ) { param->analyse.i_me_method = X264_ME_HEX; param->analyse.i_subpel_refine = 2; param->i_frame_reference = 1;//参考帧的最大帧数设为1 param->analyse.b_mixed_references = 0; param->analyse.i_trellis = 0; param->analyse.i_weighted_pred = X264_WEIGHTP_SIMPLE; param->rc.i_lookahead = 10; } else if( !strcasecmp( preset, "faster" ) ) { param->analyse.b_mixed_references = 0; param->i_frame_reference = 2;//参考帧的最大帧数设为2 param->analyse.i_subpel_refine = 4; param->analyse.i_weighted_pred = X264_WEIGHTP_SIMPLE; param->rc.i_lookahead = 20; } else if( !strcasecmp( preset, "fast" ) ) { param->i_frame_reference = 2;//参考帧的最大帧数设为2 param->analyse.i_subpel_refine = 6; param->analyse.i_weighted_pred = X264_WEIGHTP_SIMPLE; param->rc.i_lookahead = 30; } else if( !strcasecmp( preset, "medium" ) ) { /* Default is medium *默认参考 set_param_default(); */ } else if( !strcasecmp( preset, "slow" ) ) { param->analyse.i_me_method = X264_ME_UMH;//运动估算发的选择 param->analyse.i_subpel_refine = 8; param->i_frame_reference = 5;//参考帧的最大帧数设为5 param->i_bframe_adaptive = X264_B_ADAPT_TRELLIS; param->analyse.i_direct_mv_pred = X264_DIRECT_PRED_AUTO; param->rc.i_lookahead = 50; } else if( !strcasecmp( preset, "slower" ) ) { param->analyse.i_me_method = X264_ME_UMH;//运动估算发的选择 param->analyse.i_subpel_refine = 9; param->i_frame_reference = 8;//参考帧的最大帧数设为8 param->i_bframe_adaptive = X264_B_ADAPT_TRELLIS; param->analyse.i_direct_mv_pred = X264_DIRECT_PRED_AUTO; param->analyse.inter |= X264_ANALYSE_PSUB8x8; param->analyse.i_trellis = 2; param->rc.i_lookahead = 60; } else if( !strcasecmp( preset, "veryslow" ) ) { param->analyse.i_me_method = X264_ME_UMH;//运动估算发的选择 param->analyse.i_subpel_refine = 10; param->analyse.i_me_range = 24; param->i_frame_reference = 16;//参考帧的最大帧数设为16 param->i_bframe_adaptive = X264_B_ADAPT_TRELLIS; param->analyse.i_direct_mv_pred = X264_DIRECT_PRED_AUTO; param->analyse.inter |= X264_ANALYSE_PSUB8x8; param->analyse.i_trellis = 2; param->i_bframe = 8;//两个参考帧之间b帧为8 param->rc.i_lookahead = 60; } else if( !strcasecmp( preset, "placebo" ) ) { param->analyse.i_me_method = X264_ME_TESA;//运动估算发的选择 param->analyse.i_subpel_refine = 11; param->analyse.i_me_range = 24;//运动估计范围设为24 param->i_frame_reference = 16;//参考帧的最大帧数设为16 param->i_bframe_adaptive = X264_B_ADAPT_TRELLIS; param->analyse.i_direct_mv_pred = X264_DIRECT_PRED_AUTO; param->analyse.inter |= X264_ANALYSE_PSUB8x8; param->analyse.b_fast_pskip = 0; param->analyse.i_trellis = 2; param->i_bframe = 16;//参考帧之间b帧为16 param->rc.i_lookahead = 60; } else { x264_log( NULL, X264_LOG_ERROR, "invalid preset '%s'\n", preset ); return -1; } return 0; }
已加入注释。选用ultrafast会关闭b帧，但是同等质量也会增大码率。
veryfast 之后会启用各种保护质量的算法， 很大程度上降低编码速度。
- 第三步是根据 tune值，通过x264_param_apply_tune（）函数设定编码质量相关值。
static int x264_param_apply_tune( x264_param_t *param, const char *tune )
{
char tmp = x264_malloc( strlen( tune ) + 1 );
if( !tmp )
return -1;
tmp = strcpy( tmp, tune );
char s = strtok( tmp, ",./-+" );
int psy_tuning_used = 0;
while( s )
{
if( !strncasecmp( s, "film", 4 ) )
{
if( psy_tuning_used++ ) goto psy_failure;
param->i_deblocking_filter_alphac0 = -1;
param->i_deblocking_filter_beta = -1;
param->analyse.f_psy_trellis = 0.15;
}
else if( !strncasecmp( s, "animation", 9 ) )
{
if( psy_tuning_used++ ) goto psy_failure;
param->i_frame_reference = param->i_frame_reference > 1 ? param- >i_frame_reference2 : 1;
param->i_deblocking_filter_alphac0 = 1;
param->i_deblocking_filter_beta = 1;
param->analyse.f_psy_rd = 0.4;
param->rc.f_aq_strength = 0.6;
param->i_bframe += 2;//增加b帧
}
else if( !strncasecmp( s, "grain", 5 ) )
{
if( psy_tuning_used++ ) goto psy_failure;
param->i_deblocking_filter_alphac0 = -2;
param->i_deblocking_filter_beta = -2;
param->analyse.f_psy_trellis = 0.25;
param->analyse.b_dct_decimate = 0;
param->rc.f_pb_factor = 1.1;
param->rc.f_ip_factor = 1.1;
param->rc.f_aq_strength = 0.5;
param->analyse.i_luma_deadzone[0] = 6;
param->analyse.i_luma_deadzone[1] = 6;
param->rc.f_qcompress = 0.8;
}
else if( !strncasecmp( s, "stillimage", 10 ) )
{
if( psy_tuning_used++ ) goto psy_failure;
param->i_deblocking_filter_alphac0 = -3;
param->i_deblocking_filter_beta = -3;
param->analyse.f_psy_rd = 2.0;
param->analyse.f_psy_trellis = 0.7;
param->rc.f_aq_strength = 1.2;
}
else if( !strncasecmp( s, "psnr", 4 ) )
{
if( psy_tuning_used++ ) goto psy_failure;
param->rc.i_aq_mode = X264_AQ_NONE;
param->analyse.b_psy = 0;
}
else if( !strncasecmp( s, "ssim", 4 ) )
{
if( psy_tuning_used++ ) goto psy_failure;
param->rc.i_aq_mode = X264_AQ_AUTOVARIANCE;
param->analyse.b_psy = 0;
}
else if( !strncasecmp( s, "fastdecode", 10 ) )
{
param->b_deblocking_filter = 0;
param->b_cabac = 0;
param->analyse.b_weighted_bipred = 0;
param->analyse.i_weighted_pred = X264_WEIGHTP_NONE;
}
else if( !strncasecmp( s, "zerolatency", 11 ) )//关闭b帧
{
param->rc.i_lookahead = 0;
param->i_sync_lookahead = 0;
param->i_bframe = 0; // 关闭b帧
param->b_sliced_threads = 1;
param->b_vfr_input = 0;
param->rc.b_mb_tree = 0;
}
else if( !strncasecmp( s, "touhou", 6 ) )
{
if( psy_tuning_used++ ) goto psy_failure;
param->i_frame_reference = param->i_frame_reference > 1 ? param- >i_frame_reference2 : 1;
param->i_deblocking_filter_alphac0 = -1;
param->i_deblocking_filter_beta = -1;
param->analyse.f_psy_trellis = 0.2;
param->rc.f_aq_strength = 1.3;
if( param->analyse.inter & X264_ANALYSE_PSUB16x16 )
param->analyse.inter |= X264_ANALYSE_PSUB8x8;
}
else
{
x264_log( NULL, X264_LOG_ERROR, "invalid tune '%s'\n", s );
x264_free( tmp );
return -1;
}
if( 0 )
{
psy_failure:x264_log( NULL, X264_LOG_WARNING, "only 1 psy tuning can be used: ignoring tune %s\n", s );
}
s = strtok( NULL, ",./-+" );
}
x264_free( tmp );
return 0;
}
使用zerolatency编码即刻返回编码后的输出，不用flush，问题详细参看：解析ffmpeg 视频流编解码末尾丢帧问题
h->frames.i_delay = param->i_sync_lookahead + // 前向考虑帧数 max ( param->i_bframe, // B帧数量 param->rc.i_lookahead) + // 码率控制前向考虑帧数
而zerolatency 将所有设置为0；
param->rc.i_lookahead = 0;
param->i_sync_lookahead = 0;
param->i_bframe = 0; // 关闭b帧
param->b_sliced_threads = 1;
param->b_vfr_input = 0;
param->rc.b_mb_tree = 0;
命令行使用 $x264 --fullhelp 会显示出对每种模式的说明。
Presets:
--profile <string> Force the limits of an H.264 profile
Overrides all settings.
- baseline:
--no-8x8dct --bframes 0 --no-cabac
--cqm flat --weightp 0
No interlaced.
No lossless.
- main:
--no-8x8dct --cqm flat
No lossless.
- high:
No lossless.
- high10:
No lossless.
Support for bit depth 8-10.
- high422:
No lossless.
Support for bit depth 8-10.
Support for 4:2:0/4:2:2 chroma subsampling.
- high444:
Support for bit depth 8-10.
Support for 4:2:0/4:2:2/4:4:4 chroma subsampling.
--preset <string> Use a preset to select encoding settings [medium]
Overridden by user settings.
- ultrafast:
--no-8x8dct --aq-mode 0 --b-adapt 0
--bframes 0 --no-cabac --no-deblock
--no-mbtree --me dia --no-mixed-refs
--partitions none --rc-lookahead 0 --ref 1
--scenecut 0 --subme 0 --trellis 0
--no-weightb --weightp 0
- superfast:
--no-mbtree --me dia --no-mixed-refs
--partitions i8x8,i4x4 --rc-lookahead 0
--ref 1 --subme 1 --trellis 0 --weightp 1
- veryfast:
--no-mixed-refs --rc-lookahead 10
--ref 1 --subme 2 --trellis 0 --weightp 1
- faster:
--no-mixed-refs --rc-lookahead 20
--ref 2 --subme 4 --weightp 1
- fast:
--rc-lookahead 30 --ref 2 --subme 6
--weightp 1
- medium:
Default settings apply.
- slow:
--b-adapt 2 --direct auto --me umh
--rc-lookahead 50 --ref 5 --subme 8
- slower:
--b-adapt 2 --direct auto --me umh
--partitions all --rc-lookahead 60
--ref 8 --subme 9 --trellis 2
- veryslow:
--b-adapt 2 --bframes 8 --direct auto
--me umh --merange 24 --partitions all
--ref 16 --subme 10 --trellis 2
--rc-lookahead 60
- placebo:
--bframes 16 --b-adapt 2 --direct auto
--slow-firstpass --no-fast-pskip
--me tesa --merange 24 --partitions all
--rc-lookahead 60 --ref 16 --subme 11
--trellis 2
--tune <string> Tune the settings for a particular type of source
or situation
Overridden by user settings.
Multiple tunings are separated by commas.
Only one psy tuning can be used at a time.
- film (psy tuning):
--deblock -1:-1 --psy-rd <unset>:0.15
- animation (psy tuning):
--bframes {+2} --deblock 1:1
--psy-rd 0.4:<unset> --aq-strength 0.6
--ref {Double if >1 else 1}
- grain (psy tuning):
--aq-strength 0.5 --no-dct-decimate
--deadzone-inter 6 --deadzone-intra 6
--deblock -2:-2 --ipratio 1.1
--pbratio 1.1 --psy-rd <unset>:0.25
--qcomp 0.8
- stillimage (psy tuning):
--aq-strength 1.2 --deblock -3:-3
--psy-rd 2.0:0.7
- psnr (psy tuning):
--aq-mode 0 --no-psy
- ssim (psy tuning):
--aq-mode 2 --no-psy
- fastdecode:
--no-cabac --no-deblock --no-weightb
--weightp 0
- zerolatency:
--bframes 0 --force-cfr --no-mbtree
--sync-lookahead 0 --sliced-threads
--rc-lookahead 0
--slow-firstpass Don't force these faster settings with --pass 1:
--no-8x8dct --me dia --partitions none
--ref 1 --subme {2 if >2 else unchanged}
--trellis 0 --fast-psk