FFmpeg软编码（H.264）
查看libx264编码信息
ffmpeg -h encoder=libx264
Encoder libx264 [libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10]:
General capabilities: delay threads
Threading capabilities: auto
Supported pixel formats: yuv420p yuvj420p yuv422p yuvj422p yuv444p yuvj444p nv12 nv16 nv21 yuv420p10le yuv422p10le yuv444p10le nv20le
x264编码参数
libx264 AVOptions:
-preset <string> E..V..... Set the encoding preset (cf. x264 --fullhelp) (default "medium")
-tune <string> E..V..... Tune the encoding params (cf. x264 --fullhelp)
-profile <string> E..V..... Set profile restrictions (cf. x264 --fullhelp)
-fastfirstpass <boolean> E..V..... Use fast settings when encoding first pass (default true)
-level <string> E..V..... Specify level (as defined by Annex A)
-passlogfile <string> E..V..... Filename for 2 pass stats
-wpredp <string> E..V..... Weighted prediction for P-frames
-a53cc <boolean> E..V..... Use A53 Closed Captions (if available) (default true)
-x264opts <string> E..V..... x264 options
-crf <float> E..V..... Select the quality for constant quality mode (from -1 to FLT_MAX) (default -1)
-crf_max <float> E..V..... In CRF mode, prevents VBV from lowering quality beyond this point. (from -1 to FLT_MAX) (default -1)
-qp <int> E..V..... Constant quantization parameter rate control method (from -1 to INT_MAX) (default -1)
-aq-mode <int> E..V..... AQ method (from -1 to INT_MAX) (default -1)
none E..V.....
variance E..V..... Variance AQ (complexity mask)
autovariance E..V..... Auto-variance AQ
autovariance-biased E..V..... Auto-variance AQ with bias to dark scenes
-aq-strength <float> E..V..... AQ strength. Reduces blocking and blurring in flat and textured areas. (from -1 to FLT_MAX) (default -1)
-psy <boolean> E..V..... Use psychovisual optimizations. (default auto)
-psy-rd <string> E..V..... Strength of psychovisual optimization, in <psy-rd>:<psy-trellis> format.
-rc-lookahead <int> E..V..... Number of frames to look ahead for frametype and ratecontrol (from -1 to INT_MAX) (default -1)
-weightb <boolean> E..V..... Weighted prediction for B-frames. (default auto)
-weightp <int> E..V..... Weighted prediction analysis method. (from -1 to INT_MAX) (default -1)
none E..V.....
simple E..V.....
smart E..V.....
-ssim <boolean> E..V..... Calculate and print SSIM stats. (default auto)
-intra-refresh <boolean> E..V..... Use Periodic Intra Refresh instead of IDR frames. (default auto)
-bluray-compat <boolean> E..V..... Bluray compatibility workarounds. (default auto)
-b-bias <int> E..V..... Influences how often B-frames are used (from INT_MIN to INT_MAX) (default INT_MIN)
-b-pyramid <int> E..V..... Keep some B-frames as references. (from -1 to INT_MAX) (default -1)
none E..V.....
strict E..V..... Strictly hierarchical pyramid
normal E..V..... Non-strict (not Blu-ray compatible)
-mixed-refs <boolean> E..V..... One reference per partition, as opposed to one reference per macroblock (default auto)
-8x8dct <boolean> E..V..... High profile 8x8 transform. (default auto)
-fast-pskip <boolean> E..V..... (default auto)
-aud <boolean> E..V..... Use access unit delimiters. (default auto)
-mbtree <boolean> E..V..... Use macroblock tree ratecontrol. (default auto)
-deblock <string> E..V..... Loop filter parameters, in <alpha:beta> form.
-cplxblur <float> E..V..... Reduce fluctuations in QP (before curve compression) (from -1 to FLT_MAX) (default -1)
-partitions <string> E..V..... A comma-separated list of partitions to consider. Possible values: p8x8, p4x4, b8x8, i8x8, i4x4, none, all
-direct-pred <int> E..V..... Direct MV prediction mode (from -1 to INT_MAX) (default -1)
none E..V.....
spatial E..V.....
temporal E..V.....
auto E..V.....
-slice-max-size <int> E..V..... Limit the size of each slice in bytes (from -1 to INT_MAX) (default -1)
-stats <string> E..V..... Filename for 2 pass stats
-nal-hrd <int> E..V..... Signal HRD information (requires vbv-bufsize; cbr not allowed in .mp4) (from -1 to INT_MAX) (default -1)
none E..V.....
vbr E..V.....
cbr E..V.....
-avcintra-class <int> E..V..... AVC-Intra class 50/100/200 (from -1 to 200) (default -1)
-me_method <int> E..V..... Set motion estimation method (from -1 to 4) (default -1)
dia E..V.....
hex E..V.....
umh E..V.....
esa E..V.....
tesa E..V.....
-motion-est <int> E..V..... Set motion estimation method (from -1 to 4) (default -1)
dia E..V.....
hex E..V.....
umh E..V.....
esa E..V.....
tesa E..V.....
-forced-idr <boolean> E..V..... If forcing keyframes, force them as IDR frames. (default false)
-coder <int> E..V..... Coder type (from -1 to 1) (default default)
default E..V.....
cavlc E..V.....
cabac E..V.....
vlc E..V.....
ac E..V.....
-b_strategy <int> E..V..... Strategy to choose between I/P/B-frames (from -1 to 2) (default -1)
-chromaoffset <int> E..V..... QP difference between chroma and luma (from INT_MIN to INT_MAX) (default -1)
-sc_threshold <int> E..V..... Scene change threshold (from INT_MIN to INT_MAX) (default -1)
-noise_reduction <int> E..V..... Noise reduction (from INT_MIN to INT_MAX) (default -1)
-x264-params <string> E..V..... Override the x264 configuration using a :-separated list of key=value parameters
H.264编码举例：
x264 --full help
-
1. 编码器预设参数设置preset
--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:
--direct auto --rc-lookahead 50 --ref 5
--subme 8 --trellis 2
- 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
- ultrafast：最快的编码方式
- superfast：超级快速的编码方式
- veryfast：非常快速的编码方式
- faster：稍微快速的编码方式
- fast：快速的编码方式
- medium：折中的编码方式
- slow：慢的编码方式
- slower：更慢的编码方式
- veryslow：非常慢的编码方式
- placebo：最慢的编码方式
不同的预设参数对转码效率的影响不同。
设置ultrafast参数：
ffmpeg -i 222.mp4 -vcodec libx264 -preset ultrafast -b:v 2000k 333.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '222.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf56.19.100
Duration: 00:00:30.32, start: 0.000000, bitrate: 397 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 480x208 [SAR 16:15 DAR 32:13], 261 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
Metadata:
handler_name : VideoHandler
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
Metadata:
handler_name : SoundHandler
Stream mapping:
Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
Stream #0:1 -> #0:1 (aac (native) -> aac (native))
Press [q] to stop, [?] for help
[libx264 @ 0x7fc2f500fc00] using SAR=16/15
[libx264 @ 0x7fc2f500fc00] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX
[libx264 @ 0x7fc2f500fc00] profile Constrained Baseline, level 2.1
[libx264 @ 0x7fc2f500fc00] 264 - core 155 r2917 0a84d98 - H.264/MPEG-4 AVC codec - Copyleft 2003-2018 - http://www.videolan.org/x264.html - options: cabac=0 ref=1 deblock=0:0:0 analyse=0:0 me=dia subme=0 psy=1 psy_rd=1.00:0.00 mixed_ref=0 me_range=16 chroma_me=1 trellis=1 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=0 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=0 weightp=0 keyint=250 keyint_min=25 scenecut=0 intra_refresh=0 rc=abr mbtree=0 bitrate=2000 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=0
Output #0, mp4, to '333.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf58.20.100
Stream #0:0(und): Video: h264 (libx264) (avc1 / 0x31637661), yuv420p, 480x208 [SAR 16:15 DAR 32:13], q=-1--1, 2000 kb/s, 25 fps, 12800 tbn, 25 tbc (default)
Metadata:
handler_name : VideoHandler
encoder : Lavc58.35.100 libx264
Side data:
cpb: bitrate max/min/avg: 0/0/2000000 buffer size: 0 vbv_delay: -1
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
Metadata:
handler_name : SoundHandler
encoder : Lavc58.35.100 aac
frame= 149 fps=0.0 q=16.0 size= 1536kB time=00:00:06.08 bitrate=2068.4kbits/frame= 288 fps=287 q=13.0 size= 2816kB time=00:00:11.60 bitrate=1987.0kbits/frame= 452 fps=301 q=11.0 size= 4608kB time=00:00:18.18 bitrate=2076.3kbits/frame= 613 fps=306 q=11.0 size= 5888kB time=00:00:24.61 bitrate=1959.7kbits/frame= 756 fps=309 q=-1.0 Lsize= 7582kB time=00:00:30.30 bitrate=2049.7kbits/s speed=12.4x
如上，转码速度为12.4倍速。
设置medium参数：
ffmpeg -i 222.mp4 -vcodec libx264 -preset medium -b:v 2000k 333.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '222.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf56.19.100
Duration: 00:00:30.32, start: 0.000000, bitrate: 397 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 480x208 [SAR 16:15 DAR 32:13], 261 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
Metadata:
handler_name : VideoHandler
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
Metadata:
handler_name : SoundHandler
Stream mapping:
Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
Stream #0:1 -> #0:1 (aac (native) -> aac (native))
Press [q] to stop, [?] for help
[libx264 @ 0x7fd62c809200] using SAR=16/15
[libx264 @ 0x7fd62c809200] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX
[libx264 @ 0x7fd62c809200] profile High, level 2.1
[libx264 @ 0x7fd62c809200] 264 - core 155 r2917 0a84d98 - H.264/MPEG-4 AVC codec - Copyleft 2003-2018 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=abr mbtree=1 bitrate=2000 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to '333.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf58.20.100
Stream #0:0(und): Video: h264 (libx264) (avc1 / 0x31637661), yuv420p, 480x208 [SAR 16:15 DAR 32:13], q=-1--1, 2000 kb/s, 25 fps, 12800 tbn, 25 tbc (default)
Metadata:
handler_name : VideoHandler
encoder : Lavc58.35.100 libx264
Side data:
cpb: bitrate max/min/avg: 0/0/2000000 buffer size: 0 vbv_delay: -1
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
Metadata:
handler_name : SoundHandler
encoder : Lavc58.35.100 aac
frame= 116 fps=0.0 q=9.0 size= 512kB time=00:00:04.73 bitrate= 885.5kbits/sframe= 190 fps=187 q=7.0 size= 1280kB time=00:00:07.70 bitrate=1360.2kbits/sframe= 273 fps=179 q=6.0 size= 2048kB time=00:00:11.02 bitrate=1521.2kbits/sframe= 347 fps=170 q=7.0 size= 3072kB time=00:00:13.97 bitrate=1800.4kbits/sframe= 424 fps=167 q=8.0 size= 3840kB time=00:00:17.06 bitrate=1843.2kbits/sframe= 497 fps=163 q=8.0 size= 4608kB time=00:00:19.96 bitrate=1890.4kbits/sframe= 571 fps=161 q=8.0 size= 5376kB time=00:00:22.91 bitrate=1921.7kbits/sframe= 647 fps=160 q=7.0 size= 6144kB time=00:00:25.98 bitrate=1937.1kbits/sframe= 718 fps=157 q=8.0 size= 6912kB time=00:00:28.81 bitrate=1965.0kbits/sframe= 756 fps=152 q=-1.0 Lsize= 7652kB time=00:00:30.30 bitrate=2068.6kbits/s speed= 6.1x
如上，转码速度为6.1倍速，但图像质量要比ultrafast高。
-
2. H.264编码优化参数tune
--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
- film
- animation
- grain
- stillimage
- psnr
- ssim
- fastdecode
- zerolatency
如上，可使用以上几个参数进行调优编码，如使用zerolatency会提升效率，因为它降低了因编码导致的延迟。
-
3. H.264的profile(档次)与level(等级)设置
使用baseline profile编码一个H.264视频，不会包含B Slice：
ffmpeg -i input.mp4 -vcodec libx264 -profile:v baseline -level 3.1 -s 352x288 -an -y -t 10 output_baseline.ts
使用ffprobe查看output_baseline.ts中每帧包含B帧的情况：
aiputekiMacBook-Pro:bin aipu$ ffprobe -v quiet -show_frames -select_streams v output_baseline.ts |grep "pict_type=B"|wc -l
0
使用high profile编码一个H.264视频，包含B Slice：
ffmpeg -i input.mp4 -vcodec libx264 -profile:v high -level 3.1 -s 352x288 -an -y -t 10 output_high.ts
使用ffprobe查看output_high.ts中每帧包含B帧的情况：
aiputekiMacBook-Pro:bin aipu$ ffprobe -v quiet -show_frames -select_streams v output_high.ts |grep "pict_type=B"|wc -l
173
如上，baseline profile包含0个B帧，high profile包含173个B帧。
-
4. 控制场景切换关键帧插入参数sc_threshold
不使用ffmpeg命令控制编码时的GOP大小：
ffmpeg -i input.mp4 -c:v libx264 -g 50 output.mp4
如上，使用工具查看GOP间隔不是均匀的。
使用ffmpeg命令控制编码时的GOP大小：
ffmpeg -i input.mp4 -c:v libx264 -g 50 -sc_threshold 0 -y output.mp4
如上，使用工具查看GOP间隔是均匀的。
-
5. 设置x264内部参数x264opts
该参数可以控制帧的顺序及出现频率
设置GOP为50帧，并且不出现B帧：
ffmpeg -i input.mp4 -c:v libx264 -x264opts "bframes=0" -g 50 -sc_threshold 0 output.mp4
如图，使用工具查看output.mp4中并不包含B帧。
设置GOP中每2个P帧之间存放3个B帧：
ffmpeg -i input.mp4 -c:v libx264 -x264opts "bframes=3:b-adapt=0" -g 50 -sc_threshold 0 output.mp4
如图，使用工具查看output.mp4中每2个P帧之间存放了3个B帧。
-
6. CBR恒定码率设置参数nal-hrd
设置VBR(可变码率)、CBR(恒定码率)，控制码率的波动，以及设置buffer的大小：
ffmpeg -i input.mp4 -c:v libx264 -x264opts "bframes=10:b-adapt=0" -b:v 1000k -maxrate 1000k -minrate 1000k -bufsize 50k -nal-hrd cbr -g 50 -sc_threshold 0 output.ts
以上命令的具体分析如下：
- 设置B帧的个数，并且是每2个P帧之间包含10个B帧
- 设置视频码率为1000kbit/s
- 设置最大码率为1000kbit/s
- 设置最小码率为1000kbit/s
- 设置编码的buffer大小为50KB
- 设置H.264的编码HRD信号形式为CBR
- 设置GOP为50帧
- 设置场景切换不强行插入关键帧
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