我手里有一些4K HDR视频，直接用VLC在非HDR设备播放时画面特别暗还偏红，这个问题我知道是HDR转SDR的常见问题。我想用FFmpeg把它转换成SDR格式，网上搜了一套出现频率很高的滤镜组合，我自己加了缩放和锐化滤镜，用的命令是：

ffmpeg -ss 75 -i in0.mkv -map 0:0 \
-vf scale=1280x720,unsharp,zscale=t=linear:npl=100,format=gbrpf32le,\
zscale=p=bt709,tonemap=tonemap=hable,zscale=t=bt709:m=bt709:r=tv,\
format=yuv420p -t 60 -y ou0.mkv

结果转换后的视频亮度只稍微提升了一点，色调和原视频一样偏红。我对色彩空间这些概念不算陌生，但不太明白这套滤镜组合到底起什么作用，也不知道该怎么调整才能得到正常的SDR效果。

下面是FFmpeg的终端输出日志：

*****$ ffmpeg -ss 75 -i in0.mkv -map 0:0 -vf scale=1280x720,unsharp,zscale=t=linear:npl=100,format=gbrpf32le,zscale=p=bt709,tonemap=tonemap=hable,zscale=t=bt709:m=bt709:r=tv,format=yuv420p -t 60 -y ou0.mkv

ffmpeg version 4.4.4 Copyright (c) 2000-2023 the FFmpeg developers
built with Apple LLVM version 9.1.0 (clang-902.0.39.2)
configuration: --prefix=/opt/local --cc=/usr/bin/clang --mandir=/opt/local/share/man --enable-audiotoolbox --disable-indev=jack --disable-libjack --disable-libopencore-amrnb --disable-libopencore-amrwb --disable-libxcb --disable-libxcb-shm --disable-libxcb-xfixes --enable-opencl --disable-outdev=xv --enable-sdl2 --disable-securetransport --enable-videotoolbox --enable-avfilter --enable-avresample --enable-fontconfig --enable-gnutls --enable-libass --enable-libbluray --enable-libdav1d --enable-libfreetype --enable-libfribidi --enable-libmodplug --enable-libmp3lame --enable-libopenjpeg --enable-libopus --enable-librsvg --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libzimg --enable-libzvbi --enable-lzma --enable-pthreads --enable-shared --enable-swscale --enable-zlib --enable-libaom --enable-libsvtav1 --arch=x86_64 --enable-x86asm --enable-gpl --enable-libvidstab --enable-libx264 --enable-libx265 --enable-libxvid --enable-postproc
libavutil      56. 70.100 / 56. 70.100
libavcodec     58.134.100 / 58.134.100
libavformat    58. 76.100 / 58. 76.100
libavdevice    58. 13.100 / 58. 13.100
libavfilter     7.110.100 /  7.110.100
libavresample   4.  0.  0 /  4.  0.  0
libswscale      5.  9.100 /  5.  9.100
libswresample   3.  9.100 /  3.  9.100
libpostproc    55.  9.100 / 55.  9.100
Input #0, matroska,webm, from 'in0.mkv':
Metadata:
encoder         : libebml v1.4.0 + libmatroska v1.6.2
creation_time   : 2021-03-09T20:53:48.000000Z
Writing frontend: StaxRip v1.7.0.3
Duration: 00:49:10.18, start: 0.000000, bitrate: 4963 kb/s
Chapters:
[...]
Stream #0:0: Video: hevc (Main 10), yuv420p10le(tv, bt2020nc/bt2020/smpte2084), 3840x2160, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 1k tbn, 23.98 tbc (default)
Metadata:
BPS-eng         : 4003363
DURATION-eng    : 00:49:09.197000000
NUMBER_OF_FRAMES-eng: 70710
NUMBER_OF_BYTES-eng: 1475838552
_STATISTICS_WRITING_APP-eng: mkvmerge v50.0.0 ('Awakenings') 64-bit
_STATISTICS_WRITING_DATE_UTC-eng: 2021-03-09 20:53:48
_STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
[...]
Stream mapping:
Stream #0:0 -> #0:0 (hevc (native) -> h264 (libx264))
Press [q] to stop, [?] for help
[libx264 @ 0x7f9566801200] using SAR=1/1
[libx264 @ 0x7f9566801200] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX
[libx264 @ 0x7f9566801200] profile High, level 3.1, 4:2:0, 8-bit
[libx264 @ 0x7f9566801200] 264 - core 157 - 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=23 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, matroska, to 'ou0.mkv':
Metadata:
Writing frontend: StaxRip v1.7.0.3
encoder         : Lavf58.76.100
Chapters:
[...]
Stream #0:0: Video: h264 (H264 / 0x34363248), yuv420p(bt709, progressive), 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 23.98 fps, 1k tbn (default)
Metadata:
BPS-eng         : 4003363
DURATION-eng    : 00:49:09.197000000
NUMBER_OF_FRAMES-eng: 70710
NUMBER_OF_BYTES-eng: 1475838552
_STATISTICS_WRITING_APP-eng: mkvmerge v50.0.0 ('Awakenings') 64-bit
_STATISTICS_WRITING_DATE_UTC-eng: 2021-03-09 20:53:48
_STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
encoder         : Lavc58.134.100 libx264
Side data:
cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
frame= 1431 fps= 13 q=-1.0 Lsize=    6532kB time=00:00:59.89 bitrate= 893.4kbits/s speed=0.551x
video:6520kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.177061%
[libx264 @ 0x7f9566801200] frame I:12    Avg QP:17.09  size: 45016
[libx264 @ 0x7f9566801200] frame P:365   Avg QP:20.47  size:  9268
[libx264 @ 0x7f9566801200] frame B:1054  Avg QP:23.93  size:  2612
[libx264 @ 0x7f9566801200] consecutive B-frames:  1.5%  0.7%  0.2% 97.6%
[libx264 @ 0x7f9566801200] mb I  I16..4: 13.1% 68.3% 18.6%
[libx264 @ 0x7f9566801200] mb P  I16..4:  1.6%  5.9%  0.6%  P16..4: 30.9%  7.0%  3.9%  0.0%  0.0%    skip:50.1%
[libx264 @ 0x7f9566801200] mb B  I16..4:  0.1%  0.3%  0.0%  B16..8: 27.7%  1.8%  0.3%  direct: 0.7%  skip:69.0%  L0:42.9% L1:54.3% BI: 2.9%
[libx264 @ 0x7f9566801200] 8x8 transform intra:70.6% inter:84.0%
[libx264 @ 0x7f9566801200] coded y,uvDC,uvAC intra: 41.5% 51.8% 27.2% inter: 5.2% 6.5% 0.7%
[libx264 @ 0x7f9566801200] i16 v,h,dc,p: 32% 21%  8% 39%
[libx264 @ 0x7f9566801200] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 25% 16% 27%  5%  5%  6%  5%  5%  5%
[libx264 @ 0x7f9566801200] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 30% 23% 14%  5%  7%  6%  7%  5%  4%
[libx264 @ 0x7f9566801200] i8c dc,h,v,p: 59% 16% 18%  7%
[libx264 @ 0x7f9566801200] Weighted P-Frames: Y:0.5% UV:0.3%
[libx264 @ 0x7f9566801200] ref P L0: 63.9%  8.5% 18.4%  9.2%  0.1%
[libx264 @ 0x7f9566801200] ref B L0: 89.3%  8.5%  2.3%
[libx264 @ 0x7f9566801200] ref B L1: 95.5%  4.5%
[libx264 @ 0x7f9566801200] kb/s:890.50

我还试过用简单的曲线滤镜调整阴影亮度：

-vf curves=m='0/0 0.1/0.18 0.25/0.33 1/1'

效果不算太差，但色调很难调整，几乎没法把偏红的问题彻底解决。所以我还是想找个更专业的正确方法来处理。

补充测试 #

我尝试了评论区@Rotem的建议，把缩放操作整合到zscale滤镜里，亮度提升了很多，效果接近正常水平，但画面还是明显偏红。

下面是几组屏幕截图对比：

• 原视频直接播放：本来就是偏暗的场景，但实际显示比正常情况还要暗很多
• 我的初始转换结果：和原视频比有细微变化，但整体改善不大
• 按@Rotem建议调整后的结果：亮度已经接近正常，但色调还是偏红严重
• VLC内置导出的PNG截图：这个是正常的效果（场景本身偏暗，但头发细节清晰，色调自然）。我猜测VLC导出PNG时自动做了正确的色彩空间转换，而且这个PNG文件带有sRGB色彩配置文件。

希望大家能给我一些调整建议，谢谢！

备注：内容来源于stack exchange，提问作者PierU