[FFmpeg-user] color shift in swscale - slight green tint

[Paul B Mahol]

On 2/19/13, Chris Bouchard <Chris.Bouchard at framestore.com> wrote:
> I'm getting a colour shift when converting from a 422 pixel format to
> yuv420p. For example, encoding a grey card (R=128,G=128,B=128) results in a
> subtle green tint on output (R=126,G=128,B=125). The workaround to avoid
> color shift is to use " -sws_flags bilinear" or use -sws_flags
> lanczos+accurate_rnd
>
> This might be a fault in swscale or the default scaling used
>
> Default behavior results in color shift . visible as a slight green tint if
> you encode a plain grey card.
>
> Green tint apparent:
> ffmpeg -i 18percent_greyPR.mov -pix_fmt yuv420p -f mp4 -vcodec libx264 -crf
> 23 -s 1280x720 -report default_sws720.mp4
> ffmpeg started on 2013-02-18 at 13:46:14
> Report written to "ffmpeg-20130218-134614.log"
> ffmpeg version N-49601-g8f7e3e6 Copyright (c) 2000-2013 the FFmpeg
> developers
> built on Feb 13 2013 19:03:28 with gcc 4.4.5 (Debian 4.4.5-12)
> configuration: --enable-gpl --enable-libfaac --enable-libmp3lame
> --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264
> --enable-nonfree --enable-version3 --enable-libfaac --enable-libxvid
> libavutil 52. 17.101 / 52. 17.101
> libavcodec 54. 91.100 / 54. 91.100
> libavformat 54. 61.104 / 54. 61.104
> libavdevice 54. 3.103 / 54. 3.103
> libavfilter 3. 35.101 / 3. 35.101
> libswscale 2. 2.100 / 2. 2.100
> libswresample 0. 17.102 / 0. 17.102
> libpostproc 52. 2.100 / 52. 2.100
> Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '18percent_greyPR.mov':
> Metadata:
> major_brand : qt
> minor_version : 537199360
> compatible_brands: qt
> creation_time : 2013-02-18 12:42:38
> Duration: 00:00:02.08, start: 0.000000, bitrate: 9121 kb/s
> Stream #0:0(eng): Video: prores (apcn / 0x6E637061), yuv422p10le, 1920x1080,
> 9117 kb/s, 24 fps, 24 tbr, 2400 tbn, 2400 tbc
> Metadata:
> creation_time : 2013-02-18 12:42:38
> handler_name : Apple Alias Data Handler
> [graph 0 input from stream 0:0 @ 0x2195f60] w:1920 h:1080 pixfmt:yuv422p10le
> tb:1/2400 fr:24/1 sar:0/1 sws_param:flags=2
> [scaler for output stream 0:0 @ 0x218fb80] w:1280 h:720 flags:'0x4' interl:0
>
> [scaler for output stream 0:0 @ 0x218fb80] w:1920 h:1080 fmt:yuv422p10le
> sar:0/1 -> w:1280 h:720 fmt:yuv420p sar:0/1 flags:0x4
> [libx264 @ 0x219eb00] using cpu capabilities: MMX2 SSE2Slow SlowCTZ
> [libx264 @ 0x219eb00] profile High, level 3.1
> [libx264 @ 0x219eb00] 264 - core 129 r2245 bc13772 - H.264/MPEG-4 AVC codec
> - Copyleft 2003-2013 - 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=24 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, mp4, to 'default_sws720.mp4':
> Metadata:
> major_brand : qt
> minor_version : 537199360
> compatible_brands: qt
> encoder : Lavf54.61.104
> Stream #0:0(eng): Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 1280x720,
> q=-1--1, 12288 tbn, 24 tbc
> Metadata:
> creation_time : 2013-02-18 12:42:38
> handler_name : Apple Alias Data Handler
> Stream mapping:
> Stream #0:0 -> #0:0 (prores -> libx264)
> Press [q] to stop, [?] for help
> No more output streams to write to, finishing.=00:00:00.00 bitrate=N/A
> frame= 50 fps= 18 q=-1.0 Lsize= 8kB time=00:00:02.00 bitrate= 34.7kbits/s
> video:7kB audio:0kB subtitle:0 global headers:0kB muxing overhead 19.223889%
>
> [libx264 @ 0x219eb00] frame I:1 Avg QP:10.32 size: 4236
> [libx264 @ 0x219eb00] frame P:13 Avg QP:12.62 size: 73
> [libx264 @ 0x219eb00] frame B:36 Avg QP:18.67 size: 38
> [libx264 @ 0x219eb00] consecutive B-frames: 4.0% 0.0% 0.0% 96.0%
> [libx264 @ 0x219eb00] mb I I16..4: 95.2% 2.2% 2.5%
> [libx264 @ 0x219eb00] mb P I16..4: 0.2% 0.0% 0.0% P16..4: 0.8% 0.0% 0.0%
> 0.0% 0.0% skip:99.0%
> [libx264 @ 0x219eb00] mb B I16..4: 0.0% 0.0% 0.0% B16..8: 0.0% 0.0% 0.0%
> direct: 0.0% skip:100.0% L0:25.0% L1:75.0% BI: 0.0%
> [libx264 @ 0x219eb00] 8x8 transform intra:2.2% inter:0.0%
> [libx264 @ 0x219eb00] coded y,uvDC,uvAC intra: 2.8% 0.1% 0.0% inter: 0.0%
> 0.2% 0.0%
> [libx264 @ 0x219eb00] i16 v,h,dc,p: 95% 1% 5% 0%
> [libx264 @ 0x219eb00] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 59% 2% 39% 0% 0% 0% 0%
> 0% 0%
> [libx264 @ 0x219eb00] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 34% 17% 22% 5% 5% 4% 3%
> 6% 3%
> [libx264 @ 0x219eb00] i8c dc,h,v,p: 100% 0% 0% 0%
> [libx264 @ 0x219eb00] Weighted P-Frames: Y:0.0% UV:0.0%
> [libx264 @ 0x219eb00] kb/s:25.11
>
> CORRECT COLOUR was achieved by forcing "-sws_flags bilinear" which resulted
> in: R=127,G=127,B=127 as follows:
>
> ffmpeg -i 18percent_greyPR.mov -pix_fmt yuv420p -sws_flags bilinear -f mp4
> -vcodec libx264 -s 1280x720 -crf 23 -report bilinear_sws720.mp4
> ffmpeg started on 2013-02-18 at 13:47:36
> Report written to "ffmpeg-20130218-134736.log"
> ffmpeg version N-49601-g8f7e3e6 Copyright (c) 2000-2013 the FFmpeg
> developers
> built on Feb 13 2013 19:03:28 with gcc 4.4.5 (Debian 4.4.5-12)
> configuration: --enable-gpl --enable-libfaac --enable-libmp3lame
> --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264
> --enable-nonfree --enable-version3 --enable-libfaac --enable-libxvid
> libavutil 52. 17.101 / 52. 17.101
> libavcodec 54. 91.100 / 54. 91.100
> libavformat 54. 61.104 / 54. 61.104
> libavdevice 54. 3.103 / 54. 3.103
> libavfilter 3. 35.101 / 3. 35.101
> libswscale 2. 2.100 / 2. 2.100
> libswresample 0. 17.102 / 0. 17.102
> libpostproc 52. 2.100 / 52. 2.100
> Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '18percent_greyPR.mov':
> Metadata:
> major_brand : qt
> minor_version : 537199360
> compatible_brands: qt
> creation_time : 2013-02-18 12:42:38
> Duration: 00:00:02.08, start: 0.000000, bitrate: 9121 kb/s
> Stream #0:0(eng): Video: prores (apcn / 0x6E637061), yuv422p10le, 1920x1080,
> 9117 kb/s, 24 fps, 24 tbr, 2400 tbn, 2400 tbc
> Metadata:
> creation_time : 2013-02-18 12:42:38
> handler_name : Apple Alias Data Handler
> [graph 0 input from stream 0:0 @ 0x353d1e0] w:1920 h:1080 pixfmt:yuv422p10le
> tb:1/2400 fr:24/1 sar:0/1 sws_param:flags=2
> [scaler for output stream 0:0 @ 0x352dbc0] w:1280 h:720 flags:'0x2' interl:0
>
> [scaler for output stream 0:0 @ 0x352dbc0] w:1920 h:1080 fmt:yuv422p10le
> sar:0/1 -> w:1280 h:720 fmt:yuv420p sar:0/1 flags:0x2
> [libx264 @ 0x353cc80] using cpu capabilities: MMX2 SSE2Slow SlowCTZ
> [libx264 @ 0x353cc80] profile High, level 3.1
> [libx264 @ 0x353cc80] 264 - core 129 r2245 bc13772 - H.264/MPEG-4 AVC codec
> - Copyleft 2003-2013 - 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=24 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, mp4, to 'bilinear_sws720.mp4':
> Metadata:
> major_brand : qt
> minor_version : 537199360
> compatible_brands: qt
> encoder : Lavf54.61.104
> Stream #0:0(eng): Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 1280x720,
> q=-1--1, 12288 tbn, 24 tbc
> Metadata:
> creation_time : 2013-02-18 12:42:38
> handler_name : Apple Alias Data Handler
> Stream mapping:
> Stream #0:0 -> #0:0 (prores -> libx264)
> Press [q] to stop, [?] for help
> No more output streams to write to, finishing.=00:00:00.00 bitrate=N/A
> frame= 50 fps= 22 q=-1.0 Lsize= 8kB time=00:00:02.00 bitrate= 32.3kbits/s
> video:6kB audio:0kB subtitle:0 global headers:0kB muxing overhead 20.916305%
>
> [libx264 @ 0x353cc80] frame I:1 Avg QP:15.19 size: 3953
> [libx264 @ 0x353cc80] frame P:13 Avg QP:16.07 size: 49
> [libx264 @ 0x353cc80] frame B:36 Avg QP:19.67 size: 38
> [libx264 @ 0x353cc80] consecutive B-frames: 4.0% 0.0% 0.0% 96.0%
> [libx264 @ 0x353cc80] mb I I16..4: 97.5% 0.0% 2.5%
> [libx264 @ 0x353cc80] mb P I16..4: 0.0% 0.0% 0.0% P16..4: 0.0% 0.0% 0.0%
> 0.0% 0.0% skip:100.0%
> [libx264 @ 0x353cc80] mb B I16..4: 0.0% 0.0% 0.0% B16..8: 0.0% 0.0% 0.0%
> direct: 0.0% skip:100.0% L0: 0.0% L1:100.0% BI: 0.0%
> [libx264 @ 0x353cc80] 8x8 transform intra:0.0% inter:100.0%
> [libx264 @ 0x353cc80] coded y,uvDC,uvAC intra: 1.7% 0.0% 0.0% inter: 0.0%
> 0.0% 0.0%
> [libx264 @ 0x353cc80] i16 v,h,dc,p: 97% 1% 2% 0%
> [libx264 @ 0x353cc80] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 29% 21% 22% 5% 6% 4% 3%
> 7% 4%
> [libx264 @ 0x353cc80] i8c dc,h,v,p: 100% 0% 0% 0%
> [libx264 @ 0x353cc80] Weighted P-Frames: Y:0.0% UV:0.0%
> [libx264 @ 0x353cc80] ref B L1: 83.3% 16.7%
> [libx264 @ 0x353cc80] kb/s:22.85
>
> I also tried lanczos which produced a colour shift.
> I found that only 'bilinear' and 'fast_bilinear' appear to preserve source
> colour values. If using lanczos I had to also force 'accurate_rnd' to get
> perfect colour:
>
> -sws_flags lanczos+accurate_rnd
>
> Source files was a 10bit ProRes grey card R=128, G=128, B=128
>
> Colour was measured in Shake, but is visible to human eye as a slight green
> tint.
>
> Thanks!
> _______________________________________________
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> ffmpeg-user at ffmpeg.org
> http://ffmpeg.org/mailman/listinfo/ffmpeg-user
>

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