ffmpeg and crop+scale

[JosephTocco]

There is BD in 1080p. It is very old and remastered mopic, it real size is 1438×1078 (2px in top and left/rigth both 241px). I cropped to image size and want to upscale to make valid image ratio 4:3. But if I set left rectangle position to 241 I have got 1px black line on the right, if 242 there is the same line on the left. This is problem with scale. What to do with this to make valid image?

-vf "crop=1438:1078:241:2,scale=1440x1080:flags=bicubic,setsar=1:1"

[ProWo]

Maybe the real size is 1436x1078

[jagabo]

Your source is probably YUV 4:2:0 and can't be cropped on odd pixel boundaries. So ffmpeg is quietly changing your 241 x offset to 240 or 242:

-vf "crop=1438:1078:240:2,scale=1440x1080:flags=bicubic,setsar=1:1"

[JosephTocco]

Video #1
ID                             : 4113 (0x1011)
Menu ID                        : 1 (0x1)
Format                         : AVC
Format/Info                    : Advanced Video Codec
Format profile                 : High@L4.1
Format settings                : CABAC / 3 Ref Frames
Format settings, CABAC         : Yes
Format settings, Reference fra : 3 frames
Format settings, GOP           : M=3, N=24
Codec ID                       : 27
Duration                       : 1 h 3 min
Width                          : 1 920 pixels
Height                         : 1 080 pixels
Display aspect ratio           : 16:9
Frame rate                     : 23.976 (24000/1001) FPS
Color space                    : YUV
Chroma subsampling             : 4:2:0
Bit depth                      : 8 bits
Scan type                      : Progressive
format_identifier              : HDMV
Source                         : 00003.m2ts / 00003.m2ts

When I saved frame, cropped in Photoshop all what is black, in result I have got 1438×1078.
When I tried offset 240, black line on the left appeared.

Maybe above MediaInfo will be helpful? I don't understand this stream

[poisondeathray]

It's what jagabo said, the chroma subsampling prevent you from cropping by single pixels

The "dimensions" of the planes are different for 4:2:0 chroma subsampling
Y=1920x1080
Cb=960x540
Cr=960x540

A workaround is if you convert to 4:4:4, you can and crop by single pixels (and back to 4:2:0 if desired) . This "resamples" the CbCr planes so they are 1920x1080 too.

-vf "format=yuv444p,crop=1438:1078:241:2,scale=1440x1080:flags=bicubic,setsar=1:1,format=yuv420p"

[jagabo]

Note that converting to yuv 4:4:4, cropping on odd boundaries, and converting back to yuv 4:2:0, will blur the chroma. Scaling by small percentages will also create moire artifacts and blurring of chroma and luma. I hope you have a good reason to be doing this.

[ProWo]

-vf "crop=1438:1078:0:0,setsar=1:1"

works here fine on a avc movie with 4:2:0 chroma subsampling

[jagabo]

-vf "crop=1438:1078:0:0,setsar=1:1"

works here fine on a avc movie with 4:2:0 chroma subsampling

Why wouldn't it?

[jagabo]

Oh, I just noticed that ffmpeg's crop filter has an "exact" option that will let it crop YV12 at odd boundaries:

-vf "crop=1438:1078:241:2:exact=1,scale=1440x1080:flags=bicubic,setsar=1:1"

[JosephTocco]

Thanks guys! All is fixed by exact crop parameter. Now image is perfect, without any black strips or other artifacts.