[FFmpeg-user] zscale and scale with HDR video

[Paul B Mahol]

On Sat, Oct 26, 2024 at 2:15 PM Oliver Fromme <oliver at fromme.com> wrote:

> I've got a question for the experts ...
>
> So far I have successfully used zscale for scaling HDR video,
> because it is supposed to be better suited for HDR than the
> regular scale filter.
>
> However, I'm now in a situation where I need to change the
> scaling dynamically, depending on frame number -- that is,
> I have to use "n" (frame number) in the formula for width
> and/or height, along with the "eval=frame" option.
> Unfortunately, zscale does not support that, so I have to
> resort to using the regular scale filter.
>

Not true.

Zscale filter supports commands, and width/height can be changed for every
single frame, with sendcmd filter for example.


>
> In the documentation I noticed that it supports options for
> setting the color space (in_color_matrix, out_color_matrix),
> so I set those to "bt2020" for HDR.
>
> Here's the complete command, broken up in POSIX shell syntax
> using variables for better readability:
>
>   H='if(between(n\,47105\,47283)\,2160/(2032/1600)\,2160)'
>
> VF="scale=w=-1:h='$H':eval=frame:in_color_matrix=bt2020:out_color_matrix=bt2020"
>   VF="${VF},pad=3840:2160:-1:-1:eval=frame"
>   VF="${VF},crop=h=1600"
>
>   HDR="hdr10=1:hdr10-opt=1:repeat-headers=1"
>   HDR="${HDR}:colorprim=bt2020:transfer=smpte2084:colormatrix=bt2020nc"
>
> HDR="${HDR}:master-display=G(8500,39850)B(6550,2300)R(35400,14600)WP(15635,16450)L(10000000,0)"
>   HDR="${HDR}:max-cll=0,0"
>
>   ffmpeg -i input.mkv -vf "$VF" -map 0 -c copy -c:v libx265 \
>          -x265-params "$HDR" -pix_fmt yuv420p10le -preset slow -crf 20 \
>          -c:a ac3 -ac:a 6 -b:a 448k -default_mode infer_no_subs \
>          output.mkv
>
> Technically, that command works and runs without any errors
> or even warning messages.  The resulting video looks "okay"
> on first sight.
>
> However ... when pausing the video and looking closer at the
> screen, I believe there are some small artefacts that are not
> in the source video.  I'm not 100 % sure if my eyes deceive
> me or if it's real.  And: If it's real, then is it caused by
> the scale filter?  Or maybe do I just need to lower the CRF
> value, or maybe it's something else?  And of course, I still
> remember the statement that the scale filter is not really
> recommended for HDR (zscale should be used instead, but I
> can't do that, as explained above).
>
> Can somebody give a recommendation for this situation?
> Is it okay to use the scale filter as in the above example?
> Can it be improved?  Or -- is there a way to use the zscale
> filter with a dynamic formula that depends on frame number?
> Maybe I've overlooked something here.
>
> Best regards
>  -- Oliver
>
> PS:  Just in case somebody wonders why I'm doing that in the
> first place ... My source video is in 16:9 aspect ratio with
> 2160 pixel rows, but it contains black bands.  I want to crop
> the video in order to get rid of the black bands.
>
> In the above example, the actual content is only 1600 pixel
> rows, but with one exception:  There is a short section (just
> a few seconds) where the black bands are smaller.  Within that
> section, the content occupies 2032 pixel rows.
>
> So, before cropping, I have to scale that section (only that
> section, not the rest!) down, so nothing gets lost when I crop
> to 1600 pixels.  Additionally I have to insert a pad filter
> that adds black bands at the sides (left and right) within
> that section, so the width of the video stays constant.
> _______________________________________________
> ffmpeg-user mailing list
> ffmpeg-user at ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-user
>
> To unsubscribe, visit link above, or email
> ffmpeg-user-request at ffmpeg.org with subject "unsubscribe".
>