best x265 parameters to keep the quality of smartphone re-encoded videos

[fullhdfan]

I re-encode my FullHD/4K (no lower resolution !) smartphone videos with Avidemux in order to save space . I succeeded by now to reduce the video size 2-3 times using the following parameters :

- CRF quality : 23
- motion est. method : UMH
- subpixel refinement: 5
- max. motion vector search range : 32
- max. ref. frames 6
- max. consecutive B-frames : 16
- no DCT decimation
- no strong intra smoothing

Details are important to me, their loss is visible when zooming parts of the image in the video. I'm aware that every re-encode contributes to quality loss, but I want to keep it as low as possible while reducing the bitrate without noticeing it.

Does setting the "subpixel refinement" to 7 really sharpen the image (as in image/video filters) or is it just a claim of some which don't see a real benefit (marginal detail preservation over the drastically increased encoding time) ?

All videos from my phone camera are H.265 recordings (20 Mbps FullHD, 50 Mbps 4K).

It might sound silly, but what is better : conversion to H.265 or "back" to H.264 ?

Some say x264 is better in preserving details, but what would be the bitrate reduction benefit in my case ? Would 4K bitrate have "realistic values" at all ?


Thanks.

[Selur]

Does setting the "subpixel refinement" to 7 really sharpen the image (as in image/video filters) or is it just a claim of some which don't see a real benefit (marginal detail preservation over the drastically increased encoding time) ?

No. Setting subpixel refinement to 7 does not increase the sharpness, it can improve the effectiveness of the compression, which for a fixed file size could potentially improve the image quality.

It might sound silly, but what is better : conversion to H.265 or "back" to H.264 ?

For HD and upwards, I would recommend x265.

Some say x264 is better in preserving details, but what would be the bitrate reduction benefit in my case ?

I doubt it.

[rgr]

@echo off
setlocal ENABLEDELAYEDEXPANSION
chcp 1250 >NUL

dir /b *.mp4 | findstr /v /i "edited)" >list.txt

for /F "delims=;" %%F in (list.txt) do (
	echo ---------------------------------------------------------------------------------------------------
	echo File: "%%~dF%%~pF%%F"
	echo ---------------------------------------------------------------------------------------------------
	echo.
	(
		echo name1="%%F"
		echo ffms2^(name1, atrack=-1^)

		echo ApplyGradationCurves^(lumaPoints="16,16,255,235"^)
		echo TemporalDegrain2^(postFFT=3^)
		echo lsfplus^(preset="slow", strength=25^)
			REM echo Histogram
		echo Prefetch
	) >%%F.avs
	ffmpeg.exe -hide_banner -avisynth_flags +all -i "%%F.avs" -c:v libsvtav1 -preset 4 -crf 27 -c:a aac -b:a 192k "%%~nF (AV1+edited).mp4"
	del "%%F.avs"
	del "%%F.index"
)

del list.txt >NUL
pause

Remove the line with ApplyGradationCurve if your phone records in the range 16-235. The script was created with Google Pixel in mind, so there may be problems if yours is recording in VFR.
You can also use -c:a copy for the audio track (untested).