Ghost image issues with ffmpeg -filter_complex displace

Question

I've (almost) been able to apply a displacement based on 2 animated gaussian noise videos, but I'm having issues with a ghost image. A picture is worth a thousand words.

Here you have a script to replicate the issue:

ffmpeg -y -t 2 -f lavfi -i color=c=blue:s=160x120 -c:v libx264 -tune stillimage -pix_fmt rgb24 00_empty.mp4
ffmpeg -y -i 00_empty.mp4 -vf "drawtext=text=string1:y=h/2:x=w-t*w/2:fontcolor=white:fontsize=60" 01_text.mp4
ffmpeg -y -t 2 -f lavfi -i color=c=gray:s=160x120 -c:v libx264 -tune stillimage -pix_fmt rgb24 02_gray.mp4
ffmpeg -y -i 01_text.mp4 -i 02_gray.mp4 -i 02_gray.mp4 -filter_complex "[0][1][2]displace=edge=mirror" 03_displaced_text.mp4

It creates a test video with a scrolling text and a gray dummy video. Then it applies a displacement based on the gray video. If I understand correctly, because the gray video is 100% gray, it should leave the video unchanged (or maybe displace everything by a fixed ammount of pixels), but it creates a "shadow". I tried with 3 different pixel formats (yuv420p, yuv444p, rgb24) because I found this question on stackoverflow talking about that:

• Why are Cb and Cr planes displaced differently from lum by the displace complex filter in ffmpeg?

ffmpeg version 5.0.1-full_build-www.gyan.dev

Any idea will be welcome.
Thanks!

Answer 1

Solution: using -pix_fmt rgb24 and adding -c:v libx264rgb to all commands fixes it:

ffmpeg -hide_banner -y -t 2 -f lavfi -i color=c=blue:s=160x120 -tune stillimage -c:v libx264rgb -pix_fmt rgb24 00_empty.mp4
ffmpeg -hide_banner -y -i 00_empty.mp4 -vf "drawtext=text=string1:y=h/2:x=w-t*w/2:fontcolor=white:fontsize=60" -c:v libx264rgb -pix_fmt rgb24 01_text.mp4
ffmpeg -hide_banner -y -t 2 -f lavfi -i color=c=gray:s=160x120 -tune stillimage -c:v libx264rgb -pix_fmt rgb24 02_gray.mp4
ffmpeg -hide_banner -y -i 01_text.mp4 -i 02_gray.mp4 -i 02_gray.mp4 -filter_complex "[0][1][2]displace=edge=mirror" -c:v libx264rgb -pix_fmt rgb24 03_displaced_text.mp4

Thanks to poisondeathray's answer in a post at forum.videohelp.com:

the real reason is the 8it RGB to YUV gray conversion for 02_gray.mp4 becomes YUV 126,128,128 (loss of accuracy from the 8bit conversion) https://github.com/FFmpeg/FFmpeg/blob/master/libavfilter/vf_displace.c
If you supply a true gray YUV 128,128,128 video it works ok too