ffmpeg – creating an infinite side scroller of an overlay image

Goal: creating an infinite side scroller of an overlay image

• time and speed adjustable.

my code,

ffmpeg -y -i “storage/0000-0000/Android/data/com.silentlexx.ffmpeggui/Sub/cybercity_3840_h1080.png” -filter_complex nullsrc=d=96:size=1920×1080,drawbox=c=red:t=fill[bg];[bg][0:v]overlay=x=’min(0,-(t-0)*40)’:y=0 -r 30 -c:v libx264 -crf 10 “storage/0000-0000/Android/data/com.silentlexx.ffmpeggui/Sub/slide_b.mp4”

png starts at x=0, then moves at the speed of 40pixel to the left.

How to stack the png endless so that the nullsrc is never seen?

Furthermore, please explain the min expression,

overlay=x=’min(0,-(t-0)*40)’:y=0