How to add a watermark as a loop using ffmpeg sliding overlay
Solution 1:
Use
ffmpeg -i test.mkv -loop 1 -i watermark.png \
-filter_complex "overlay=x=0:y='if(gte(t,0), ((H+h)\/5)\*mod(t\,5)-h, NAN)':shortest=1" \
output.mkv
5 seconds is the period of the above expression. Change both 5s to alter the period.
Unless you're working with files with negative timestamps such as segmented or files split using ffmpeg -ss in copy mode, you can skip the gte function.
ffmpeg -i test.mkv -loop 1 -i watermark.png \
-filter_complex "overlay=x=0:y='((H+h)\/5)\*mod(t\,5)-h':shortest=1" \
output.mkv