ffmpeg -f lavfi -i zoneplate=kx2=320:ky2=240,format=gray -frames 1 -y
out.png
When you draw a line through the center, you have
black - gray - white - gray - white - gray -black
which means the sine wave changes its sign in the center.
That's why zoneplates are normally calculated with cos() function.
See also: https://en.wikipedia.org/wiki/Zone_plate#Continuous_zone_plates
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