Some commands I’ve not seen before so had to investigate 
The ‘sponge’ command is random based, which lead me to wonder about other approaches and the possibility of controlling it (range 0 to 1):
example_pointoutline : skip ${1=0.12}
normalize[-1] 0,255
blur[-1] 2
isophotes[-1] 6
100%,100%,1,1
noise[-1] 0.5,1
lt. {$1-0.5}
mul[-1,-2]
_circle 5
dilate[-2] [-1]
rm.
Two problems though; I’ve cheated by using the internal ‘_circle’ command and the distribution won’t be quite like your line method!