$img  = conv2d( $img, $kernel ) == 1;
$img += 48;

my $s = ${ $img-> byte-> get_dataref };

my @checks = (              # increments to
-$w_ - 1 ,1, 1,             # 8 neighbours
 $w_ - 2,    2,
 $w_ - 2, 1, 1,
);

my $i = CORE::index $s, 1;  # 1st point
my @list = ( $i );
substr $s, $i, 1, 0;

push @list, $i
    while first { substr $s, $i += $_, 1, 0 } @checks;