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# I assume you are using 2 different pdls, not rows of the same pdl as
+ you mentioned in your post
# If it's just one pdl, 'dog' it
...
# my $upper_edges = $indices + 1 != $indices->rotate(-1)

# now slice and dice up your piddles knowing the regions you want.

use PDL::ImageND;
my $edge_detector_kern = pdl [ -1, 1 ];
my $upper_edges = convolveND( $mask, $edge_detector_kern );
# where it is -1 is the upper edge of a region of zeroes
# where it is 1 is the upper edge of a region of ones