http://www.perlmonks.org?node_id=11115354


in reply to How to use a pdl as an index into another pdl

It's histogram you are looking for, not index. Actually, indexND example pretty clear shows that it does something else. In your case, some selected elements from all-zeroes piddle became incremented from 0 to 1, that's all. Rather (demo output looks correct to me):

use strict; use warnings; use feature 'say'; use PDL; use PDL::IO::Image; use open IO => 'raw'; my $data = `convert -size 5x5 radial-gradient:red-green bmp:-`; my $im = PDL::IO::Image-> new_from_file( \$data ) -> pixels_to_pdl -> long; my $flat = $im-> copy; $flat-> slice( [],[],2 )-> inplace-> shiftleft( 16, 0 ); $flat-> slice( [],[],1 )-> inplace-> shiftleft( 8, 0 ); my $cube = $flat-> mv( 2, 0 ) -> sumover -> flat -> hist( 0, 0x1000000, 1 ) -> reshape( 256, 256, 256 ); ### Demo say $im; say my $uniq_reds = $im-> slice( [],[],0 )-> uniq; say my $uniq_greens = $im-> slice( [],[],1 )-> uniq; say $cube-> dice( $uniq_reds, $uniq_greens, 0 ); __END__ [ [ [ 0 0 0 0 0] [ 0 75 128 75 0] [ 0 128 255 128 0] [ 0 75 128 75 0] [ 0 0 0 0 0] ] [ [128 128 128 128 128] [128 91 64 91 128] [128 64 0 64 128] [128 91 64 91 128] [128 128 128 128 128] ] [ [0 0 0 0 0] [0 0 0 0 0] [0 0 0 0 0] [0 0 0 0 0] [0 0 0 0 0] ] ] [0 75 128 255] [0 64 91 128] [ [ [ 0 0 0 1] [ 0 0 4 0] [ 0 4 0 0] [16 0 0 0] ] ]