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]
 ]
]

[download]

In reply to Re: How to use a pdl as an index into another pdl by vr
in thread How to use a pdl as an index into another pdl by Barrabas

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