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]
 ]
]

[download]

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Re^2: How to use a pdl as an index into another pdl
by etj (Priest) on Apr 24, 2022 at 15:03 UTC
    vr++ correctly identifies this is a histogram type of problem. This note is just about an alternative approach I stumbled on very recently.

    The new PDL::IO::STL, in the imminent PDL 2.079, uses PDL::VectorValued if installed, to deduplicate its set of colour-vectors, then point the triangles' vertex-indices at the correct entry in the vertex-vector. It uses vsearchvec of PDL::VectorValued::Utils for this. Summarised from https://github.com/PDLPorters/pdl/blob/32a99b563358f7ad3ee26fba22fb24b5c02380e8/IO/STL/STL.pm#L150-L158: 

    use PDL::VectorValued::Utils;
# $pdl is ncoords=3,nvertices=3,ntriangles
my $uniqv = $pdl->uniqvec; # ncoords=3,nunique
my $face_indices = vsearchvec($pdl, $uniqv); # nvertices=3,ntriangles

    [download]
    From this, if you histogram the vertex-indices to get a count of each, you can then turn this into a pair of ndarrays ($coords, $count), and do 
    $cube->indexND($coords) .= $count;

    [download]
    This will work because then you are only writing into each cell once. As the "CAVEAT" in range of PDL::Slices notes:
    It's quite possible to select multiple ranges that intersect. In that case, modifying the ranges doesn't have a guaranteed result in the original PDL -- the result is an arbitrary choice among the valid values. For some things that's OK; but for others it's not. In particular, this doesn't work: 
    pdl> $photon_list = new PDL::RandVar->sample(500)->reshape(2,250)*10
pdl> $histogram = zeroes(10,10)
pdl> $histogram->range($photon_list,1)++;  #not what you wanted

    [download]
    The reason is that if two photons land in the same bin, then that bin doesn't get incremented twice. (That may get fixed in a later version...)

    Another alternative approach would be to try PDL::NDBin.

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