How to use a pdl as an index into another pdl

Barrabas has asked for the wisdom of the Perl Monks concerning the following question:

Oh great and venerable ones...

I am trying to create a PDL color "Cube" array of the counts of each color in an image.

The image is a PDL of dimension(width,height,3) of RGB values:

> my $Image  = PDL::IO::Image->new_from_file("SomeImage.jpg");
> my $Pixels = $Image->pixels_to_pdl;
> print $Pixels->info;

PDL: Byte D [1920,1920,3]
[download]

The color count is a pdl cube of dimension 256,256,256:

> my $Colors = zeroes(long,256,256,256);
[download]

I would like to iterate over the pixels in the image, incrementing the element in the $Colors cube indexed by the RGB value of each pixel.

> my $P2 = $Pixels->clump(2);  # Collapses $Pixels to [3686400,3]
> my $P3 = $P2->mv(0,1);       # Reverses  $Pixels to [3,3686400]

> $Colors->indexND($P3)++;     # Thread over $P3 2nd dimension, index 
+into $Colors, and increment
> print $Colors->at(0,0,0);    # Print count of black pixels

1
[download]

There are many black pixels in the image, so the count at RGB(0,0,0) should be fairly large. Looking through the $Color pdl, it *appears* that colors are being set to 1, but not incremented. IOW, the cube is sprinkled with 1 values where I would expect it to be sprinkled with color counts.

My two questions are:

1) What is my mistake, and

2) Is there a straightforward PDL way to do this? (Ie - thread one PDL as an index into another for incrementing)

Back to Seekers of Perl Wisdom