As part of working on a PDL version of Animated Heatmap, it was necessary to make PDL able to turn a field of floating-point numbers into a heatmap, like Imager::Heatmap's draw method does. Cue PDL::Graphics::ColorSpace!

use PDL;
use PDL::Graphics::Simple;
use PDL::Graphics::ColorSpace;
sub as_heatmap {
  my ($d) = @_;
  my $max = $d->max;
  die "as_heatmap: can't work if max == 0" if $max == 0;
  $d /= $max; # negative OK
  my $hue   = (1 - $d)*240;
  $d = cat($hue, pdl(1), pdl(1));
  (hsv_to_rgb($d->mv(-1,0)) * 255)->byte->mv(0,-1);
}
imag as_heatmap(rvals 300,300);
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