StumpyCore/PNG form_hsl function example:

spectrum.cr

require "stumpy_png"
include StumpyPNG

width = 361
height = 101
spectrum = Canvas.new(width, height)
(0..width - 1).each do |x|
  (0..height - 1).each do |y|
    # RGBA.from_hsla_n(hue, saturation, lightness, alpha) is an internal helper method
    color = RGBA.from_hsl([x, 64, y]) # Here from_hsl(x, 100, y) would work exactly the same
    spectrum[x, y] = color
  end
end

StumpyPNG.write(spectrum, "spectrum.png")