granger_rainbow.py

#!/usr/local/bin/python3

import png # pip3 install pypng
import colorsys # h_ue,s_aturation,v_alue

filename = "rainbow.png"

# MacBook1,1 - MacBook7,1: 1280x800
# iMac 27" 5K: 5120x2880
# borderless A4 @ 600ppi: 4960x7016 -> https://www.papersizes.org/a-sizes-in-pixels.htm
# AllRGB size: 4096x4096

info = {
    "width":    4096,
    "height":   4096,
    "bitdepth":   8
    }

channel_max = pow(2, info["bitdepth"]) - 1

rows = list()

halfway = int(info["height"] / 2)

for y in range(0, halfway):
    row = []
    saturation = 2.0 * float(y) / float(info["height"])
    
    for x in range(0, info["width"]):
        for channel in colorsys.hsv_to_rgb(float(x)/info["width"], saturation, 1.0):
            row.append(int(round(channel * channel_max)))
    
    rows.append(row)

for y in range(halfway, info["height"]):
    row = []
    value = 2.0 * (1.0 - (float(y) / float(info["height"])))
    
    for x in range(0, info["width"]):
        for channel in colorsys.hsv_to_rgb(float(x)/info["width"], 1.0, value):
            row.append(int(round(channel * channel_max)))
    
    rows.append(row)

my_png = png.Image(rows, info)
my_png.save(filename)

Used to generate these Granger Rainbow's:

• https://www.flickr.com/photos/henkpoley/44200095760/
• https://www.flickr.com/photos/henkpoley/31079466547/
• https://www.flickr.com/photos/henkpoley/44213709670/

Posted publicly because Fulvio Baccaglini asked nicely. His image is here: https://www.flickr.com/photos/185533101@N05/51488653555/

A Granger Rainbow is a test image that shows any smoothness problems in the color handling of your system. If you have a very high end display or printer it should look smooth. Usually you will see bands on clumps when certain colours cannot be represented.

8 bit per color channel means this is only just regular computer RGB. The files don't define a color space. So usually it will attempt to 'hang' the image as some ideal RGB color space, or just stretch it among your system's full range.