Colour gets arty

This project looks at colour the way a website developer or graphic artist might. It draws a colour wheel using hue and lightness. It makes use of named and default arguments to functions. It is helpful to have done the RGB colour project, but not essential.

RGB and HSL colour

Colour is stored in your computer in the same way that it is put on your screen: as amounts of red, green and blue light. This is called the RGB model. You may already have looked at this in the RGB colour project.

If you are a web designer, or choosing colours for your room, the RGB model is not the best way to think about colour. You might prefer to think in these terms:

  • hue: the actual colour, where it belongs in the rainbow.
  • saturation: how strong the colour is; how different it is from grey.
  • lightness: a scale from black (0),
    through brightly-coloured (0.5), to white (1).

This is called the HSL model. In this project you will use the Python library module called colorsys to convert from HSL to the RGB model that turtle needs.

A colour wheel

Start the project by making an empty file Right-click and open it with IDLE.

You’re going to make a sort of circular rainbow, where the hue goes from zero to one. At each hue, you will draw a thin pie-slice (a sector) in that colour. Type in the editor:

# Explore HSL colour
from turtle import *
import colorsys

def sector(r, angle, c):
    "Sector filled with colour c"
    circle(r, angle)

Save and run this file. Test this function in the shell by typing:

>>> sector(200, 15, (0,0,1))

You should see a blue sector:


(If it didn’t work, debug your code.) In the function call, we specified the colour as an RGB tuple (0,0,1), meaning no red, no green, and 1 full unit of blue. (The numbers in a colour tuple are fractions that run from zero to one.)

Now we want to piece together a circle from sectors of different colour. The angle of the sectors has to add up to 360 degrees. Add this function to your code after the sector function:

def wheel(r, sat=1.0, light=0.5, N=24):
    "A colour wheel of radius r with N sectors"
    for i in range(N):
        c = colorsys.hls_to_rgb(i/N, light, sat)
        sector(r, 360/N, c)

Notice that the Python library likes to call the HSL model the HLS model. (Both are ok, just be careful with the order of the arguments, when you use it.) And when we call it, we have to mention the module colorsys in front: this is because we used a different kind of import statement. As a test, save and run, then try wheel(200) at the shell prompt.

Add this program at the end of your code:

# Program

K = 50
for k in range(K):
    r = 300*(1-k/K)
    wheel(r, light=(K-k-1)/K, sat=1.0, N=120)

tracer(0) makes the program run fast by putting off drawing until the call to update.

Save and run this. You should see a wheel of all the hues, like the one in the sidebar. Lightness runs from zero in the middle (black), to 1 at the rim (white).

To learn how saturation affects colour, try altering sat=1.0 to sat=0.5. How would you change the program to display varying saturation, at a lightness you choose? (Remember, a lightness of 0.5 is the most colourful.)