Python turtle.setup() truncates canvas to screen size - how to avoid that?

Question

To measure the rotation speed of a rod, I need to make a dial with a large number of alternating dark / transparent segments arranged in a circle. The rotating dial will interrupt the light on a photosensor, and then I only need to measure the frequency of light pulses. Python turtle graphics seems like a good idea for drawing this dial.

I need to draw this image very large, so as to avoid the stair-step effect on the edges of the segments - I need smooth edges. But if I do turtle.setup(x, y) with either x or y bigger than the screen, the canvas is truncated to fit the screen. How to avoid that?

My code is included at the end. See here a screenshot with the truncated canvas for x = y = 1420

EDIT: Just to make it clear - getscreen() / getcanvas() at the end capture this truncated canvas image and save it as-is (truncated) into an EPS file. That's what bothers me. I need the whole circle captured in a high-resolution image file.

I am using python-2.7.4 on Ubuntu 13.04

This is the code:

#!/usr/bin/python

# set this to 1 to troubleshoot
debug = 0

import turtle
import math

# image file with the result
fname="dial.eps"

# number of lines
n = 100
# external radius
r2 = 700
# length of each line
l = round(r2 / 10)

r1 = r2 - l

# pen thickness
# tuned for 50% fill factor at the inner end of each line
# (dark stripe and transparent stripe have equal width there)
thick = 2 * math.pi * r1 / float(2 * n)
print "thickness =", thick

# setup screen size to contain the whole circle, plus a little extra
border = 20 + thick
turtle.setup(2 * r2 + border, 2 * r2 + border)

dot = turtle.Turtle()
dot.speed(0)
dot.hideturtle()

# draw crosshairs in the center
dot.setpos(l, 0)
dot.setpos(-l, 0)
dot.home()
dot.setpos(0, l)
dot.setpos(0, -l)
dot.penup()

# thickness of lines
dot.pensize(thick)

for step in range(0, n):
    a = 360.0 * step / float(n)
    arad = math.radians(a)
    x1 = r1 * math.cos(arad)
    y1 = r1 * math.sin(arad)
    x2 = r2 * math.cos(arad)
    y2 = r2 * math.sin(arad)
    if debug == 1:
        print "a =", a, "\t x1 =", x1, "\t y1 =", y1, "\t x2 =", x2, "\t y2 =", y2
    dot.penup()
    dot.setpos(x1, y1)
    dot.pendown()
    dot.setpos(x2, y2)

ts = turtle.getscreen()
ts.getcanvas().postscript(file=fname)

print "Saved image to: ", fname
print "All done. Click image to exit."

turtle.exitonclick()

Answer 1

• You can't set the turtle canvas bigger than your screen
• The EPS file saved by getcanvas().postscript is resolution-independent; you can print it at any size and it will still come out at your printer's native resolution.

I modified your code to read the screen size and change the circle radius accordingly:

#!/usr/bin/python

# set this to 1 to troubleshoot

debug = 0

import turtle
import math
ts = turtle.getscreen()
max_size = 0
if ts.window_width > ts.window_height:
    max_size = ts.window_height()
else:
    max_size = ts.window_width()

# image file with the result

fname = 'dial.eps'

# number of lines

n = 100

# external radius
# r2 = 700

r2 = 0.8 * max_size / 2

# length of each line - changed from 'l', which looks too much like 1

line_length = round(r2 / 10)

r1 = r2 - line_length

# pen thickness
# tuned for 50% fill factor at the inner end of each line
# (dark stripe and transparent stripe have equal width there)

thick = 2 * math.pi * r1 / float(2 * n)
print 'thickness =', thick

# setup screen size to contain the whole circle, plus a little extra

border = 20 + thick

# turtle.setup(2 * r2 + border, 2 * r2 + border)

dot = turtle.Turtle()
dot.speed(0)
dot.hideturtle()

# draw crosshairs in the center

dot.setpos(line_length, 0)
dot.setpos(-line_length, 0)
dot.home()
dot.setpos(0, line_length)
dot.setpos(0, -line_length)
dot.penup()

# thickness of lines

dot.pensize(thick)

for step in range(0, n):
    a = 360.0 * step / float(n)
    arad = math.radians(a)
    x1 = r1 * math.cos(arad)
    y1 = r1 * math.sin(arad)
    x2 = r2 * math.cos(arad)
    y2 = r2 * math.sin(arad)
    if debug == 1:
        print 'a =', a, '\t x1 =', x1, '\t y1 =', y1, '\t x2 =', x2, \
            '\t y2 =', y2
    dot.penup()
    dot.setpos(x1, y1)
    dot.pendown()
    dot.setpos(x2, y2)

ts.getcanvas().postscript(file=fname)

print 'Saved image to: ', fname
print 'All done. Click image to exit.'

turtle.exitonclick()

Zoomed in at 500%, there are no jaggies:

Oh, and please never use l as a variable name; it looks too much like 1. Bad programmer, no biscuit … ☺