Turtle Graphics

Turtle graphics refers to a technique of allowing graphics to be drawn, typically on a computer screen, by controlling a virtual "turtle" with simple commands like forward and turn. Turtle graphics were first described using the programming language LOGO, and were created by Seymour Papert as part of constructivist approach to teaching kids geometry by creating simulation environments or "microworlds", where principles could be approached and explored in a direct way, as opposed to classical formal education using pre-determined abstractions.

Turtle graphics are a kind of vector-based graphics system, in contrast to a traditional systems raster-based system like a bitmap.

http://www.greenteapress.com/thinkpython/swampy/

see Vector graphics

Turtle Graphics Inkscape Extension

#!/usr/bin/env python
#-*- coding:utf-8 -*-
"""
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"""

import random, lxml, sys
sys.path.append('/usr/share/inkscape/extensions') # or another path, as necessary
import inkex
from simplestyle import *

#########################################################
# lxml utils

def collecttext (elt):
    ret = elt.text or ""
    ret += "\n".join([collecttext(child) for child in elt])
    return ret

#def getcontents (element):
#    return (element.text or "") + "".join(map(lxml.etree.tostring, element))

#########################################################
# the main turtle action

import math

defaultpathstyles = {
    'fill':'none',
    'fill-rule':'evenodd',
    'stroke':'#000000',
    'stroke-width':'1px',
    'stroke-linecap':'butt',
    'stroke-linejoin':'miter',
    'stroke-opacity':'1'
}

def rgb (r, g, b):
    return '#%02x%02x%02x' % (r,g,b)

# <path style="fill:none;fill-rule:evenodd;stroke:#000000;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1" d="%s" />
def pointstodata (points):
    try:
        data = "M %0.6f,%0.6f" % points[0]
        for p in points[1:]:
            data += " L %0.6f,%0.6f" % p
        return data
    except IndexError:
        return ""

class Turtle (object):
    def __init__(self, x=100.0, y=100.0, heading=0.0, pendown=False, layer=None):
        self.x = x
        self.y = y
        self.heading = heading
        self._pendown = pendown
        self.points = []
        self.stack = []
        if layer is not None:
            self.stack.append(layer)
        self.styles = {}

    def goto(self, x, y):
        self.x = x
        self.y = y
    
    def face(self, d):
        self.heading = (d/180.0)*math.pi

    def forward (self, steps):
        self.x += math.cos(self.heading) * steps
        self.y -= math.sin(self.heading) * steps
        if self._pendown:
            self.points.append((self.x, self.y))

    def left (self, d):
        d = (d/180.0)*math.pi
        self.heading += d

    def backward (self, steps):
        self.forward(-steps)

    def right (self, d):
        self.left(-d)

    def backward (self, steps):
        self.bk(steps)

    def pendown (self):
        self._pendown = True
        self.points.append((self.x, self.y))
        
    def penup (self):
        self._pendown = False
        if len(self.points) and len(self.stack):
            path = inkex.etree.Element(inkex.addNS('path', 'svg'))
            # http://stackoverflow.com/questions/1551666/how-can-2-python-dictionaries-become-1/1551878#1551878
            styles =  dict(defaultpathstyles, **self.styles)
            path.set('style', formatStyle(styles))
            path.set('d', pointstodata(self.points))
            self.stack[-1].append(path)
            self.points = []

    def startgroup (self, id=None):
        g = inkex.etree.Element(inkex.addNS('g', 'svg'))
        if id is not None:
            g.set('id', str(id))
        self.stack[-1].append(g)
        self.stack.append(g)

    def endgroup (self):
        self.stack.pop()

    ## aliases
    def fd (self, steps): self.forward(steps)
    def bk (self, steps): self.backward(steps)
    def lt (self, d): self.left(d)
    def rt (self, d): self.right(d)
    def pd (self): self.pendown()
    def pu (self): self.penup()

    def export_to_dict (self, d):
        d['forward'] = self.forward
        d['backward'] = self.backward
        d['left'] = self.left
        d['right'] = self.right
        d['pendown'] = self.pendown
        d['penup'] = self.penup
        d['goto'] = self.goto
        d['face'] = self.face
        d['startgroup'] = self.startgroup
        d['endgroup'] = self.endgroup
        d['fd'] = self.fd
        d['bk'] = self.bk
        d['lt'] = self.lt
        d['rt'] = self.rt
        d['pd'] = self.pd
        d['pu'] = self.pu
        d['styles'] = self.styles

#########################################################
# The Inkscape Effect interface

class SeymourEffect(inkex.Effect):
    def __init__(self):
        inkex.Effect.__init__(self)
        # http://docs.python.org/lib/module-optparse.html
        self.OptionParser.add_option('-c', '--codeid', action = 'store',
          type = 'string', dest = 'codeid', default = 'code',
          help = 'ID of code element')

    def effect (self):
        codeid = self.options.codeid
        svg = self.document.getroot()
        # svg = self.document.xpath('//svg:svg', namespace = inkex.NSS)[0]

        code = self.document.xpath("//*[@id='%s']" % codeid)[0]
        src = collecttext(code)
        # print src
        
        width = inkex.unittouu(svg.get('width'))
        height = inkex.unittouu(svg.attrib['height'])
        outputlayer = inkex.etree.SubElement(svg, 'g')
        outputlayer.set(inkex.addNS('label', 'inkscape'), 'Seymour Output')
        outputlayer.set(inkex.addNS('groupmode', 'inkscape'), 'layer')
        
        turtle = Turtle(x=width/2, y=height/2, layer=outputlayer)
        d = {}
        turtle.export_to_dict(d)
        d['rgb'] = rgb
        exec src in d

##############################################################################
# This file can be directly applied to an SVG on the commandline using:
# python seymour.py path/to/your.svg

if __name__ == "__main__":
    effect = SeymourEffect()
    effect.affect()