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First release

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+Copyright (c) 2012 Francois Dion (francois.dion@gmail.com)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+Pi-A-Sketch
+===========
+
+A Project that was featured at PYPTUG PyHack Workshop #02
+---------------------------------------------------------
+This is the code for the project featured here:
+http://www.raspberry-python.blogspot.com/2013/01/pi-sketch.html
+
+
+
+See http://www.pyptug.org and http://raspberry-python.blogspot.com
+
+The Pi-A-Sketch is distributed under the MIT license (see LICENSE)

File piasketch.py

+#!/usr/bin/env python
+""" piasketch - The Pi-A-Sketch program draws a few circles
+It is also a basic library for steppers controlling an X/Y device.
+
+The main functions are line(x,y) and circle(radius).
+"""
+import RPi.GPIO as gpio
+import time
+
+PINS_A = [4, 25, 24, 23]
+PINS_B = [22, 21, 18, 17]
+PINS = PINS_A + PINS_B
+SEQA = [(4, ), (4, 25), (25, ), (25, 24), (24, ), (24, 23), (23, ), (23, 4)]
+RSEQA = SEQA[::-1][1:] + SEQA[::-1][:1]
+SEQB = [(22, ), (22, 21), (21, ), (21, 18), (18, ), (18, 17), (17, ), (17, 22)]
+RSEQB = SEQB[::-1][1:] + SEQB[::-1][:1]
+
+CORRECTION = 22
+
+
+def setallpins():
+    """
+    Before we can use them, we need to set the pins for both motors
+    """
+    gpio.setmode(gpio.BCM)
+    for pin in PINS:
+        gpio.setup(pin, gpio.OUT)
+
+
+def stepper(sequence, pins, delay=0.002):
+    """
+    One full step, based on an ordered sequence and corresponding pins
+    """
+    for step in sequence:
+        for pin in pins:
+            gpio.output(pin, gpio.HIGH if pin in step else gpio.LOW)
+        time.sleep(delay)
+
+
+def move(steps, axis="x"):
+    """
+    Move a certain number of steps on a specific axis. sign for direction
+    """
+    (seq, pins) = (SEQA, PINS_A) if axis == "x" else (SEQB, PINS_B)
+    if steps < 0:
+        steps = -steps
+        seq = RSEQA if axis == "x" else RSEQB
+    for _ in range(steps):
+        stepper(seq, pins)
+
+
+def line(x1, y1):
+    """
+    Bresenham's line algorithm from rosetta code (ADA/Python),
+    adapted to relative positioning. It will be drawn from current
+    position to x1,y1. Negative number goes in other direction.
+    """
+    dx = abs(x1)
+    dy = abs(y1)
+    x, y = 0, 0
+    px, py = x, y
+    sx = -1 if 0 > x1 else 1
+    sy = -1 if 0 > y1 else 1
+    if dx > dy:
+        err = dx / 2.0
+        while x != x1:
+            if x - px is not 0:
+                move(x - px)
+            if y - py is not 0:
+                move(y - py, "y")
+            px, py = x, y
+            err -= dy
+            if err < 0:
+                y += sy
+                err += dx
+            x += sx
+    else:
+        err = dy / 2.0
+        while y != y1:
+            if x - px is not 0:
+                move(x - px)
+            if y - py is not 0:
+                move(y - py, "y")
+            px, py = x, y
+            err -= dx
+            if err < 0:
+                x += sx
+                err += dy
+            y += sy
+    if x - px is not 0:
+        move(x - px)
+    if y - py is not 0:
+        move(y - py, "y")
+
+
+def _circlepoints(radius):
+    """
+    mid point circle drawing algorithm - basically Bresenham's.
+    It was converted for use with relative. Cant use the bitmap
+    approach of the original algorithm. Points are sequential
+    clockwise. It works, but it is ugly. I only had a few
+    minutes to write this. It needs to be cleaned up.
+    """
+    points = []
+    segment = []
+    for seg in range(8):
+        x0, y0 = 0, -radius
+        f = 1 - radius
+        ddf_x, ddf_y = 1, -2 * radius
+        x, y = 0, radius
+
+        segment = []
+        while x < y:
+            if seg == 0:
+                segment.append((x0 + x, y0 + y))
+            elif seg == 1:
+                segment.append((x0 + y, y0 + x))
+            elif seg == 2:
+                segment.append((x0 + y, y0 - x))
+            elif seg == 3:
+                segment.append((x0 + x, y0 - y))
+            elif seg == 4:
+                segment.append((x0 - x, y0 - y))
+            elif seg == 5:
+                segment.append((x0 - y, y0 - x))
+            elif seg == 6:
+                segment.append((x0 - y, y0 + x))
+            elif seg == 7:
+                segment.append((x0 - x, y0 + y))
+
+            if f >= 0:
+                y -= 1
+                ddf_y += 2
+                f += ddf_y
+            x += 1
+            ddf_x += 2
+            f += ddf_x
+        if seg % 2:
+            points.extend(segment[::-1])
+        else:
+            points.extend(segment)
+    return points
+
+
+def circle(radius):
+    """
+    Draw a circle of specified radius. It is not centered. It will start
+    drawing a circle at its current position, going right and down.
+    """
+    points = _circlepoints(radius)
+    #print points
+    dx, dy = 1, 1
+    px, py = 0, 0
+    counter = 0
+    total = len(points)
+    quarter = total / 4
+    #print total, quarter
+    for (x, y) in points:
+        counter += 1
+        dx, dy = x - px, y - py
+        px, py = x, y
+        # print dx, dy
+        if dx is not 0:
+            move(dx)
+        if dy is not 0:
+            move(dy, "y")
+        if counter == quarter:
+            move(-CORRECTION)
+        elif counter == quarter * 2:
+            move(CORRECTION, "y")
+        elif counter == quarter * 3:
+            move(CORRECTION)
+        elif counter == total:
+            move(-CORRECTION, "y")
+
+
+def main():
+    """
+    As a standalone app, draw a few circles
+    """
+    setallpins()
+    #line(-50, -200)
+    #line(5,0)
+    #circle(350)
+    for radius in range(100,400,25):
+        circle(radius)
+    gpio.cleanup()
+
+if __name__ == "__main__":
+    main()