Shortest Path Algorithm (Part 1)
This is the first part of a study on how shortest path algorithms work. This study is divided into three parts.
- 1) Draw an empty board
- 2) Add obstacles randomly
- 3) Implement the shortest path algorithm
The division intends to make it easier to understand and visualize the algorithm.
1 - Dependencies
Before we start writing code, we need to install the python library Pillow.
pip install Pillow
2 - Imports
To draw the board, we need to import
ImageDraw classes from the Pillow library.
from PIL import Image, ImageDraw
PIL is short for Python Image Library
3 - Classes
Now that we have our dependencies installed, let’s start defining the Node class.
The Node class will hold each position of the board.
class Node: """ Each node on the board """ # size of each node SIZE=30 def __init__(self, x, y, fill='#FFFFFF', outline='#000000'): # define node position self.x = x self.y = y # define node colors self.fill = fill self.outline = outline def draw(self, base): # define the start of the image left = self.x * self.SIZE top = self.y * self.SIZE # define the end of the image right = left + self.SIZE bottom = top + self.SIZE # shape shape = [(left, top), (right, bottom)] node = ImageDraw.Draw(base) node.rectangle(shape, fill=self.fill, outline=self.outline)
Inside the draw method, we use the ImageDraw class to create the image of the node.
The most important part is the first parameter of the rectangle method. This parameter is responsible for the positions of the pixels that the class will use to draw our node.
The shape is an array with two positions. In the first position, we define where is the top/left edge of the rectangle and in the second, the bottom/right edge.
With these definitions, the method will draw:
- One vertical line from the left/top edge to the relative left/bottom edge;
- One vertical line from the right/bottom edge to the relative right/top edge;
- One horizontal line from the left/top edge to the relative right/top edge;
- One horizontal line from the right/bottom edge to the relative left/bottom edge.
In the end we have a rectangle.
Now we need to create the Board class, to hold the matrix of Nodes and draw the final image.
class Board: """ Matrix of Nodes """ def __init__(self, rows, cols): # size of the board self.rows = rows self.cols = cols def create(self): # start empty board self.board = [None] * self.rows for row in range(0, self.rows): self.board[row] = [None] * self.cols for col in range(0, self.cols): self.board[row][col] = Node(col, row) def draw(self): w = self.cols * 30 h = self.rows * 30 # define base image img = Image.new("RGB", (w, h), (173, 173, 173)) for row in range(0, self.rows): for col in range(0, self.cols): self.board[row][col].draw(img) # draw final image img.show()
4 - Drawing
With the classes defined, now everything we need to do is create an instance of the Board class with the number of rows and cols the board has.
board = Board(9, 9)
Then, we just need to call the methods create and draw.