A Graphical Hierarchy

Announcements

• Problem Set 2 will be posted today!
  • You should be good to go on the first 2 problems
  • We’ll have most of the tools for the third problem after today
• Problem Set 1 feedback ideally within 24 hours.

Review Question

Which of the below blocks of code would print something different than the others?

for n in range(10):
    if n % 2 == 0:
        print(n)

for i in range(0,10,2):
    if i > 0:
        print(i)

for j in range(0,20,4):
    L = j // 2
    print(L)

for k in range(0,10):
    if not k % 2 > 0:
        print(k)

A quick regression

if condition_1:
    # Run this code if 
    # condition 1 is true
elif condition_2:
    # This code runs if 
    # condition 1 is false
    # but condition 2 is true
elif condition_3:
    # Runs if both condition 1
    # and 2 fail but condition
    # 3 is true
else:
    # This code runs if 
    # all above conditions 
    # fail

The Portable Graphics Library

• Built atop Tkinter
• The library (pgl.py) is available on the website
  • Put it in the same folder as your code, and then you can import it
• Operates on the idea of a collage or cork-board

• Note that newer objects can obscure older objects. This layering arrangement is called the stacking order.

The Pieces

• At its simplest then, we have two main parts:
  • The window (or felt-board/cork-board)
    • Created with the GWindow function
    • Takes two arguments: a width and a height in pixels
  • The contents
    • A wide assortment of shapes and lines that can be added to the scene
    • Control over where they are placed, how large they are, what color they are, etc

Blue Rectangle!

from pgl import GWindow, GRect

GW_WIDTH = 500
GW_HEIGHT = 200

gw = GWindow(GW_WIDTH, GW_HEIGHT)
rect = GRect(150, 50 ,200, 100)
rect.set_color("Blue")
rect.set_filled(True)
gw.add(rect)

The Coordinate System

• Positions and distances on the screen are measured in terms of pixels
• The location of the origin and orientation of the y-axis are different from math!
  • Origin is in the upper left instead of lower left
  • Y-values increase as you move downwards

Other Simple Objects

Functions to create simple geometric objects:

• Rectangles!
  • GRect( x, y, width, height )
  • Creates a rectangle whose upper left corner is at (x,y) of the specified size
• Circles/Ovals!
  • GOval( x, y, width, height )
  • Creates an oval that fits inside the rectangle with the same dimensions
  • Placement based on the upper left corner of that enclosing rectangle
• Lines!
  • GLine( x1, y1, x2, y2 )
  • Creates a line extending from (x1, y1) to (x2, y2)

Understanding Check

Which of the below images would be produced by the following code?

gw = GWindow(200,200)
for c in range(0,10):
    for r in range(0,10):
        rect = GRect(20*c,20*r,20,20)
        if (r+c) % 2 != 0:
            rect.set_filled(True)
    gw.add(rect)

Classification

• In the mid-18th century, Scandinavian botanist Carl Linnaeus revolutionized the field of biology by developing a new system for classifying plants and animals in a way that revealed their structural relationships.
• Recognized that organisms fit into a hierarchy in which the placement of individual species reflects their anatomical similarities.
• Paved the way for Darwin’s theory of evolution a century later

Biological Hierarchy

Instance vs Pattern

• Important to distinguish between a class of object and a specific instance of that class
  • Iridomyrmex purpureus is a class of ant, not an individual ant
  • There can be many individual ants, all sharing the same class
• Each ant is said to be an instance of a particular class
• An individual ant belongs to a species class of purpureus, family of Formicidae, Phylum of Arthropoda, etc

The GObject Hierarchy

• The types of graphical objects form a hierarchy:

• The GObject class represents the collection of all graphical objects
• The GFillableObject class represents those that have a fillable interior

Interacting with the GWindow

• We’ve already shown creation:

gw = GWindow(width, height)

• You have several more operations that you can apply to the GWindow object:

Interacting with GObjects

• The following operations apply to all GObjects, where object is the name of any specific instance.

• All coordinates and distances are measured in pixels

Interacting with GFillableObjects

• Fillable GObjects have a smaller subset of commands that also apply to them.
• Initially the only fillable objects available to you are rectangles and ovals

Smile!

gw = GWindow(400, 400)

head = GOval(20, 20, 360, 360)
head.set_fill_color("yellow")
head.set_filled(True)
gw.add(head)

reye = GOval(110, 100, 40, 40)
reye.set_filled(True)
gw.add(reye)

leye = GOval(250, 100, 40, 40)
leye.set_filled(True)
gw.add(leye)

mouth = GLine(150, 250, 250, 250)
mouth.set_line_width(5)
gw.add(mouth)

Label It!

• Sometimes you need to add some text to the window
• Can display any string using GLabel using the following format:

msg = GLabel(string_to_add, x_location, y_location)

• Here string_to_add is the text you want to display, and x_location and y_location are the (x,y) coordinates of where you want to place the string

Label Geometry

• The GLabel class relies on some geometrical concepts that are derived from classical typesetting
  • The baseline is the imaginary line on which the characters rest
  • The origin is the point on the baseline at which the text begins
  • The width is the horizontal distance from the origin to the end of the text
  • The height of the font is the distance between adjacent baselines
  • The ascent is the distance the characters rise above the baseline
  • The descent is the distance the characters drop below the baseline
    
    

Interacting with Labels

• A GLabel has several special methods that you can use to interact with it
  • You can use: get_width(), get_height(), get_ascent(), and get_descent() methods to obtain the geometric properties
  • You can set a special font for the label using

  labelname.set_font(font)

  • The font is a string comprised of the following elements:
    • The font style, which is usually blank or italic
    • The font weight, which is usually blank or bold
    • The font size, which is a number followed by the units (typically pt, px, or em)
    • The font family, which is the name of the font. Because what fonts are available can differ from machine to machine, the family is usually a sequence of fonts separated by commas
    • The font family sequence usually ends with a standard family (serif, sans-serif, or monospace) to ensure that the label can display

Label Example

gw = GWindow(500, 200)
msg = GLabel("hello world!", 50, 100)
msg.set_font("italic bold 80px 'times new roman'")
gw.add(msg)

Centering a GLabel

• Frequently useful to center within the window or some shape
• To center properly, you need to know the label dimensions, but you can’t determine the dimensions until after you’ve created the label!
• The main idea then is to:
  • Create a GLabel without setting its location
  • Call the .set_font() method to set the desired font (which could change the size)
  • Determine the horizontal position of the origin by subtracting half the width from the desired location x
  • Determine the vertical position of the baseline by adding half the ascent to the desired location y
  • Add the GLabel at the newly calculated position

Centering Example

gw = GWindow(500, 200)
msg = GLabel("hello world!")
msg.set_font("italic bold 20px 'times new roman'")
x = 250 - msg.get_width() / 2
y = 100 + msg.get_ascent() / 2
gw.add(msg, x, y)