Lecture 06: Graphics and Objects

COSC 225: Algorithms and Visualization

Spring, 2023


  1. Assignment 4 Notes
  2. Scalable Vector Graphics
  3. Activity: Draw a Cat
  4. JavaScript Objects

Steps for CA Simulation


  1. Rule = # from 0 to 255
  2. Configuration = 0/1 array

Compute: updated configuration


  1. convert rule number to binary to get update rules
  2. apply update rule to each 3 consecutive entries of configuration

Another CA Example

Pick a random number:

Draw rules:

Simulate execution:

JavaScript Typing

Numerical values are Numbers

  • does not distinguish floating point vs integer formats

Example: 4 == 4.0?

So 4 / 2 is the same is 4.0 / 2

What about 5 / 2?

  • to do integer division, use Math.floor()
  • E.g., Math.floor(5 / 2) gives 2

So Far

HTML + CSS + JavaScript

  • HTML for document structure, content, semantics
  • CSS for styling based on semantics
  • JavaScript to generate/interact with elements

This is all good for text-based documents

  • simple graphics for including images, drawing boxes

Next Steps

Course Goal: Visualizations of algorithmic processes

  1. Graphics
    • want to depict things other than static images and interactive boxes
    • Tool: Scalable Vector Graphics (SVG)
    • Another Tool: Canvas API
  2. Objects
    • algorithms/processes have intermediate states that we want to visualize
    • Tool: JavaScript Objects

Connection: visualize the states of objects as computation progresses

Scalable Vector Graphics (SVG)

What is SVG?

Scalable Vector Graphics

  • format for representing graphical objects
  • vector graphics: image defines instructions for how to draw
    • not just pixels (e.g., png, jpg, tiff)
  • specify shapes, shapes
  • XML-based—structured like HTML:
    • elements and attributes
    • can be styled with CSS
    • can be manipulated with JavaScript
  • standalone file .svg or embedded in HTML

Structure of SVG

Create an SVG element with <svg> tag:

<svg width="600" height="400" xmlns="http://www.w3.org/2000/svg">

Must specify width, height, and xmlns

  • xmlns is “xml namespace”, used to avoid naming conflicts with other types of XML (e.g., HTML)
  • don’t worry about this

What Can SVG Do?


<rect width="100%" height="100%" 
fill="white" stroke="black" stroke-width="5"/>


<line x1="100" y1="100" x2="500" y2="300" 
stroke="black" stroke-width="5"/>


<circle cx="300" cy="200" r="100" stroke="black" 
fill="pink" stroke-width="5"/>


<polygon points="100 100 300 100 400 200 500 300 200 350" 
stroke="black" stroke-width="5" fill="pink"/>


<path d="M120,230 Q220,0 400,100 T300,300"
stroke="black" stroke-width="5" fill="transparent"/>


Draw This (or something better)

Dealing with Repetition

All circles have same radius, stroke, stroke-width, fill

SVG elements can be styled using CSS!

circle {
    r: "25";
	fill: "pink";
	stroke: "black";
	stroke-width: "5";

Now must only specify the location of each circle!

Styling by class, id

SVG elements can be given class and id just like HTML elements!

<circle cx="300" cy="200" class="dot" id="special-dot"/>
<circle cx="120" cy="300" class="dot"/>
<circle cx="40" cy="40" class="dot"/>
<circle cx="260" cy="260" class="dot"/>
<circle cx="350" cy="85" class="dot"/>
.dot {
    r: 25;
	fill: pink;
	stroke: black;
	stroke-width: 5;
#special-dot {
    fill: red;


SVG + JavaScript

SVG can be accessed and modified with JavaScript!

<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8">
    <link rel="stylesheet" href="style.css">
    <script src="dots.js" defer></script>
    <div id="root">
      <svg id="dots"
	   width="600" height="400"
	<rect id="dot-background" width="100%" height="100%"/>	
      <button onclick="moveDots()">Move Dots!</button>

Creating Elements

HTML contains:

      <svg id="dots"
	   width="600" height="400"
	<rect id="dot-background" width="100%" height="100%"/>	

Slightly different that html elements

  • must include namespace
const ns = 'http://www.w3.org/2000/svg';
const svg = document.querySelector('#dots');

let circle = document.createElementNS(ns, 'circle');

Modifying Elements

Again, different from HTML

let circle = document.createElementNS(ns, 'circle');
circle.setAttributeNS(null, 'cx', this.cx);
circle.setAttributeNS(null, 'cy', this.cy);
circle.setAttributeNS(null, 'class', 'dot');
.dot {
    r: 10px;
    fill: rgb(50, 120, 255);
    stroke: black;
    stroke-width: 2;

Objects in JavaScript

What are Objects?

Collection of

  • attributes and associated values
  • methods

Example dot class

  • attributes:
    • cx x position of center
    • cy y position of center
  • methods:
    • updateLocation(cx, cy) moves dot to a new location

Object Constructors

In JS, object types can be defined by defining a constructor

  • function that creates the object
  • keyword this defines attributes and methods

By convention, constructor names are Capitalized:

function Dot(cx, cy) {
    this.cx = cx;
    this.cy = cy;
    this.circle = document.createElementNS(ns, 'circle');
    this.circle.setAttributeNS(null, 'cx', this.cx);
    this.circle.setAttributeNS(null, 'cy', this.cy);
    this.circle.setAttributeNS(null, 'class', 'dot');

To make one dot

let someDot = new Dot(100,100);
let anotherDot = new Dot(200,200);

Now to make some dots…

dots = []; // an array of dots

function makeDots() {
    for(let i = 0; i < 10; i++) {
	let x = Math.floor(600 * Math.random());
	let y = Math.floor(400 * Math.random());
	dots.push(new Dot(x, y));

Defining Methods

You can include method definitions in the constructor as well!

function Dot(cx, cy) {
    this.updateLocation = function (cx, cy) {
	this.cx = cx;
	this.cy = cy;
	this.circle.setAttributeNS(null, 'cx', this.cx);
	this.circle.setAttributeNS(null, 'cy', this.cy);

Now we can move dots around

dots = [];

//...create dots...

function moveDots() {
    for(let i = 0; i < 10; i++) {
	let x = Math.floor(600 * Math.random());
	let y = Math.floor(400 * Math.random());
	dots[i].updateLocation(x, y);

Dots Demo

Next Time

  1. Representing more interesting data types
  2. Visualizing algorithms