Processing Tutorial – Abridged

Before we dive in:

What You’ll Need

  • Download Processing for your specific operating system.
  • The confidence, curiosity and enthusiasm to learn and the drive to teach yourself and the patience to help those around you.

What you need to do:

  • Do the One hour of code with Dan Shiffman to get familiar with Processing and the concepts we’ll dive further into today.

Processing Workshop

In this workshop we will be introduced to Processing – a programming language and environment developed “to promote literacy within the visual arts and visual literacy within technology”. Processing is used across a variety of communities but has had particular success within the art and design community (and data visualization community) for its strength in generating visual and interactive output. The purpose of this workshop is to:

  1. Understand the basic concepts of programming visual outputs with code.
  2. Become familiar with writing simple code to draw shapes, lines, and colours in Processing.
  3. Explore the difference between static and dynamic displays as created by our code.
  4. Become acquainted with the sources of information and help available to ‘part-time’ coders.

We will use the concepts learned in this workshop throughout the rest of the course to learn how to systematically approach data visualization problems and explore other tools and methods of producing visuals with data.

Learning Outcome / Assignment:

At the end of this workshop, you will be equipped with the fundamental tools to draw with code. With your new skills you will be able to produce all sorts of visual output using Processing’s primitive shapes and programming concepts such as variables, conditionals, and loops. For your assignment choose one these three Proun drawings by El Lissitzsky.


You will develop 2 versions:

  1. The first will be a version as close to the image as possible.
  2. The second will be a remix of the image – playing with the composition through the use of alternative colors, strokeWeight, object positioning, etc. – save your code in a separate file!

Be sure to try and use variables – ones that you define and in-built variables like mouseX and mouseY – to affect the composition.

NOTE: You are encouraged to work together on the initial composition, but your remixes should all look different.

Turning in the assignment:

  • create a blog (that you create from wordpress, tublr, cargo collective, blogspot, etc)
  • you will post your two images AND your code to your blog.
  • send the link to your blog to Andras and Sally.
  • Watch the video below to successfully copy your Processing code to your blog such that it is formatted with syntax highlighting.

how-to-post-code-wordpress from Joey Lee on Vimeo.


  • S1. The Processing Environment
    • The Text Editor & Display Window
  • S2: Fundamentals
    • S2.1: Structure
      • S2.1.1: Comments
      • S2.1.2: Functions
      • S2.1.3: Expressions & Statements
  • S3.1: Shape
    • S3.1.1: coordinates
    • S3.1.2: primitives
    • S3.1.3: properties
    • Basic Color properties
    • Basic Stroke Properties
    • S3.1.4: custom shapes
  • S4: A tiny bit more Fundamentals
    • Variables and Data Types
  • S5. Dynamic Sketches
    • A dynamic sketch
    • Dynamic Sketch Remix
    • Saving your work
  • Recap & Future Directions:
    • What we learned today
  • Future Directions
    • Endless learning
  • Web Searching
  • Resources
  • References
  • Appendix

S1. The Processing Environment

The Text Editor & Display Window

The Processing Development Environment (IDE) looks like this: pde

  • Text editor: a nice place to put your code
  • Play Button: Run your “sketch”
  • Stop Button: Stop your “sketch”
  • Message Area: One-line messages are printed here – usually error messages.
  • Console: The console will print out feedback – usually text that you instruct it to print out while you are building your sketch.
  • Display Window: “A computer screen is a grid of light elements called pixels. Each pixel has a position within the grid defined by coordinates.” This is where your sketches will be shown.

S2: Fundamentals

S2.1: Structure

Voltaire once said, “If you wish to converse with me, define your terms”. In this section, we’re going to define a number of terms so we can all speak the same language when referring to the different elements of a program.

What you should learn and remember from this section are:

  2. functions can have a number of parameters that are passed arguments
    • e.g. size() is a function that has two parameters: width and height. We can pass 2 numbers in like 700 and 500 which are the arguments.
  3. expressions & statements

S2.1.1: Comments

In code, comments are things you write that the computer ignores and does not evaluate.   In Processing, comments are indicated by beginning a line with //. For example:

// ellipse(1, 1, 1, 1);  This is a commented line because of the '//'
ellipse(250, 100, 50, 50);

These are useful when programming to help you (and others who read your code) keep track of what’s happening in your program and allows you to test different functions and statements.

We can use two methods of comments:

For single line comments: //

// fill(255, 0, 0);
// ellipse(250, 100, 50, 50);   


Block comments for multiple lines of code:

fill(255, 0, 0);
ellipse(250, 100, 50, 50);

TIP: We can use comments to help us structure our logical steps when writing code. For example:

// first set the canvas size

// next set the canvas color

// next draw a circle in the center of the canvas. Since the canvas is 700 pixels wide and 500 pixels tall, the center must be at (350, 250). 

// next draw a triangle about 1/3 the width of the canvas

// ... 

S2.1.2: Functions

Functions allow you to draw shapes, set colors, calculate numbers, and to execute many other types of actions. A function’s name is usually a lowercase word followed by parentheses. Functions have parameters in which you fill in with arguments.

The comma-separated elements between the parentheses are the parameters that you fill with arguments, and they affect the way the function works. Some functions have no parameters and others have many. This program demonstrates the size() and background() functions. cont.

// The size function has two parameters. 
// The first sets the width of the display window and the second sets the height
// the arguments here are 200, 200
size(200, 200);

// This version of the background function has one parameter.
// It sets the gray value for the background of the display window in the
// range of 0 (black) to 255 (white). The argument here is 102.

S2.1.3: Expressions & Statements:

Using an analogy to human languages, a software expression is like a phrase. Software expressions are often combinations of operators such as +, *, and / that operate on the values to their left and right. A software expression can be as basic as a single number or can be a long combination of elements. An expression always has a value, determined by evaluating its contents.

// expression       // value
5 + 5               10

// expression       // value
10 < 50             true

A statement – composed of a set of expressions – is like a sentence that gets translated into machine readable code that instructs the computer to do something. For example we see that the collection of these expressions makes a statement (below) that tell the computer to do a particular task.

size(200, 200);     // Runs the size() function to set the canvas size
int x;              // Declares a new variable x as an integer type
x = 102;            // Assigns the value 102 to the variable x
background(x);      // Runs the background() function

S3.1: Shape

Let’s dive into how we can draw shapes with Processing.

S3.1.1: coordinates

The coordinate space of a Processing canvas is set using the size() function. The parameters for width and height sets the number of pixels that will be in the x-coordinate space and y-coordinate space.

The Processing canvas starts at (0,0) at the top-left corner of the canvas.


Credits: “A Programming Handbook for Visual Artists and Designers” – Reas & Fry

* for your reference: Processing also comes with a number of other renderers including a 3D renderer, but we won’t get into this now – just something to think about!

S3.1.2: primitives

Processing’s primitive shapes are the bread and butter of making visual output with code. As we saw in our first program in which we made a circle, triangle, and square with one function, there are other primitive shapes that we can use. These primitives are listed below:



Experiment: draw 5 points in a row on your screen

line(x1, y1, x2, y2)


Experiment: draw 3 “x’s” on your screen using lines

triangle(x1, y1, x2, y2, x3, y3)


Experiment: draw 3 triangles if different sizes slightly overlapping each other

quad(x1, y1, x2, y2, x3, y3, x4, y4)


Experiment: draw 1 funky quadrilateral

rect(x, y, width, height)


Experiment: draw 2 rectangles with exactly the same arguments – use the rectMode(CENTER) function for 1 of the rectangles and see what it does.

ellipse(x, y, width, height)


Experiment: draw a “bullseye” target with 6 ellipses. Play will fill colors to change the color of each ellipse.

bezier(x1, y1, cx1, cy1, cx2, cy2, x2, y2)


Experiment: draw 1 bezier curve to see how it works.

S3.1.3: properties

In Processing we can change the properties of the shapes to affect their fill color, stroke color, and drawing mode. Let’s go over a few of them here:

Basic Color properties

If we use RGB color space:

* value1 = red          (between 0 - 255)
* value2 = green        (between 0 - 255)
* value3 = blue         (between 0 - 255)
* alpha = transparency  (between 0 - 100)

Processing comes with a handy Color tool to select colors


NOTE: if you want to save a color to a variable, you must use the color type:

color pink = color(255, 8, 152);


fill(value1, value2, value3)
fill(value1, value2, value3, alpha)


Use the noFill() function if you don’t want a fill color

size(700, 500);

// with fill
fill(242, 172, 20);
ellipse(width*0.25, height * 0.5, 150, 150);

// no fill
ellipse(width*0.75, height * 0.5, 150, 150);

Experiment: make your own example using noFill() with another set of shapes like a triangle and rectangle.

Basic Stroke Properties


stroke(value1, value2, value3)
stroke(value1, value2, value3, alpha)


Use the noStroke() function if you don’t want a stroke color

size(700, 500);

// no stroke
fill(242, 172, 20);
ellipse(width*0.25, height * 0.5, 150, 150);

// black stroke
fill(242, 172, 20);
ellipse(width*0.75, height * 0.5, 150, 150);

Experiment: make your own example using stroke() and noStroke() with another set of shapes like a triangle and rectangle.


You can use the strokeWeight() function to change the width of the stroke.

size(700, 500);

line(width*0.25, height * 0.25, width*0.75, height * 0.25); 

line(width*0.25, height * 0.35, width*0.75, height * 0.35); 

line(width*0.25, height * 0.45, width*0.75, height * 0.45); 

line(width*0.25, height * 0.55, width*0.75, height * 0.55);

line(width*0.25, height * 0.65, width*0.75, height * 0.65);

line(width*0.25, height * 0.75, width*0.75, height * 0.75);

Experiment: make your own example using strokeWeight() AND noFill() with a set of slightly overlapping rectangles.

S3.1.4: custom shapes

In Processing we can create our own shapes from a series of vertices. We can do by sandwiching a series of vertex(x,y) functions in between the beginShape() and endShape() functions.

For example – let’s make a funny fish:

size(700, 500);

// right eye
ellipse(width*0.6, height*0.2, 50, 100);
ellipse(width*0.6, height*0.15, 25, 50);

// body
vertex(width*0.25, height*0.75);
vertex(width*0.35, height*0.15);
vertex(width*0.45, height*0.25);
vertex(width*0.65, height*0.25);
vertex(width*0.55, height*0.85);

// left eye
ellipse(width*0.5, height*0.35, 50, 100);
ellipse(width*0.5, height*0.3, 25, 50);

processing_beginshape-011There are a number of shape mode parameters that can be passed into the beginShape() function in order to have more control over the custom shapes being produced. These include:


// Draws a point at each vertex 
vertex(30, 20);
vertex(85, 20);
vertex(85, 75);
vertex(30, 75);


// Draws a line between each pair of vertices 
vertex(30, 20);
vertex(85, 20);
vertex(85, 75);
vertex(30, 75);


// Connects each grouping of three vertices as a triangle 
vertex(75, 30);
vertex(10, 20);
vertex(75, 50);
vertex(20, 60);
vertex(90, 70);
vertex(35, 85);


// Starting with the third vertex, connects each subsequent
// vertex to the previous two 
vertex(75, 30);
vertex(10, 20);
vertex(75, 50);
vertex(20, 60);
vertex(90, 70);
vertex(35, 85);


vertex(10, 20);
vertex(75, 30);
vertex(75, 50);
vertex(90, 70);
vertex(10, 20);


vertex(30, 25);
vertex(85, 30);
vertex(85, 50);
vertex(30, 45);
vertex(30, 60);
vertex(85, 65);
vertex(85, 85);
vertex(30, 80);


vertex(30, 25);
vertex(85, 30);
vertex(30, 45);
vertex(85, 50);
vertex(30, 60);
vertex(85, 65);
vertex(30, 80);
vertex(85, 85);

All of these shape modes are super exciting and interesting, but we won’t go into much more detail here. For more info, check out the Processing Documentation and/or the “Programming Handbook for Visual Artists and Designers” by Reas & Fry.

S4. A tiny bit more Fundamentals

Variables and Data Types

In the earlier example, we will see the use of variables like mouseX, mouseY , and mousePressed. What we know about these variables is that they store data like the location of the x- and y- coordinate of the mouse or if the mouse is pressed.

In programming languages, we can define our own variables to store data and in turn work with that data running calculations, statistics and/or visualizing it.

Before we work through an example, here are the fundamental data types in Processing, how to declare them, and assign them a value:

  • integer numbers

    • Integers are whole numbers such as 12, -120, 8, and 934.
    • Processing represents integer data with the “int” data type. With variables, we can:
    • Assign single values to a variable:
      // x is an integer equal to 10
      int x = 10;
      • This reads: x is an integer equal to 10
    • Assign an expression to a variable:
      // y is an integer equal to 5 times 5
      int y = 5 * 5;
      • This reads: y is an integer equal to the product of 5 and 5
    • Assign an expression involving other variables to a variable:
      // z is an integer equal to the sum of x and y
      int z = x - y ;
      • This reads: z is an integer equal to the sum of variable x and variable y
  • floating numbers

    • Floating-point numbers have a decimal point for creating fractions of whole numbers such as 12.8, -120.75, 8.125, and 934.82736.
    • Processing represents floating-point data with the “float” data type:
    • Assign single values to a variable:
      // i is an floating number equal to 95.7
      float i = 10.5
    • Assign an expression to a variable:
      // j is an float equal to 5.0 times 5.0
      float j = 5.0 * 5.0;
    • Assign an expression involving other variables to a variable:
      // k is a normalized difference between variables i and j
      float k = i - j / i + j ;
      • This reads: k is an float equal to the difference of variable i and variable j divided by the sum of variable i and varible j
  • strings

    • A string is a sequence of characters.
    • Processing represents string data with the “String” data type:
    • Assign single values to a variable:
      // exclamation is a string containing the sentence "hello world!"
      String exclamation = "My Gosh! ";
    • Assign an expression to a variable:
      // task is a String expression "Hey yo"
      String task = "I'm" + " " + "programming.";
      • you can combine strings using the “+” operator.
    • Assign an expression involving other variables to a variable:
      // realization is a string concatenating the variable exclamation and task
      String realization = exclamation + task;
      // use the text() function to render the text to the canvas
      text(realization, width/2, height/2);
  • boolean

    • A boolean is a true or false value. (we will see in the mousePressed function’s value)
    • Processing represents boolean data with the “boolean” data type:
    • Assign single values to a variable:
      // trueLove is a a boolean type set to true
      boolean trueLove = true;
    • Assign an expression to a variable:
      // falseExpression is a boolean type showing false for the expression 10 is less than 5
      boolean falseExpression = 10 < 5;
      • 10 is NOT less than 5, therefore the value of falseExpression is false.
    • Assign an expression involving other variables to a variable:
      // trueExpression is a boolean type that is equal to the expression that trueLove does not equal falseExpression
      boolean trueExpression = trueLove != falseExpression;
  • arrays

    • An array is a list of data. It is possible to have an array of any type of data. BUT they must all be the same data type.
    • Each piece of data in an array is identified by an index number representing its position in the array. The first element in the array is [0], the second element is [1], and so on (because Processing is “zero indexed”).
    • There are a few ways to create an array.
    • Assign a list values to a variable:
      // an array of integers:
      int[] d1 = {1, 2, 3, 4, 5, 6};
      // calculate the maximum value of the array
      int d1_max = max(d1);
      // use the printArray() function to print it
      • this reads: d1 is an array of integers (int[]) with the values – 1,2,3,4,5,6. The max value printed to the console is 6.
      • We use the printArray() function to print the numbers in the array.
    • Create an empty array, then append values to it:
      // an empty array:
      int[] d2 = {};
      // add numbers to it
      d2 = append(d2, 20);
      d2 = append(d2, 40);
      d2 = append(d2, 60);
      • Keep this in mind when starting to work with data and you need to iteratively add values to an array.
    • Create an array of a known size, then append values to it:
      int[] numbers = new int[3];
      numbers[0] = 90;  // Assign value to first element in the array
      numbers[1] = 150; // Assign value to second element in the array
      numbers[2] = 30;  // Assign value to third element in the array
      int a = numbers[0] + numbers[1]; // Sets variable 'a' to 240
      int b = numbers[1] + numbers[2]; // Sets variable 'b' to 180 

Don’t worry if I just lost you with arrays – they can be a little intimidating, but they are awesome, and they will hold onto our data so we can make stuff like this:


S5. Dynamic Sketches

A dynamic sketch

A dynamic sketch makes use of Processing’s setup() and draw() functions to structure a logic for interaction and animations.

Together they look something like this:


credits: “Learning Processing” – Shiffman

A Responsive Ellipse-drawing tool dynamic-sketch1

Let’s break down this code line by line:

void setup() {
    size(700, 500);

void draw() {
  if (mousePressed) {
  ellipse(mouseX, mouseY, 40, 40);

setup() Set up your sketch by calling all statements that only need to occur once during your drawing – this might be things like the size() of your canvas.

draw() Continuously repeat the statements within this function – continually evaluate the statements in a loop, over and over and over, forever.

Notice how within the draw() function there is something new; an if-statement.  If statements generally look like this:

// this is not 'real' code, it's just trying to describe an if-statement!
if  ('this is true') {
  then do this

So, what the computer is doing when it reaches the if-statement line is asking: “Is the mousePressed?  If it is, run background(255), which will clear the canvas.  If the mouse is not being pressed, the computer will not run the code within the curly brackets {}, and will skip to drawing the ellipses.  Because this code is within draw(), the computer will constantly run and re-run, and re-re-run this code.  The result on the canvas should make sense when you think of it like this!

NOTE: mousePressed is equal to TRUE whenever your mouse is pressed and FALSE otherwise. Therefore it is a variable which stores a boolean of either TRUE or FALSE in response to a mouse event.

What the above example shows is:

  • our basic understanding of conditional statement using the if statement.
  • the use of variables – in this case the in-built variables called mouseX, mouseY, and mousePressed.

Expanding ‘if’ to ‘if/else’

But let’s say we want more control over our program… what can we do? The if/else statement allows you to say:

*“if a condition is met, then execute the statements within the brackets of the”if” statement, HOWEVER if those conditions are NOT met, then run the statements within the brackets of the “else” statement.“*

// this is not 'real' code, it's just trying to describe an if/else-statement!
if ('this is true') {
  then do this
} else {
  do this

Taking our code from above, we can write an if/else statement that draws ellipses with orange stroke when the mouse is pressed and rectangles with purple stroke if it is not pressed.

void setup() {
    size(700, 500);

void draw() {
  if (mousePressed) {
    stroke(255, 202, 26, 50);
    ellipse(mouseX, mouseY, 40, 40);
  } else {
    stroke(250, 15, 247, 50);
    rect(mouseX, mouseY, 40, 40);

Another dynamic example!

Here’s a sample of a sketch that tracks the speed of your mouse in the X and Y direction and increases the size of the drawn ellipse with faster mouse speed:


// sets up values that do not need to change in the canvas 
void setup() {
  // setting the size of the canvas  
  size(700, 500);
  // setting the background colour

// the code that will be run continuously
void draw() {
  // if the mouse is pressed, set the background to 255,
  // otherwise draw an ellipse
  if (mousePressed) {
  // create a variable that calculates the speed of the mouse
  // mouseX is the current location of the mouse
  // pmouseX is the previous location of the mouse
  int ellipseRX = abs(mouseX - pmouseX);
  // set the stroke colour
  stroke(30, 160, 200);
  // draw an ellipse where the X and Y radii are the value
  // of the mouse speed
  ellipse(mouseX, mouseY, ellipseRX, ellipseRX);

Dynamic Sketch Remix

Take a few minutes now and remix the code from one of the dynamic sketches above:

  • add 1 more new shape that responds to your mouseX and mouseY and
  • Use the conditional to affect the color of the shapes by the mousePressed.

Saving your work

When you’re programming, saving your work is super important. Let’s save our first program.

  • From the PDE, go to “save…”
  • Navigate to a folder called “Sketches” (or create one if there isn’t already)
  • Save your work with a name that doesn’t include spaces. The convention is to use “camel case”, or to replace spaces with underscores, like:
    • “myFirstSketch”
    • “my_first_sketch”

You’ll notice when you save your “sketch” the file type will be added to the end of whatever you called your program. So if you named your project “my_first_sketch”, you will see in your folder “my_first_sketch.pde”. The “.pde” is the processing file type.

How do you feel? Empowered? Confused? Excited?

Let’s learn more! *** Now that we’ve reviewed the One hour of code, let’s learn more about the fundamental concepts of programming. These fundamentals will help you to navigate Processing as well as other languages you may encounter in the future, like Javascript, python, or R (which you will encounter in a few weeks).***

Recap & Future Directions:

What we learned today

  • The fundamentals of programming in Processing – and programming in general. We covered:
    • Processing’s primitive shapes & how to make some of our own.
    • Fundamental Data types
    • Mathematical operations in Processing
    • Variable declarations and assignments
    • Conditionals (if / else if / else) and controlling program flow using logical operators
  • We learned:
    • That we can program!
    • How to begin thinking through code – how to break down problems into steps.
    • We need to practice this stuff often to get better.
  • What we didn’t learn:
    • We learned a lot of things today, but it certainly is only the tip of iceberg. There are tons of more functions and programmatic control structures we didn’t get into today. It will be up to you to learn more about these things. There are a ton of great resources out there which are listed in the references section bewow.
    • Here’s a list of some of the major things I think we didn’t cover today:
      • Different renderers (e.g. P3D, OPENGL, etc)
      • For loops
      • While loops
      • reading in tables, dictionaries, JSON
      • Reading and writing files
      • exporting to PDF or saving frames as PNG
      • sharing your sketch online
      • and the list goes on and on… 🙂

Future Directions

Endless learning

There’s so much more to learn and so much cool stuff we haven’t yet done with data! Throughout the course I hope you can reference back to this workshop as you learn more about Processing and as a way to help you understand some of the other tools we will be using like R or Mapbox Studio, and others.

Web Searching

“Programming is just about how good you are at google searching.”


Learning Processing With Dan Shiffman – this dude is handsdown the best guy ever.

“Programming Handbook for Visual Artists and Designers” by Casey Reas and Ben Fry

P5.js tutorials

“Getting Started with Processing” – Ben Fry

“Visualzing Data” – Ben Fry


“Processing, A Programming Handbook for Visual Artists and Designers” by Casey Reas and Ben Fry

“Getting Started with Processing” – Ben Fry

“Learning Processing” – Dan Shiffman


Here’s a bunch of reference material from other versions of this workshop that just didn’t make it in. Feel free to go through some of these examples to help you understand some of the concepts we brushed over.

A1: Printing to the console:

The most basic way to get feedback from a computer is by “printing to the console.” We can do this by using the print() or println() functions. The difference between print() and println() is that print() does not start a new line break after each function call. If this seems like a bunch of hogwash, try out the examples below 🙂


print("Hello World");
print("My name is Joey");
print("It is raining outside");


println("Hello World");
println("My name is Joey");
println("It is raining outside");

A2: Data & Variables & Computation

What is data? Data could be anything from the time of day, the location of a bus stop, the color of your hair, and even the location of your mouse on the screen and the keys you pressed to login to your email. In general, data often consists of measurements of physical characteristics whether it be a digital photo of your dog or the precipitation in Vancouver.

Processing can store, modify, and manipulate many different kinds of data, including numbers, letters, words, colors, images, fonts, and boolean values (true, false).

One of the key features of programming is the ability to store values and/or data to variables. Below are the main data types in Processing – and programming in general – and examples of how to store them to a variable in Processing. NOTE: Processing (the java version) is strongly typed and thus we must declare what the data type will be when assigning data to a variable. If this sounds crazy, have a look at the examples below 🙂

In the following sections we are going to quickly go through:

  1. Arithmetic and mathematical functions
  2. Relational expressions (remember, what does the crocodile want to eat?)
  3. Variables & data types

Arithmetic & Functions

Here’s a list of the main arithmetic functions in Processing. There are others including functions for making exponents, doing square roots, making log transformations, etc – for a full list check the Processing Documentation.

Just give them a good look for now, we will use a number of these in the examples below. Most of these you’ve seen before.

+ (add)
- (subtract) 
* (multiply)
/ (divide)
% (modulus)
() (parentheses)
++ (increment)
-- (decrement)
+= (add assign)
-= (subtract assign)
*= (multiply assign)
/= (divide assign)
- (negation)

Relational Expressions

A relational expression is made up of two values that are compared with a relational operator. In Processing, two values can be compared with relational operators as follows:

    Expression          Evaluation

    3> 5                false

    3< 5                true

    5< 3                false
    5> 3                true

In Processing, we have these relational operators at our fingertips in order to evaluate truthy or falsy values:

    Operator            Meaning
    >                   greater than

    <                   less than

    >=                  greater than or equal to <= less than or equal to

    ==                  equivalent to
    !=                  not equivalent to

We will clear the air once we go through some control structures like conditionals and looping.

A3: Control structures

A3.1: Conditionals: if/else/else if statements

Conditionals allow a program to make decisions about which lines of code run and which do not. They let actions take place only when a specific condition is met. Conditionals allow a program to behave differently depending on the values of their variables.

if/else statements are ways to control the behavior of your program. This allows us to make decisions about what happens in our code. if/else statements might allow us to filter out data, change the size of an ellipse or rectangle depending on the value of a variable, or react to a mouse or key input (oooh fancy!). *** ### A simple if statement So this is the basic structure of an if statement. It basically says: “if a condition is met, execute the statements within the brackets.”

// psuedo code - this wont run in Processing
if (test) {

In our first sketch example (below), we played with conditional statements by using the if/else statement in conjunction with the mousePressed variable to tell the computer:

“if the mouse is pressed, then change the background color and clear the ellipses from the canvas.”

void setup() {
    size(700, 500);

void draw() {
  if (mousePressed) {

A simple if/else if/else statment

We’ve now made an if statement, and if/else statement, and now we want to add 1 more layer of control to our decision making. Here we introduce the “else if” conditional.

The else if statement allows us to say “if a condition is met, then execute the statements within the brackets of the if statement, HOWEVER if those conditions are NOT met, then evaluate whether or not the condition is met for the”else if” statement. If the condition is met in the “else if statement”, then run the statements within those brackets, if not, then check if the conditions are met in the next else if statement and so on and so forth. And if none of those are met, then run the else statement.”

// psuedo code - this wont run in Processing
if (test) {
} else if (test) {
} else if {

... (as many else if statements as you'd like)...


Challenge 2: Deconstructing an if/else if/else statement

ifelseifelse-11 Here’s some kind of an 80’s pattern you might have seen on a fanny pack.

Get into groups or talk to your neighbor and break this sketch down. Notice we introduce what looks like a new in-built variable we haven’t seen yet called frameCount and have a modulo (%) that seems to be doing something.

void setup() {
  size(700, 500);

void draw() {
  if (mousePressed) {
    if (frameCount % 4 == 0) {
      ellipse(mouseX, mouseY, 60, 60);
    } else if (frameCount % 4 == 1) {
      rect(mouseX, mouseY, 60, 60);
    } else if (frameCount % 4 == 2) {
      triangle(mouseX, mouseY, mouseX+ 60, mouseY+60, mouseX-60, mouseY+60);
    } else {
      quad(mouseX, mouseY, mouseX, mouseY+60, mouseX-75, mouseY+60, mouseX+75, mouseY);

Wasn’t that fun?! This may seem trivial, but essentially you can now write code that is responding to data and producing visuals based on the control structures you’ve put in place.

A3.2:Logical Operators

We can have multiple “tests” conditions in an if or else if statement. Using the logical operators (such as the ones you see below) allow you to combine test conditions in an if/else if/ else statement makes it possible to do this.

Operator            Meaning

&&                  AND 

||                  OR 

!                   NOT 

Expression          Evaluation

true && true        true
true && false       false
false && false      false
true || true        true
true || false       true
false || false      false
!true               false
!false              true

Challenge 3: A simple drawing and erasing tool


Run this code and see what happens when you:

  1. press the mouse
  2. Press any key on your keyboard
  3. press both the mouse and keys on your keyboardvoid setup() { size(700, 500); background(0); rectMode(CENTER); noStroke(); }void draw() { if (mousePressed && keyPressed) { background(0);} else if (mousePressed){ fill(255); rect(mouseX, mouseY, 60, 60); } else if (keyPressed) { fill(0); rect(mouseX, mouseY, 60, 60); }}

Add comments to the code at each conditional statement in the draw() function telling us what will happen if that stement is TRUE.

A3.3: repetition

Computers are excellent at executing repetitive tasks accurately and quickly. Modern computers are also logic machines. Building on the work of the logicians Leibniz and Boole, modern computers use logical operations such as AND, OR, and NOT to determine which lines of code are run and which are not.

A3.3.1: The for loop

The for loop is probably the coolest thing since ever. At the most basic level for loops allow your to repeat a statement or statements over and over again until a condition is met.

Why are for loops so awesome? Let’s look at an example of how a loop can be used to take a program which takes 14 lines of code to make 14 lines…

// original hard-coded program
size(200, 200);
line(20, 20, 20, 180);
line(30, 20, 30, 180);
line(40, 20, 40, 180);
line(50, 20, 50, 180);
line(60, 20, 60, 180);
line(70, 20, 70, 180);
line(80, 20, 80, 180);
line(90, 20, 90, 180);
line(100, 20, 100, 180);
line(110, 20, 110, 180);
line(120, 20, 120, 180);
line(130, 20, 130, 180);
line(140, 20, 140, 180);

… and make the program 4 lines of code:

size(200, 200);
for (int i = 20; i < 150; i += 10) {
  line(i, 20, i, 180);

The general structure if described in the image below.

forloops1 Credits: “A Programming Handbook for Visual Artists and Designers” – Reas & Fry

Let’s look closer at the specific “for” structure:

for (int i = 20; i < 80; i += 5) {
        line(20, i, 80, i+15);

If you were to read this out loud it would sound like: “for the integer i equals 20, as long as i is less than 80, increment i by 5, drawing a line each time with new y coordinates based on i.”

Let’s get a little crazy and make a nested loop. A nested loop? You can do that?! Sure!

size(400, 400);

for (int i = 20; i < width - 20; i += 20) {
  for (int j = 20; j < height - 20; j += 20) {
        fill(random(255), random(255), random(255));
        ellipse(i, j, 10, 10);

How does a nested loop work? First, the outer loop starts. Then the inner loop runs until the condition is met at which point the outer loop increments, repeating the inner loop again until the condition is met at which point the outer loop increments, and over and over until the outer loop condition is met. This means that columns of ellipses are drawn from left to right with each complete loop.

*NOTE: the random() function generates a random number from 0 to the value of the input parameter. In this case, for each iteration in the loop, a random number is generated between 0 and 255 for each parameter in the fill() function.

A4: Synthesis:

As our last examples, we will work with arrays to make some simple animations. We will use the following concepts we’ve picked up in this workshop:

  • variables:
    • mousePressed, keyPressed
  • conditionals
    • if statements
  • increment
    • i++
  • shape properties
    • e.g. stroke, strokeWeight
  • shapes
    • point, line
  • random()
  • looping:
    • for loops!
    • draw function

A4.1 Synthesis 1: Animated data line

// Declare the arrays - globally
int[] y = {};
int[] x = {};

// declare a counter variable i to increment through the arrays later
int i = 0;

// set the size and background color
void setup() {
  size(700, 500);

// continuously evaluate the draw function
void draw() {
  if (mousePressed){
    // append the x & y coordinates of the mouse to array x and y
    x = append(x, mouseX);
    y = append(y, mouseY);
    // draw a point with a strokeWeight 2 where the mouse is
    stroke(150, 150, 150, 80);
    point(mouseX, mouseY);
    // when you press a key and 
    // as long as the index is less than the length of the array 
    // so we don't run out of data points to draw
  if (keyPressed && i+1 < y.length) {
    // increment i by 1 
    // choose a random stroke color
    stroke(random(255), random(255), random(255), 60);
    // use the modulo to select a strokeWeight from 0 to 8 
    // as the numbers increment
    strokeWeight(i % 8);
    // draw a line using the data from the array by 
    // selecting the index positions 
    line(x[i-1], y[i-1], x[i], y[i]);
  • up until this point, we haven’t seen the difference between global and local variable declaration. This is what we call variable scope. While global variables can be accessed across your entire program, local variables are limited to the functions that contain them.
  • in this case, if we declared our array x and y in the setup fuction, we wouldnt be able to access that data in the draw function because it would be “stuck” inside the setup function and therefore not accessible in the draw function.

A4.2 Synthesis 2: Animated taxi trail

Let’s make an animation of a taxi cab in Vancouver.

*** before you continue: 1. save your sketch as “taxi_trail”, 2. copy the “assets/data” folder into your “taxi_trail” folder you just created.***

// To read in an image: 
// 1. instantiate the PImage class & 
// 2. declare the image object 
PImage backgroundmap;

// An array of arrays of coordinates of a taxi in Vancouver
int[][] coords = {{274,487},{274,487},{318,441},{328,428},{341,419},{353,410},{362,399},{365,392},{407,426},{447,446},{462,455},{506,473},{517,478},{526,484},{547,493},{551,497},{556,503},{559,507},{559,507},{566,512},{566,512},{573,518},{573,518},{581,524},{586,533},{598,542},{598,547},{584,560},{578,568},{567,578},{556,586},{549,597},{529,613},{523,619},{523,619},{521,622},{548,646},{562,658},{566,667},{556,681},{551,685},{534,668},{517,657},{508,648},{501,642},{491,656},{468,680},{457,671},{445,653},{430,636},{414,622},{398,607},{382,595},{350,571},{334,556},{346,544},{359,533},{339,504},{329,493},{319,483},{326,476},{335,468},{342,460},{336,448},{327,443},{315,451},{304,459},{293,466},{288,471},{278,479},{271,488},{267,476},{264,466},{257,456},{247,448},{237,442},{237,442},{231,436},{231,436},{224,430},{201,416},{200,415},{197,396},{200,381},{200,369},{200,369},{198,355},{189,345},{184,339},{201,335},{214,328},{228,324},{248,321},{261,316},{278,308},{314,302},{324,310},{326,313},{336,322},{339,316},{354,310},{361,311},{386,304},{394,298},{420,307},{422,310},{430,321},{437,321},{445,307},{454,296},{476,291},{481,293},{498,294},{507,288},{514,286},{533,284},{548,287},{556,281},{561,277},{561,270},{561,248},{552,254},{547,259},{532,262},{529,261},{511,254},{505,252},{476,244},{463,235},{452,223},{449,222},{419,214},{411,212},{403,207},{390,192},{383,176},{374,161},{353,148},{346,139},{341,128},{338,102},{330,88},{317,76},{306,64},{295,51},{288,84},{285,109},{263,108},{254,97},{260,85},{265,77},{268,63},{262,54},{260,42},{254,36},{237,39},{222,53},{206,62},{186,72},{178,90},{166,99},{153,105},{135,125},{130,150},{140,182},{140,205},{131,222},{120,230},{101,236},{89,256},{86,282},{93,293},{113,302},{133,303},{156,317},{170,328},{178,332},{196,333},{233,324},{264,322},{274,316},{289,301},{306,298},{320,303},{329,323},{331,345},{344,367},{355,378},{372,392},{390,409},{396,420},{414,438},{431,450},{439,460},{452,467},{460,476},{461,478},{448,502},{433,498},{424,490},{409,471},{401,463},{395,459},{393,467},{383,472},{375,467},{369,462},{363,467},{360,471},{360,476},{374,484},{386,495},{388,498},{392,504},{401,512},{402,513},{415,522},{422,532},{423,539},{413,544},{405,550},{396,556},{394,562},{402,567},{416,578},{421,582},{417,598},{397,616},{388,624},{373,632},{369,639},{366,647},{366,652},{376,661},{383,666},{377,676},{365,679},{348,688},{340,693},{333,697},{307,709},{291,717},{269,728},{256,735},{236,743},{228,757},{229,771},{228,782},{241,783},{265,783},{272,783},{275,784},{276,785},{281,800},{284,815},{299,826},{302,824},{310,805},{321,799},{345,784},{355,772},{357,770},{384,744},{387,736},{391,730},{399,725},{404,724},{417,721},{423,716},{432,706},{435,705},{446,698},{452,692},{478,665},{485,658},{487,655},{501,647},{513,632},{525,621},{538,608},{549,598},{556,591},{576,573},{589,551},{595,544},{609,552},{620,573},{632,582},{653,593},{681,599},{701,604},{737,606},{764,603},{765,613},{765,632},{763,650},{764,657},{783,658},{803,664},{804,679},{785,671},{752,663},{727,669},{709,671},{680,671},{661,666},{651,662},{638,649},{619,639},{601,624},{596,620},{589,614},{578,603},{571,594},{564,586}};

// global counter variable i to increment the animation
int i = 0;

// set the size and background color
void setup() {
  size(960, 840);
  // read in the map image
  // Images must be in the "data" directory to load correctly
  backgroundmap = loadImage("backgroundmap_960x840.png");
  // reduce the frame rate to slow the animation

// the draw function
void draw(){
  // load the image using the image() function 
  //so the map is drawn every frame of the animation
  image(backgroundmap, 0, 0);
  // add annotations
  //  fill(255);
  //  stroke(255);
  //  rect(width*0.60, 35, 500, 50);
  //  fill(255, 0, 0);
  //  textSize(32);
  //  text("click to show the route", width*0.62, 40, width*0.75, 200);

  // animate the taxi as long as there data data available
  // by incrementing i by 1 each frame
  if(i+1 < coords.length){
    fill(255, 214, 46, 75);
    stroke(255, 214, 46);
    ellipse(coords[i][0], coords[i][1], 20, 20);
  // if the mouse is pressed, draw the entire route of the taxi in red
    // using a for loop, we iterate through all the data points
    // to draw the whole taxi route 
    for(int j = 0; j < coords.length - 1; j++){
      stroke(255, 0, 0, 85);
      // draw a line from position x1, y1 to x2, y2
      line(coords[j][0], coords[j][1], coords[j+1][0], coords[j+1][1]);

  • we add some new elements, one of which is the PImage class. Don’t worry too much about this now as it relates to object oriented programming. The important thing to know is that the PImage class allows us to read in images and work with them in our sketches.

Whew! That’s a lot of stuff. Take some time to play around with the code, decompress, have some coffee and a snack. If you made it this far you are amazing. I don’t know about you but my mind is blown!

S8: More on math

Math can be an important aspect of programming, but it’s not necessary to be good at math to understand or enjoy programming.

A5.1: arithmetic & functions – more examples

A5.1.1: Add

 int x;
 int y;
 int z;
 x = 10;
 y = 5;
 z = x + y;
 println(z);  // z will equal 15
 // let's take z and set the background color
 // then create an ellipse and set the fill(z, z+100, z+20)
 fill(z, z+100, z+20);
 ellipse(width/2, height/2, 50, 50);

Experiment: Using addition and variable declarations, come up with your own exmple of how you might use a the combination to parameterize your drawing *** ###S8.1.2:subtract

int x;
int y;
int z;
x = 10;
y = 5;
z = x - y;
println(z);  // z will equal 5

// let's make a yellow ellipse with a strokeWeight(z)
ellipse(width/2, height/2, 50, 50);

Experiment: Change the numbers in the sketch to see how changing the value of the varibles affects your shapes. *** ###S8.1.3:multiply int x; int y; int z;

x = 10;
y = 5;
z = x * y;
println(z);  // z will equal 50

// let's make a rectangle that's z pixels wide and z*2 pixels high
size(400, 400);
rect(width/2, height/2, z, z*2);

Experiment: Change the numbers in the sketch to see how changing the value of the varibles affects your shapes.

A5.1.2: divide

int x;
int y;
int z;
x = 10;
y = 5;
z = x / y;
println(z);  // z will equal 2

// let's make 3 ellipses adjusting their width and height by the z value, in this case just dividing by 2
size(400, 400);

fill(z+240, z+15, z*4);
ellipse(width/2, height/2, 100/z, 100/z);

fill(z, z*60, z+60);
ellipse(width/2, height*0.25, 50/z, 50/z);

fill(z, z*35, z+200);
ellipse(width/2, height*0.75, 50/z, 50/z);

A5.1.3: modulo

The modulo returns the remainder of the division between two numbers

int x; 
int y;
int z;
int j;
int i;

x = 10;
y = 5;
j = 3;

z = x % y;
i = x % j;

println(z); // z will equal 0 since the remainder of 10/5 is 0
println(i); // i will return 1 since the remainder of 10/3 is 1

// Here's an if/else statement - we're going to learn about these in section: Control!
if (z == 1){
    fill(#00F9FF); // if z == 1, then color the ellipse turqoise
} else {
    fill(#FE00FF); // if z does not equal 1, then color the ellipse purple

ellipse(width/2, height/2, 50, 50); // we'll see a purple ellipse! 

A5.1.4: parentheses

The parentheses can be used to set up your mathematical order of operations (remember PEMDAS?). Be sure to keep the order of operations in mind when writing your statements.

int x = 3 + 4 * 5; // Assign 23 to x 
int y = (3 + 4) * 5; // Assign 35 to y


A5.1.5: increment

The increment is a shortcut for adding 1 value to an existing variable.

// incrementing the long way;
int x = 0;
x = x + 1;
println(x); // x will print as 1;

// incrementing the short way:
int y = 0;
println(y); // y will print as 1;

Experiment: use the increment to change the stroke size of 3 lines

A5.1.6: decrement

// decrementing the long way;
int x = 10;
x = x - 1;
println(x); // x will print as 9;

// incrementing the short way:
int y = 10;
println(y); // y will print as 9;

Experiment: use the increment to change the stroke size of 3 lines

A5.1.7: += (add assign), -= (subtract assign), *= (multiply assign), /= (divide assign)

The add assign, subtract assign, multiply assign, and divide assign allow you to add, subtract, multiply, or divide a variable by a specified value:

// add assign
float a = 10.0;
a += 10;
println(a);     // a will print as 20.0;

// subtract assign
float b = 35.6;
b -= 0.6;
println(b);     // b will print as 35.0

// multiply assign
int c = 100;
c *= 3;
println(c);     // c will print as 300

// divide assign
int d = 36;
d /= 4;
println(d);     // d will print as 8

A5.1.8: ceil()

The ceil() function rounds whatever parameter up to the nearest whole number

    // round 32.4 up to 33.0
    float y = ceil(32.4);
    // round 67.7 up to 68
    float x = ceil(67.7);

A5.1.9: floor()

The floor() function rounds whatever parameter down to the nearest whole number

    // round 32.4 down to 32.0
    float y = floor(32.4);
    // round 67.7 up to 67.0
    float x = floor(67.7);

A5.1.10: round()

The round() function returns the closest value.

    // round 32.4 to 32.0
    float y = round(32.4);
    // round 67.7 up to 68.0
    float x = round(67.7);

A5.1.11: min(), max()

The min() function returns the minimum value from a sequence of at least 2, but maximum 3 parameter. You can also apply the min() function on an array to return the minimum value in an array of numbers.

    int u = min(5, 9); // Assign 5 to u
int v = min(-4, -12, -9); // Assign -12 to v 

float w = min(12.3, 230.24); // Assign 12.3 to w 

The max() function returns the maximum value from a sequence of at least 2, but maximum 3 parameters. You can also apply the max() function on an array to return the maximum value in an array of numbers.

int x = max(5, 9); // Assign 9 to x

int y = max(-4, -12, -9); // Assign -4 to y 

float z = max(12.3, 230.24); // Assign 230.24 to z

A6: More on Control

The programs we’ve seen so far run each line of code in sequence. They run the first line, then the second, then the third, etc. The program stops when the last line is run. It’s often beneficial to change this order-sometimes skipping lines or repeating lines many times to perform a repetitive action. Although the lines of code that comprise a program are always positioned in an order from top to bottom on the page, this doesn’t necessarily define the order in which each line is run. This order is called the flow of the program. Flow can be changed by adding elements of code called control structures.

A6.1 conditionals & decision making

A6.1.1:Relational Expressions

A few examples of these:


// 3 is greater than 5 is FALSE
println(3 > 5); // Prints "false"


// 3 is less than 5 is TRUE
println(3 < 5); // Prints "true"


// 3 is less than or equal to 5 is TRUE
println(3 <= 5); // Prints "true"


// 3 is greater than or equal to 5 is FALSE
println(3 >= 5); // Prints "false"


// 3 is equal 5 is FALSE
println(3 == 5); // Prints "false"

!= // 3 does not equal 5 is TRUE println(3 != 5); // Prints “true”



A7: Processing for websites!


I have to mention P5.js which is the javascript library inspired by Processing not only because it is super cool, but because I, along with many others, believe the age of the web is here! P5.js is a javascript library that takes the soul of Processing and implements it in javascript meaning that you can sketch with code using web technologies (html, css, & javascript). I would argue that the syntax is friendlier and is more flexible in terms of sharing your work. Besides the fact that the browser can crash if you send it a ton of data, P5.js may be an interesting way to move out of your desktop and onto the web.

*NOTE: I decided not to teach P5.js since I thought introducing html, css, and javascript would be maybe too much for the first class. Please let me know if you have other thoughts!

P5.js website:

P5.js tutorials: