Post-it Clock

In this mini-project, you will create a clock where the numbers are represented using photographs made ​​of sticky notes or Post-its. To do this, you will learn how to use images in your programs and store them into arrays. We will go through this step-by-step.

  1. Displaying an image

    First, let’s learn how to create a program capable of displaying an image. In Processing, there is a type of variable or class called PImage. Variables of this datatype can store an image loaded from a file by using the function loadImage().

    Download the photo here.


    New commands:

    • PImage: a datatype that can store an image.
    • loadImage(imageFile): Loads  imageFile located in the data directory of the current sketch. The name of imageFile must be spelled out exactly. See below for instructions on how to place the file in the right folder.
    • image(imageName, x, y): Displays the PImage imageName. The top left corner is placed at the coordinates xy.
  2. Adding Images

    For the program to have access to the image, drag and drop the image file onto the Processing IDE as shown in the following screenshot:

    The image is stored in the program’s subfolder called data . This screenshot shows the file structure where you can find your image file. You can also find it by going to the menu ‘Sketch –> Show Sketch Folder’.

    When running the program, the picture below is what you should see; a collage of four Post-it notes as the background image for your program window:

    In this program we have a PImage variable called im. In setup(), we store an image called foto.jpg in im using loadImage(). In draw() we display the image using image(). Since we set our program window to the same size as our image, the image will fit in the window perfectly. You can replace the image with another one that you take yourself or download from the internet. For best results, make sure that the image size and window size are the same.

  3. Displaying two images

    To put two pictures next to each other, you will need to declare a variable of the same type PImage , but with a different name. You will then need to set the coordinates of the second image.

    Download the photos foto.jpg and foto2.jpg


    Here, we’ve simply added another PImage called im2. We will need to double the window size in the x axis to fit the images side-by-side.

  4. Arrays

    When working with several images, we can use arrays to more easily access to them. An array is like a container with many compartments. Each compartment can hold different objects and can be called up by referencing that specific compartment.

    For example, we can have an array called FruitContainer that has 3 compartments and can hold 3 objects. Compartment 1 has a banana, compartment 2 has a strawberry, and compartment 3 has a cherry. The numbering of arrays always start with 0. So if we look in FruitContainer[0], we’ll find the banana. Similarly, in FruitContainer[1] is the strawberry and in FruitContainer[2]  is the cherry.

    Arrays can be of a certain type such as an integer (int)or a string (String). The maximum number of variables that can be stored must be  declared for the array – 3 in the example above. The following program, which is a modification of the above, shows how to use a type Array PImage to store two images.


    New commands:

    • The declaration of the images looks a bit different than before: PImage im[] = new PImage[2]. This means that we declare an array of the data type PImage with two blocks (or compartments) of data.
    • When displaying and loading the images we will refer to img[0] for the first container of the array and img[1] for the second container.

    Even though the programs have been changed, there is no difference between the result.

  5. Using two Arrays

    To further simplify the image variable assignment, we will store the names of the images in an array of strings. Here’s an example:


    We’ve added an array of the data type String where we store the file names of the images, String imFile[] = {"foto.jpg", "foto2.jpg"}. This means that imFile[0] equals to “foto.jpg” and imFile[1] equals to “foto2.jpg”. When we load the images we can use the String array to get the names of the files.

    As you will see when you run this program, the result is again the same as the others. The big question is, Why then use this to do the same? The answer is simple – it makes it easier if we need to make changes. If you need to, changes can be made in one place and not throughout the entire program. When adding more images this becomes more important.

  6. The for() loop

    In this step we will increase the number of images to display from 2 to 4. We will still use 2 arrays as in the previous steps, but to make the code even more efficient we will introduce a new programming statement: the for()loop.

    This is one of the most useful functions in programming. It does exactly what it sounds like – for X number of times, it loops through its statements. Let’s see the example before we explain it further.

    Download the photos banana.jpgpeninsula.jpgpostit.jpg, and tortilla.jpg


    New commands:

    • for(initializer; test; update){ statements }: This function allows you to repeat a piece of code for as many times as you need. The first thing to run is the initiating statement, initializer. In this example, it’s the declaration if an integer, i. Next, it checks if the test is true or false, in this case  if i is less than 4 . This is similar to the if  statement in the previous project, remember? If this is true, the code within the curly brackets will be executed. Next thing that happens is the update, in this case it is to add 1 to i. Next the test is checked again and if it’s still true the same procedure will follow again. Since we are always updating by adding 1 to ii will eventually be equal to 4 which means that the test will be false (4 is not less than 4). In that case the program leaves the loop.

    Here is the result:

    What we’ve done is to make 2 arrays with 4 positions each; one for the images and one for the image names.

    Next thing we use the for() loop will run 4 times and load each image into the array . We can use the variable i to access the positions in our arrays. The first time we go through the loop()i equals 0 which means that we load the image imFile[0] (“postit.jpg”) and store it in im[0]. Next time it loops, i equals 1, then 2, and then finally 3.

    Though not so obvious, the for() loop makes the code much shorter in the long run. Imagine if we had to load 100 pictures and had to write each upload statement rather than use a for() loop. 

  7. Multiple images

    Since our objective is to create a clock, the next step is to use images to represent numbers. For your convenience, we have a series of images that have numbers made from Post-it notes.

    Download the images representing the numbers 012345, 678 and 9.


    No new commands are introduced here.  Instead, we’ve just expanded the arrays to 10 positions. We’ve made the window size 700×95 to fit the 10 images which are 70×95 in size. The for() loops iterate ten times – setup(), it loads the images and in draw(), it displays the images. Since the images is 70 pixels wide we can set the x coordinate of each image to 70*i.

  8. The time

    In this step we will show you how we can display the time. As we’ve mentioned before, Processing can access a number of system variables. You’ve already used mouseX, mouseY and frameRate. There are three variables we can use to get the current time: hour ()minute () and second (). Let’s get started with the time by displaying the current hour:


    New commands:

    • hour(): returns the current hour as a number from 0 to 23.
    • int(data): Converts the data to an integer. Eg int(2.545) returns 2, int(233.9999) returns 233 etc.

    The first thing we do in draw() is to store the number of the hour in the variable h. The hour is a two digit number but our images only contains one digit each so we need to calculate which digits to display. The first digit is stored in h_dec and calculated with int(h / 10). The second digit is stored in h_uni and calculated with h - h_dec * 10.

    Say that the current hour is 14. This means that h equals 14, h_dec equals 1 and h_uni equals 4. These values are then used to decide which image to display.

  9. The final clock

    This is the last step of this project. In this step we will add minutes and seconds to our clock using the same method as in the previous step with the hours.

    New commands:

    • minute(): returns the current minute as a value from 0 to 59.
    • second(): returns the current second as a value from 0 to 59.

    We declare variables for the minutes and seconds and calculate what digits to display similar to the previous step.

Experiment further

  • Using the system variables of time, create a kitchen timer/countdown image clock.