In-Class Activity: Drawing Shapes on Pictures by Using Functions

 

Introduction

In a previous activity, you have created an image object from existing JPEG image files. In this activity you will learn how to create an empty image to use as a blank canvas, and how to draw on it using functions provided in the ImageDraw module of the PIL library.

Creating an empty picture

  1. Start new notebook: Create a new notebook, called DrawingWithFunctions.ipynb in Google Colab.
  2. Add your heading: Add a Text cell and put your name, the date, and a brief description of this activity.
  3. Mount drive, import libraries: Add a Code cell and include the following statements to import the Python libraries we will be using in this activity:
    from google.colab import drive
    drive.mount('/drive')
    from PIL import Image, ImageDraw
    from matplotlib import pyplot as plt
  4. Make an empty image: We can create a new image in Python by using the new function from the Image library. The following statements create and show a black image of size 100x150:
    myImage = Image.new('RGB',(100,150))
    myImage.show()
    Add a Code cell and copy these statements. Run the Code cell. What happens? Is it what you expect? Add a Text cell before this Code cell to explain what you are doing.
    5. Add a Text cell and create a new image of size 100x150 of any color you wish. You may use any of the common colors used in webpages.
  5. Specify a color: You can pass a third parameter to the new function to specify a background color. In the same Code cell, create a new yellow image of size 100x150 and show this image.
  6. Any size, any color: In the same Code cell, create several new images, experimenting with sizes and colors. You may use any of the common colors used in webpages.

    Analysis Questions: What color is an empty picture if you don't provide a third parameter? What happens if you provide only one parameter? If you provide more than three parameters? If you provide the parameters in a different order?

Locations in a picture

JES provides several functions that you can use to add text, lines, rectangles, and ovals to a picture. When you add a new drawing element to a picture, you need to specify where to add it. The location is the number of pixels over from the left edge and the number of pixels down from the top. Location (0, 0) is the pixel in the top left corner. If w is the width of your picture and h is its height, then location (w - 1, 0) is the pixel in the top right corner, and location (0, h - 1) is the pixel in the bottom left corner.

The two numbers that make up a pixel's location are called its x and y coordinates, but note that, unlike in mathematical graphs, the y coordinates start at the top and grow as they go down, not as they go up.

Analysis Questions: What is the location of the pixel in the bottom right corner? What is the location of the pixel in the center of the picture?

Drawing on a blank canvas

Next you will be using the following drawing functions provided by JES to create an interesting picture:

  1. In the program area of JES, define a function that takes three parameters: a width, a height, and a color. Your function should make an empty picture with the specified dimensions and color.
     
  2. Use the addLine function to draw a line in your picture. The addLine function takes 6 parameters: the picture in which you want to draw, the x and y coordinates of one endpoint, the x and y coordinates of the other endpoint, and the color you want the line.
     
  3. Add a return statement at the end of your function to return your picture. To test your function, Load it, call it from the command area, and then call show on the picture it returns.
    Debugging Tip: Remember that you must capture the picture that gets returned from your function in a variable so you can use it in the show command!
  4. Use the addRect function to draw an outline of a box (an unfilled rectangle) in your picture. The addRect function also takes 6 parameters, but only the first and the last have the same meaning as in the addLine function. The second and third parameters represent the x and y coordinates of the upper-left corner of the box, while the fourth and fifth parameters represent the box's width and height.
    Challenge Suggestion: If you want a little more challenge, figure out how to center your box in the picture. What should the coordinates of the upper-left corner of the box be?
     
  5. Load and test your function. If your rectangle isn't where you thought it would be, go back and change the arguments in your call to addRect.
    Debugging Tip: If your rectangle didn't show up on your picture, make sure that the statement to draw the rectangle is before the return statement in your function.
  6. Use the addRectFilled function to draw a filled box in your picture. The parameters passed to addRectFilled are the same as those passed to addRect, except that addRectFilled fills the rectangle with the specified color. Test your function.
    Challenge Suggestion: Make your filled box smaller than the box outline, and nest it in the center of the outline.
     
  7. Use the addText function to add a string (such as your name) to your picture. In this case, the x and y coordinates represent the left edge of an invisible baseline for the text string, similar to the line on a sheet of lined paper. Test your function.
    Analysis Questions: What happens if you add the text "This is silly" to your picture at location (0, 0)? At location (1, 20)? At location (w - 1, 0), where w is the width of your picture? At location (0, w - 40)?
  8. Be creative. Add new statements that use the drawing functions to create an interesting picture. You may choose to keep or delete the statements we added to the function in exercises 2-7, but be sure to KEEP the return statement!
     
  9. Make sure that you remembered to write comments at the top with a description of the file, your name, and the date.
     
  10. When you are done, submit your file via Kit. (If you have time to do the extra exercise below, delay submitting your file until you have completed both.)

If you have time: Drawing on existing pictures

  1. Write another function that takes a picture object as its single parameter. Draw the same picture as above, or another one, on the picture passed in a parameter. If you want to know the width or height of the picture, to determine where you should place drawing elements, you can use two more JES functions that we haven't seen yet: getWidth(picture) and getHeight(picture). You may wish to pass the same picture in every time you test it, or you may wish to pass in a blank canvas each time.
     
  2. Test your new function by passing it a blank canvas. Then test it by passing in an existing picture obtained by calling pickAFile and makePicture! (Alternatively, you can use the pickAndShow function if you copy and paste it from pickAndShow.py into your new file.) Finally, if the drawing created in this function is different from the drawing created in the previous exercise, try drawing one picture on top of the other.