This week you will explore another useful feature of the Graphics module: creating and manipulating images. You will create Instagram-style filters. This will give you a hands-on introduction to the subfield of computer science called image processing.

The goals this week are to:

• Practice working with objects.
• Practice with the Graphics library, this time with Image objects.
• Use command-line arguments to control aspects of a program.
• Practice reading code documentation.

• Lab 6: Images
  • Tasks
    • L0. The setup
    • L1. Downloads
      • L1a. Code
      • L1b. Images
    • L2. Create an Instagram-style image filter
      • L2a. Create a filter function swapRedBlue that swaps red and blue colors in an image
      • L2b. Test out your filter
  • Get started with the project
    

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Tasks

L0. The setup

Create a Lab06 folder on your computer where you store CS151 projects and labs, hopefully somewhere that is backed up (e.g. in Dropbox, Google Drive, Microsoft OneDrive, etc.)

Open up a Terminal (e.g. Control+` in VS Code) and change the directory (cd) to the Lab06 folder.

L1. Downloads

L1a. Code

Copy each other following files to your Lab06 folder.

Description	Links
Graphics Module	Graphics module Documentation (PDF)

Important: The Graphics documentation PDF contains a listing of all Graphics object types, their methods, and how they work. It is helpful to have this open as you work.

L1b. Images

Download at least two of the following PPM images to use in the lab exercises. You should right-click on the Image and do "Save Link As..."

L2. Create an Instagram-style image filter

Now you're going to play with the pixels in an image to create different effects.

Recall that each pixel in a color image has three values (r, g, b). Each pixel is addressed by its row ("y") and column ("x"). The height of the image tells us the number of rows and the width of the image tells us the number of columns.

The Graphics module contains two functions that make it easy to get and set pixel values if you have an Image object. The three lines below demo how you would:

• read in a Image from a file (which you did above)
• get the RGB values at the pixel at x = 42 and y = 35.
• replace the RGB values of the pixel at x = 42 and y = 35 by twice their current values.

img = graphics.Image((0,0), 'myImage.ppm')
(r, g, b) = img.getPixel(42, 35)
img.setPixel(42, 35,(2*r, 2*g, 2*b))

L2a. Create a filter function swapRedBlue that swaps red and blue colors in an image

Create a new file called filters.py, import graphics and sys.

Copy-and-paste and fill in the below function signature and docstring for the function named swapRedBlue that takes in one parameter, which is an Image object that you will be modifying.

def swapRedBlue(img):
    '''Swaps the red and blue values of every pixel of a image `img`'''

Hint: You will need to write a nested loop. Also, take a look at lecture notes and/or the documentation PDF for Image methods.

L2b. Test out your filter

Create a test function that should take one parameter, the filename (a string) of the image (will be passed in by command-line argument). Except for applying the filter and saving the resulting image, the code should be very similar to what you did in show.py:

• Load in the image.
• Apply the swapRedBlue image filter to the image.
• Save the filtered image as a .ppm file in your Lab06 folder (the filename should not be the same as the original image!). Look at the documentation to figure out how to save an Image object as a image file on your computer.
• Show the result until the user clicks.

Add main code that calls your test function and pass in the filename from command-line arguments.

Running filters.py should show you the filtered image (red parts of the original image are blue-ish and vice-versa).

You should also see a new PPM file in your Lab06 folder that is the filtered image. Double check that the saved image file looks right (e.g. pass the filename as a command-line argument to show.py from last week).

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Get started with the project

Once you are done with each of the lab tasks, you may start on the project.

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