Images lab

Log into a lab computer, mount COURSES, and navigate to your STUWORK directory. Look back at the lab from the first day of class or get help from Anna or Rachel if you need a reminder on how to complete these steps.

Create a folder in your STUWORK directory called imagesLab. Open VSCode, and drag the imagesLab folder into the VSCode window. Download the starter code and move all files into your new folder.

The download should include two Python files, imageLab.py and cImage.py, as well as three sample images.

One additional setup note: if you're running this on your own computer after class, you might need to install a library called pillow. You can do so with the following command in terminal:

pip3 install pillow

If that doesn't work, try pip install pillow. If that still doesn't work, reach out for help and in the meantime work on one of the lab computers that already has pillow installed.

This section describes the actions you should take with the code; while programming, you may also find it useful to read the section further down in the document where I summarize functions that might be helpful to use.

These functions can be applied to Image objects and may be helpful when writing your code.

• getHeight (no parameters): returns an integer representing how tall the image is, i.e., the number of rows of pixels in the image.
• getWidth (no parameters): returns an integer representing how wide the image is, i.e., the number of columns of pixels in the image.
• getPixel (two parameters: column (int) and row (int)): returns a reference to the pixel object at coorindates (column, row). E.g., if column=2 and row=0, this would return the pixel in the top row, three pixels from the left.
• setPixel (three parameters: column (int), row (int), and pixel (Pixel object)): set the pixel at coordinates (column, row) to the given pixel. Helpful for when you've modified a copy of a pixel and want to place that modified pixel into the new image.

There are also some functions that can be Pixel objects that may be useful.

• getRed (no parameters): returns the integer in [0, 255] representing the level of red in the pixel.
• getGreen (no parameters): returns the integer in [0, 255] representing the level of green in the pixel.
• getBlue (no parameters): returns the integer in [0, 255] representing the level of blue in the pixel.

Here are some more exercises to practice using nested loops and while loops. For each of these, I recommend writing the solution paper before typing it up and testing it. We'll go through the solutions to these in class on Friday, so please try them before then.

1. Use nested loops to write a snippet of code that will print the following pattern to the screen:

   *
   **
   ***
   ****
   *****
   

2. Use nested loops to write a snippet of code that will print the following to the screen:

   012345
   123456
   234567
   345678
   

3. Write a function called multiplication that will compute and print a multiplication table nested loops. You should print 5 rows and 5 columns (but feel free to make your function more advanced by having it accept parameters indicating how many rows and columns to print). I.e.,

   1   2   3   4   5
   2   4   6   8   10
   3   6   9   12  15
   4   8   12  15  18
   5   10  15  20  25
   

   Note: you'll want to use the print() function, but instead of printing a newline at the end, you may want to print a tab. You can do that with the following syntax: print(1, end='\t') where '\t' is the newline character.

4. What will be the output of each of the following code snippets?

   a.

   i = 9
   while i > 0:
       print(i)
       i -= 2
   print(f"After loop, i is {i}")
   

   b.

   for i in range(4):
       j = 0
       while j <= i:
           print(i, j)
           j = j + 1
       print()
   

This lab was adapted from labs used by Anna Rafferty and Layla Oesper. Thanks for sharing!