Programming, problem solving, and algorithms

CPSC 203, 2025 W2

January 16, 2026

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Colour and Code

Make the poppies pop!

PC: https://www.epicgardening.com/spring-wildflowers-from-seed/

Examine each pixel. If its red color channel is high (over 200, say), then leave it as is. Otherwise, change the pixel to grey by setting all 3 color channels to the average of their original values.

Colour and Code

import numpy as np
from skimage import io
import matplotlib.pyplot as plt

Pixel = tuple[np.uint8, np.uint8, np.uint8]  # (r, g, b)

'''Input: Pixel p
   Output: Pixel
   Function: If the pixel has high red value, then don't change it, otherwise
   fade the pixel to grey.  Note, to grey a pixel, set all color channels to
   the average of the 3.
'''
def boost_red_pixel(p: Pixel) -> Pixel:
    # setup -- extract the color channels as ints
    red = int(p[0])
    green = int(p[1])
    blue = int(p[2])
    # compute grey
    grey = (red + green + blue)//3

    # conditionally change the pixel to grey
    if red < 200:
        red = green = blue = grey

    # return updated pixel
    return (red, green, blue)

def fancify(img: np.ndarray) -> np.ndarray:
    H, W = img.shape[:2] # grab the height and width of the image
    out = img.copy()     # write to a copy

    # traverse every pixel in the image and change it!
    for row in range(H):
        for col in range(W):
            out[row, col] = boost_red_pixel(img[row,col])

    # return image with changed pixels
    return out

def main() -> None:

    img = io.imread('https://raw.githubusercontent.com/UBC-CS/cpsc203/refs/heads/main/images/poppyfield.jpg')[..., :3]     # keep first 3 channels (RGB)

    out = fancify(img)

    plt.imshow(out)
    plt.axis('off')
    plt.show()

main()

Your turn: Change image or boost

Experiment with the code! Try one or more of these:

Find a different image URL and boost a different color (green? blue?)
Change the threshold value (try 150 instead of 200)
Instead of greying, try inverting: red = 255 - red
What happens if you boost pixels that are LOW in red instead of high?

12:00

A little math puzzle break

\(1 + 3 + 5 + \ldots + 1337\) = ____________

How many terms?

puzzle continued…

English description:

The sum of the first \(k\) odd integers is ________

What about \(11 + 13 + \ldots + 23\)?

And \(2 + 4 + 6 + \ldots + 1338\)?

Python Demo: Lists

Exploratory code lives at https://us.prairielearn.com/pl/course_instance/193300/assessment/2587679

Don’t worry if you haven’t set up your own machine. The infrastructure on PL is adequate for most things we’ll do.

Back to handcraft

A quilt in different colors—green and pink—and patterns—paisley and flowers.

A white doily with a spiral pattern on a wooden surface.

A pen made from pinecone.

A vibrant woven textile with intricate geometric patterns in various colors the main being red, displayed against a mossy wooden background.

A violin made from intricately quilled green paper with a bow.

A bicycle covered in crocheted yarn in various colors.

Representing handcraft

How do we represent a craft project in code?

We need to think about:

What data describes the object?
What operations can we perform on it?

Let’s start with something simpler than a full quilt…

A bead bracelet

What data describes a bead?

What data describes a bracelet?

Dataclasses in Python

A dataclass bundles related data together into a custom type.

This creates a new type called Bead with two attributes.

Creating Bead objects

Your turn: Beads

What would these print?

02:00

Adding behavior: methods

A dataclass can also have methods.

Calling methods

What you learned today

The sum of the first k odds is _____
Lists are more complex than they first appear.
Dataclasses let us define our own types
Attributes are the data associated with an instance of a class
Methods are functions that belong to a class

Next time: The Bracelet