Introduction to images

Understanding images

Pixels

• Digital images consist of pixels
• A pixel is a small square, showing a single color

Download and view pebbles.jpeg with an image viewer, zoom in, and see the pixels it uses.

RGB

• each pixel can be controlled using a combination of red, green and blue
• each color ranges from a value of 0 (minimum) to 255 (maximum)
• these colors mix to form the color of the pixel
• visit rgb-explorer to see color mixing in action

The byuimage library

• code we have written to make it easier to work with images
• similar to byubit, but in for images
• visit your CS110 conda environment, and in a PyCharm terminal install this:

pip install byuimage

Loading an image

from byuimage import Image

image = Image('flower.jpeg')
image.show()

Looping over the pixels

• the pixels in the image are a collection
• this collection is ordered -> they go from left to right and top to bottom
  • (0,0) -> (1,0) -> (2,0) …
  • (0,1) -> (1,1) -> (2,1) …

Looping over the pixels and changing them

for pixel in image:
    ...

• In this loop, we now have a pixel variable that is set to the current pixel (fro top to bottom, left to right)
• pixel.green allows us to change the green value of the variable
  • the same for pixel.green and pixel.blue

for pixel in image
    pixel.green = 50

Modifying an image

• Let’s set the blue and green values in each pixel of the image to 0
• this will leave just the red values — the “red channel” of the image

image = Image("flower.jpeg")
for pixel in image:
    pixel.green = 0
    pixel.blue = 0
image.show()

Looping over the pixels

• each time through the for loop, the pixel variable is set to the current pixel
  • the variable could be called truck, but we like our variables to make sense

image = Image("flower.jpeg")
for truck in image:
    truck.green = 0
    truck.blue = 0
image.show()

image = Image("flower.jpeg")
for pixel in image:
    pixel.green = 0
    pixel.blue = 0
image.show()

Play around with the code

• what about a green channel?
• making all pixels black?
• inverting the pixels?
• swapping red, green, and blue?

image = Image("flower.jpeg")
for pixel in image:
    pixel.green = 0
    pixel.blue = 0
image.show()

Invert the pixels

image = Image("flower.jpeg")
for pixel in image:
    pixel.red = 255 - pixel.red
    pixel.green = 255 - pixel.green
    pixel.blue = 255 - pixel.blue
image.show()

Swap red, green, blue colors

image = Image("flower.jpeg")
for pixel in image:
    red = pixel.red
    pixel.red = pixel.green
    pixel.green = pixel.blue
    pixel.blue = red
image.show()

Image functions

Write a function for red channel

def red_channel(filename):
    image = Image(filename)
    for pixel in image:
        pixel.green = 0
        pixel.blue = 0
    return image

red_flower = red_channel("flower.jpeg")
red_flower.show()

How many variables are used in this code?

def red_channel(filename):
    image = Image(filename)
    for pixel in image:
        pixel.green = 0
        pixel.blue = 0
    return image

red_flower = red_channel("flower.jpeg")
red_flower.show()

• filename — function parameter becomes a variable inside the function
• image — variable inside the function
• red_flower — variable outside the function

Darkening an image

def darken(filename):
    image = Image(filename)
    for pixel in image:
        pixel.red = pixel.red * 0.5
        pixel.green = pixel.green * 0.5
        pixel.blue = pixel.blue * 0.5
    return image

darkened = darken("flower.jpeg")
darkened.show()

if statements inside a while loop

Remove the green

Change pixels that are green to black

from byuimage import Image

def remove_green(filename):
    image = Image(filename)
    image.show()
    for pixel in image:
        if pixel.green > 100:
            pixel.red = 0
            pixel.blue = 0
            pixel.green = 0
    return image

img = remove_green("flower.jpeg")
img.show()

Remove the green

Use a better method to identify green pixels: take the average of red, blue, and green and a green pixel has its green value bigger than the average

def remove_green(filename):
    image = Image(filename)
    image.show()
    for pixel in image:
        average = (pixel.red + pixel.blue + pixel.green) / 3
        if pixel.green > average:
            pixel.red = 0
            pixel.blue = 0
            pixel.green = 0
    return image


img = remove_green("flower.jpeg")
img.show()