1. CMPT 120 Lecture 22 – Unit 4 – Computer Vision
Python – Nested Loops, Tuples and Fruitful Functions

2. to find out how to load images
Last Lecture, we did …
Look at the PIL
documentation
to find out how to load images
and access
image pixels.
Can you print
the values
of the pixels
in your image?

3. This is how I access this pixel:
Last Lecture, we saw …
2D table called my_image
This is how I access this pixel:
my_image[10, 8]

4. Write the code that prints what is at
Testing … Testing …
What is at coordinates (0,0)?
Write the code that prints what is at
coordinates (0,0)

5. Tuples
Like a list, but not. Tuples are like read-only lists.

6. Let’s have a closer look at …
Tuples

7. Back to our “combining images” problem …
Let’s translate this
comment into
Python code!
A few things to
keep in mind:
How to go through each pixel of A?
How to know if a pixel is green?
How to find the corresponding pixel in B?

8. How to go through each pixel of A?
Nested loops are useful for traversing 2D tables

9. How to know if a pixel is green?
255 is maximum intensity

10. Two ways to access a pixel tuple's rgb values
# Get aPixel at (0,0)
aPixel = imageKidGreen[0,0]
# Way 1 - Get this pixel’s r value
r = aPixel[0]
# Way 2 - Get this pixel’s g value
g = imageKidGreen[0,0][1]
# Way 2 - Get this pixel’s b value
b = imageKidGreen[0,0][2]

11. How to find the corresponding pixel in B?

12. Fruitful Functions Problem Statement:
Write a function that returns True when a pixel is green and False otherwise
How would we generalize this function i.e., given a pixel and a colour, the function returns True if the pixel has this colour

13. Let’s give this function a try!…
Can you complete this function?

14. Can we improve our program?
Tada!!! Hum…
Can we improve our program?

15. Review How do you create or initialize a tuple?
In an image contained in a table called awesome_image, how would you access the pixel located 5 pixels in, and 8 pixels down from the top left?
What can nested loops be used for in the context of image processing?
What are 2 differences between tuples and lists?
What can computer vision be used for?

16. Next Lecture
New practice exercises
Bring your laptop! 