# Section #1: Nested Loops and Images

January 17th, 2021

Written by Nick Parlante and Juliette Woodrow

## Image Range-Loop Problems

You might find nested y,x range loops helpful in solving these. Problems like this are well suited to making a drawing first to think through the algorithm before coding, and especially to get the arithmetic for each coordinate exactly right. These algorithms are a magnet for off-by-one errors.

A good first step, as in lecture, is to sketch out the pixel grid x,y coordinates as in lecture to plan how your algorithm will access the pixels.

## Understanding Nested For Loops

What does this code print?


for i in range(2):
for j in range(4):
print(i, j)



## Aqua Bars

In this problem, your job is to write a function aqua_bars(filename, bar_height) which takes as input a filename of an image and an integer bar height. In the function, create a new out image tall enough to hold a copy of the original image, plus extra space at the top and bottom for horizontal, aqua colored bars. Each bar should have have the height given in the bar_height parameter. A white pixel can be changed to aqua by setting its red value to 0. Return the out image when done.

Here is an example run of the function for the image 'yotter-100.jpg' shown below:

Input Image:

 aqua_bars('yotter-100.jpg', 5) 

Output Image:

## Stack2

In this problem, you job is to define a function stack2(filename) which creates a new out image double the height of the original and places two copies of the original image inside of it, with one on top of the other.

Here is an example run of the function for the image 'stop-200.jpg' shown below:

Input Image:

 stack2('stop-200.jpg') 

Output Image:

## Flip

Write a function flip_vertical(filename) that creates and returns a new image which is the original image flipped vertically but unchaged horizontally.

Here is an example run of the function for the image 'poppy-200.jpg' shown below:

Input Image:

 flip_vertical('poppy-200.jpg') 

Output Image: