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Learning Math with Robotics
Flip an Image
Overview
In this lesson, students will understand how to flip an image over a line using code.
Learning Objectives
Students will be able to:
  • Flip an image over a line using code.
Lesson Description

 

1. The flip image Block

The flip image block, available in the Image menu, will take the image that is stored in an image object and flip it across a specified line, such as the $y$-axis:

 

2. Flipping Works Like a Mirror

The specified flipping line acts like a mirror. The result is easiest to see when the line is the $y$-axis (like a vertical mirror). If the initial image is centered at the point $x = 6$, $y = 6$, the flip image ($y$-axis) block will change the image to its mirror image at the point $x = -6$, $y = 6$. Here's the code and result (with the labels drawn separately):

 

If we instead flip it across the $x$-axis, it looks like this (the $y$ values of the image are changed from positive to negative):

 

Example 1 shows how it works using a dolphin image. It's recommended to use the  Step  button to step through the code one block at a time to see how the flipping works.

 IMPORTANT: When we use the flip image block to flip an image stored in an image object, the stored image is actually changed. That is, it remains in the new flipped orientation and location. So if we do a second flip with the image object, the flip will affect the image at its current flipped orientation and location, not the original image. 

Example 1 demonstrates this by creating and displaying an initial image of a dolphin, and then flipping it three times, displaying it after each flip.

 

3. Flipping Across the $y = x$ and $y = -x$ Lines

The $y = x$ line is a diagonal line at an angle of $45$ degrees. When we flip an image across that line it looks like this:

     


And flipping across the $y = -x$ line looks like this:



Examples 2 and 3 show how it works. Example 3 also places three colored dots on the image to help see how the flips work.

 

4. Flipping Across Any Vertical or Horizontal Line

We can use any vertical or horizontal line for the flipping line, not just the $x$-axis or $y$-axis. This code specifies the vertical line $x = 2$ as the flipping line, with the result shown below (with the blue dotted line and labels drawn separately):


Here are two more examples using the horizontal line $y = 4$:

     


Example 4 shows how it works.

 

 
Vocabulary
  • CS: flip image Flip the image along the y-axis, the x-axis, or the xy-axis.

 
Key Points
  1. The flip image block reflects an image object across a specified line, such as the \(x\)-axis, \(y\)-axis, \(y = x\), or \(y = -x\). The line acts like a mirror, so the image appears on the opposite side at the same distance from the line.
  2. Flipping an image changes its position and orientation based on the line of reflection. For example, flipping across the \(y\)-axis changes the sign of the \(x\)-coordinate, and flipping across the \(x\)-axis changes the sign of the \(y\)-coordinate.
  3. When an image object is flipped, the stored image is actually changed. This means each new flip works on the image in its current location and orientation, not on the original version.
Warm-Up: Reflect a Heart about the $x$-Axis
Reflect a heart about the $x$-axis. What do you notice?
Example 1: Flip an Image Three Times and Display It Each Time

The code creates an image object with a dolphin image and displays it. It then flips the image across the y-axis, displays it, flips that new version of the image across the $x$-axis and displays it, and finally flips that new version of the image across the $y$-axis and displays it. Use the  Step  button to step through the code one block at a time to see how it works.
Example 2: Flip an Image Across the $y = x$ and $y = -x$ LInes
The code creates an image object with a cat image and displays it. It then flips the image across the $y = x$ line and displays it. It uses the flip image block again to flip the image back across the $y = x$ line to change back to the original version. Then it flips the image across the $y = -x$ line and displays it. Use the  Step  button to step through the code one block at a time to see how it works.
Example 3: Flip an Image with Points Added
The code creates an image object with a dog image, adds three colored points to it, and displays it. It then flips the image across the $y = x$ line and displays it. It uses the flip image block again to flip the image back across the $y = x$ line to change back to the original version. Then it flips the image across the $y = -x$ line and displays it. The three points help to show how the flips work. Use the  Step  button to step through the code one block at a time.
Example 4: Flip an Image Across the Vertical Line $x = 2$ and the Horizontal Line y = 4
The code creates an image object with a cat image, adds a yellow point to it, and displays it. It flips the image across the vertical line $x = 2$ and displays, and then it flips it again across the horizontal line $y = 4$ and displays it. The yellow point helps to show how the flips work. Use the  Step  button to step through the code one block at a time.
Student Talk
  • Click  New Problem . How should we change the code to match what is displayed on the grid?
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