🎮 Functions for Character Animation

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(Also available in Pyret)

Students define functions that control the movement of the target and danger in their games.

Lesson Goals

Students will be able to:

Apply the Design Recipe to create a function given the constraints of a word problem.
Explain the basics of animation.

Student-Facing Lesson Goals

Let’s use the Design Recipe to help us breakdown word problems that will get our game characters moving!

Materials

Preparation

This lesson involves a roleplay in which students will act out character animation along a number line representing the x-axis. Decide where this roleplay will take place in your classroom and whether you will project, draw or make a number-line (from 0 to 1000) for your volunteers to move along.

Key Points for the Facilitator

Encourage students to take their time in understanding why we want to fix update-danger and update-target.
Students might be confused as to how the animation is working. The make-game function at the bottom of the file has many inputs - including update-danger and update-target. make-game takes in all those inputs, including the functions we’ll write, and creates the interactive window that we see when we click the "Run" button!
Note: This lesson does not result in the player (avatar) moving! This lesson is strictly about automated characters (called Non-Player Characters, or 'NPCs'). Player movement will be covered in a future lesson.

🔗Animation 45 minutes

Overview

Students connect the behavior of functions with changing coordinate values, ultimately leading to animation.

Launch

How does a flip-book animation work?
- Each page of the book is slightly different, and the pages go so fast that the motion looks smooth.
Why do we see movement from still images?
- Our eyes fill in the gaps between rapidly changing images.
How might this apply to our game?
- If we change image coordinates a little bit at a time, they will appear to move.

We are going to write a function, called update-target, that will be responsible for changing the x-coordinate of the TARGET location.

Note: In our game, there is hidden code that will randomly generate the y-coordinate for where the TARGET enters the screen.

Draw a number line on the board, running from 0 to 1000 (you can also lay tape on the floor, or use a tile floor as a coordinate plane!) Make sure spacing is such that a student will be able to visibly move along it in increments of 50.
Select a student volunteer to be the TARGET.

Before we write our update-target function, let’s have a volunteer act it out!
For this roleplay, we will have our update-target function move our TARGET left across the x-axis by 50 (pixels) each time the function is called.
- When the TARGET hears "update target" followed by their current location, they take a step in the negative direction, moving left across the x-axis by 50 (pixels).
Let’s have our TARGET start at 800.
To get them moving, we’ll call (update-target 800).
To make them move again, we’ll need to call the function and new location. What will our new function call be?

Crowdsource the function calls to move the TARGET several more times.

How could we change our update-target function to move the TARGET across the classroom more quickly?
- We could have the TARGET move by more pixels each time, for example by 100 instead of 50.
What did you notice about the movement of TARGET? What was changing?
- Answers will vary: they were moving horizontally, their x-coordinates were changing, they were moving one step at a time, etc.
If we want to write another function called update-danger and have it move the DANGER 50 pixels to the right, how will the function work?
- It will take in the x and y-coordinate and add 50 to x.

Investigate

Sign in to WeScheme and open your saved Game Starter Files, or make a new copy.
Examine the update-danger function. Identify the Contract, and interpret what the function is currently doing.
With your partner, complete the first word problem on Danger and Target Movement.
Transfer the code to your Game Starter File and click "Run" to load the code.
The working update-danger function should automatically move the DANGER image across the screen!
Then complete the second word problem on Danger and Target Movement.
Transfer the code to your Game Starter File and click "Run".
Now you should see the DANGER and TARGET moving across the screen independently!

Extension Activities

Once students have successfully gotten update-target and update-danger working, they can change those functions to make their targets and dangers move in whichever direction and whatever speed they want to make the game the most fun to play! They should be sure to modify their purpose statements and examples if they change their functions.

Students may ask about animating their player and we won’t get to that until a future lesson on 🎮 Player Animation. You can point out that the player will need to be able to move both up and down, and right now they don’t know the math or programming for a function to have two completely different behaviors.

Synthesize

Connecting the code to the underlying math is important - especially if you want to customize your game!

What part of the function controls the character’s speed?
What part of the function controls the character’s direction?
If you wanted the characters to move in 2 dimensions (diagonally, for example), would anything have to change about the Domain? What about the Range?

These materials were developed partly through support of the National Science Foundation, (awards 1042210, 1535276, 1648684, 1738598, 2031479, and 1501927). Bootstrap by the Bootstrap Community is licensed under a Creative Commons 4.0 Unported License. This license does not grant permission to run training or professional development. Offering training or professional development with materials substantially derived from Bootstrap must be approved in writing by a Bootstrap Director. Permissions beyond the scope of this license, such as to run training, may be available by contacting contact@BootstrapWorld.org.