Platform Game Programming with AI

Lessons

Senior 3 · ICT

Platform Game Programming with AI

4 lessons

Lesson 3 of 4

Obstacles, Scoring and an Enemy That Decides

60 min

Rewards, risks and a rival

Your player can now run and jump, but nothing is at stake. A game becomes a game when there is something to win (points), something to lose (lives), something in the way (hazards), a goal, and a rival that reacts to what the player does. This lesson adds all five. The rival, an enemy with a small finite state machine, is the AI.

Definition

Variable: a named store for a value that can change while the program runs, such as score or lives. A variable for all sprites (global) can be read and changed by every sprite; one for this sprite only (local) belongs to one sprite.

Broadcast: a message sent to all sprites, which starts any script beginning with when I receive [message].

Sense–decide–act: the cycle of an AI character: sense the world (distance to the player), decide what to do (which state), then act (move).

Score and lives

Make two variables for all sprites: score and lives. Their monitors on the stage are the game's HUD. The Stage resets them when the game starts:

A coin adds to the score once, then disappears:

To make more coins, right-click the coin sprite → duplicate, and move each copy to its place (each has its own go to x: y: at the start of its script). A gem worth more is the same script with change [score] by (25).

Hazards, lives and game over

Add this script to the Player. The red parts of the Level costume (spikes, lava) and the Enemy both cost a life. The one-second wait gives the player a moment of safety after returning to the start.

Add a Game Over backdrop and a stage script when I receive [game over] → switch backdrop to [Game Over]. Also make the falling script from Lesson 2 subtract a life.

The goal

A Flag sprite at the end of the level runs: when green flag clicked → wait until <touching [Player]?> → broadcast [next level]. The Level sprite answers when I receive [next level] → next costume, and the Player answers when I receive [next level] → go to x: (-200) y: (-100). One message, three sprites, a new level.

Worked example 1: a scoring rule

A game gives 10 points per coin and 25 per gem; at the goal it adds a bonus of 50 points for every life left. The player starts with 3 lives, collects 7 coins and 2 gems, and loses one life on the spikes.

$\text{score} = 7 \times 10 + 2 \times 25 + 2 \times 50 = 70 + 50 + 100 = 220$

In Scratch, the bonus is one block in the goal script, placed before the broadcast: change [score] by ((lives) * (50)). Because lives is a global variable, the Flag sprite can read it.

The enemy: sense, decide, act

Paint an Enemy sprite and place it on the ground between x = 60 and x = 220 (adjust these numbers to your level). Make a variable state, for this sprite only. Its script is a two-state version of the Lesson 1 state machine:

Stage of the cycle	Blocks	Meaning
Sense	(distance to [Player]) from Sensing	How far away is the player, in pixels?
Decide	set [state] to [chase] or [patrol]	Closer than 120: chase. Further than 200: patrol. In between: keep the current state.
Act (chase)	compare x positions, change x by (3) or (−3)	Walk towards the player along the ground.
Act (patrol)	move (2) steps; turn round at x = 60 and x = 220	Walk to and fro on its section of ground.

The enemy moves only along x, so it stays on its platform. Its chase speed, 3, is a little below the player's walking speed, 4, so a skilful player can escape. That choice is game design, not luck.

Worked example 2: tracing the enemy

The enemy is patrolling at x = 150. The player lands on the same ground at x = 40 and stands still, so the distance is $150 - 40 = 110$ .

Sense: 110 is less than 120.
Decide: state becomes chase.
Act: the player's x (40) is not greater than the enemy's x (150), so the else branch runs: change x by (−3).

The enemy closes 3 pixels a frame. To cover 110 pixels it needs $\frac{110}{3} \approx 36.7$ , so 37 frames, about $\frac{37}{30} \approx 1.2$ s. In practice the sprites touch a little sooner, because each costume has a width. If the player walks away at 4 pixels a frame, the gap grows by $4 - 3 = 1$ pixel per frame, and after the gap passes 200 the enemy returns to patrol.

Warning

Common mistakes

Score never resets: no set [score] to (0) at the green flag, so the score carries on from the last game.
Wrong variable type: score made “for this sprite only” on the coin; the player's HUD then shows a different variable. Score and lives must be for all sprites.
touching color never true: the hazard colour in the costume is slightly different from the colour in the block. Use the block's eyedropper to pick the colour from the stage.
Lives drop by 30 at once: the player stays touching the enemy for many frames. The wait after going back to the start fixes it.
An unbeatable enemy: chasing faster than the player can run, or from anywhere on the level, is not hard; it is unfair.

THE WASD CODER: how to make a chasing AI in Scratch, an enemy that follows the player. Compare it with this lesson's enemy, which chases only when the player is near.

✦

Hands-on: finish level 1

Make score and lives for all sprites and add the stage script.
Add at least four coins and one gem from your plan. Test that each counts once.
Colour your hazards red in the Level costume and add the hazards script to the Player. Make falling into a gap cost a life too.
Add the Game Over backdrop and the Flag with the next level broadcast. Draw a second Level costume, so the flag leads somewhere.
Add the Enemy and its state machine. Show the state monitor while you test, and watch it change as you approach.
Tune the numbers (sensing distances 120 and 200, enemy speeds 2 and 3) until a classmate can win, but not too easily.
Extension: add the third state from Lesson 1, return: when the player is lost, the enemy walks back to x = 140 before patrolling again.
Save. Ask two classmates to play and to say one thing that felt unfair. Fix one of them.

Practice

Define a global variable and a broadcast, with one use of each in the platform game.
Write the coin script from memory, in words.
Why does the hazards script include wait (1) seconds?
Describe the sense, decide and act parts of the enemy script.
Using Worked example 1's rule, a player collects 5 coins and 3 gems and finishes with 1 life. What is the score?
An enemy chasing at 3 pixels per frame is 96 pixels from a player who stands still. About how many seconds until it reaches the player, at 30 frames per second?
A pupil's enemy switches between chase and patrol many times a second when the player stands about 150 pixels away. The pupil used one limit, 150, for both. Analyse the bug and fix it.
Evaluate two designs for the enemy: (A) always moves towards the player from anywhere on the level at 6 pixels a frame; (B) the design in this lesson. Which is better for the player, and why?

Show answersHide answers

Global variable: one that every sprite can use, such as score or lives. Broadcast: a message to all sprites, such as “next level”, which makes the Level change costume and the Player go to the start.
When the green flag is clicked: show; wait until touching Player; change score by 10; play the coin sound; hide.
After a hit, the player would still be touching the enemy or hazard for several frames and lose several lives. The wait gives a moment of safety.
Sense: measure the distance to the Player. Decide: set the state to chase if closer than 120, to patrol if further than 200. Act: in chase, move towards the player along x; in patrol, move 2 steps and turn round at the ends of its route.
$5 \times 10 + 3 \times 25 + 1 \times 50 = 50 + 75 + 50 = 175$ .
$\frac{96}{3} = 32$ frames; $\frac{32}{30} \approx 1.1$ s.
With one limit, a distance just under 150 sets chase, and the next small change to just over 150 sets patrol, so the state flips every frame. Use two limits with a gap, for example chase below 120 and patrol above 200, so the enemy keeps its decision in between.
B is better. A cannot be escaped, since it is faster than the player and senses from anywhere: the player always loses, which is unfair and not fun. B only chases when the player is near, gives up when the player gets away, and is slower than the player, so skill decides the outcome.

Sources and credits

Variable and Artificial Intelligence, Scratch Wiki contributors, Scratch Wiki, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Scratch is a project of the Scratch Foundation; block names and colours follow Scratch 3.
Video: How to Make Chasing AI in Scratch, THE WASD CODER, embedded from YouTube.
The block diagrams, tables, worked examples, hands-on task and practice items were written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Rewards, risks and a rival

2Score and lives

3Hazards, lives and game over

4The goal

5The enemy: sense, decide, act

6Hands-on: finish level 1