PLECK lesson 123 · suggested week 25, day 3
Add distance-based detection
Objective. Use world-space distance and a clear detection cue.
Godot Engine Standard desktop editor 4.7.2-stable, GDScript (Godot 4), Compatibility renderer. Headless Godot 4.7.2.stable.official.ed1daf0bf passed 106 assertions on the supplied scaffolds, including malformed JSON saves. Audio used the Dummy driver and a synthetic WAV, not a speaker. The visual editor, browser export, and signed-in submission were not tested. The desk still does not run a student's own upload.
About 55 minutes. The due date is chosen by a parent.
Patrol, detection, chase, return and readable fairness
Suggested pace: Recall and predict 5 min; Read instruction and scaffold 12 min; Build or design the assigned artifact 25 min; Run acceptance tests 8 min; Explain and checkpoint 5 min.
Materials: Godot Standard 4.7.2 desktop editor (GDScript, Compatibility renderer), Local course folder and prior checkpoint, No paid assets or public account required.
Comes after VGC-L122.
Distance compares positions in the same coordinate space. C-GUARD-AI uses global_position for both guard and target. Detection below 120 pixels enters Chase. Start by displaying the current state in a Label or changing a steady icon, not by adding damage immediately. Assign the Player to the exported target field. If the target is missing or freed, the guard stops safely. The detection radius is a game rule; this simple example can see through obstacles unless you add a separately tested visibility system.
Worked example
A patrol guard follows two points. A separate manager handles damage, leaving steering reusable in another level with different round rules. Scaffold C-GUARD-AI — A readable patrol/chase/return state machine Setup: Create CharacterBody2D Guard with visible circle or polygon and matching collider. Layer 4 mask 2 for walls. Attach script and assign target to your Player instance in Inspector. Level is one open rectangle; this is steering, not navigation around walls. Damage remains a separate sensor/manager event. Draw detection radii on a design diagram before tuning values. ```gdscript extends CharacterBody2D enum Mode { PATROL, CHASE, RETURN } @export var target: Node2D @export var speed: float = 70.0 var mode: Mode = Mode.PATROL var home: Vector2 var patrol_target: Vector2 func _ready() -> void: home = global_position patrol_target = home + Vector2(80, 0) func _physics_process(delta: float) -> void: if not is_instance_valid(target): velocity = Vector2.ZERO return var distance: float = global_position.distance_to(target.global_position) if mode != Mode.CHASE and distance < 120.0: mode = Mode.CHASE elif mode == Mode.CHASE and distance > 180.0: mode = Mode.RETURN var destination: Vector2 if mode == Mode.CHASE: destination = target.global_position elif mode == Mode.RETURN: destination = home if global_position.distance_to(home) <= maxf(3.0, speed * delta): mode = Mode.PATROL else: destination = patrol_target if global_position.distance_to(patrol_target) <= maxf(3.0, speed * delta): patrol_target = home if patrol_target != home else home + Vector2(80, 0) var offset := destination - global_position velocity = offset.normalized() * minf(speed, offset.length() / maxf(delta, 0.001)) move_and_slide() ```
A readable patrol/chase/return state machine
GDScript (Godot 4, not Scratch, Python, JavaScript or C#). Scaffold C-GUARD-AI. Copy starts from this text. The desk does not run it.
Scene and node setup
Create CharacterBody2D Guard with visible circle or polygon and matching collider. Layer 4 mask 2 for walls. Attach script and assign target to your Player instance in Inspector. Level is one open rectangle; this is steering, not navigation around walls. Damage remains a separate sensor/manager event. Draw detection radii on a design diagram before tuning values.
extends CharacterBody2D
enum Mode { PATROL, CHASE, RETURN }
@export var target: Node2D
@export var speed: float = 70.0
var mode: Mode = Mode.PATROL
var home: Vector2
var patrol_target: Vector2
func _ready() -> void:
home = global_position
patrol_target = home + Vector2(80, 0)
func _physics_process(delta: float) -> void:
if not is_instance_valid(target):
velocity = Vector2.ZERO
return
var distance: float = global_position.distance_to(target.global_position)
if mode != Mode.CHASE and distance < 120.0:
mode = Mode.CHASE
elif mode == Mode.CHASE and distance > 180.0:
mode = Mode.RETURN
var destination: Vector2
if mode == Mode.CHASE:
destination = target.global_position
elif mode == Mode.RETURN:
destination = home
if global_position.distance_to(home) <= maxf(3.0, speed * delta):
mode = Mode.PATROL
else:
destination = patrol_target
if global_position.distance_to(patrol_target) <= maxf(3.0, speed * delta):
patrol_target = home if patrol_target != home else home + Vector2(80, 0)
var offset := destination - global_position
velocity = offset.normalized() * minf(speed, offset.length() / maxf(delta, 0.001))
move_and_slide()
Checks to run in the editor
- Far player: guard patrols 80 pixels from home.
- Within 120 pixels: chase.
- Beyond 180 pixels: return, then patrol.
- Missing target stops safely rather than throwing an invalid-instance error.
Native Godot 4.7.2.stable.official.ed1daf0bf headless compile and representative node-tree/runtime tests passed; see native-qa-results.json. Visual UI, audible output, exports and website submission still need separate checks.
Add distance-based detection — evidence task
- 1. Add detection using C-GUARD-AI and a visible state cue.
- 2. Test at distances clearly inside and outside 120 pixels.
Deliverable. Detection checkpoint and distance test table.
Acceptance checks
- VGC-L123-T1. Inside radius enters Chase.
- VGC-L123-T2. Missing target stops without invalid-instance errors.
Understanding question. Why compare both positions in global space?
Include a downloadable project ZIP with project.godot, .tscn, .gd and original asset files when code changes. A private, teacher-accessible download link to that ZIP is equivalent. Screenshots or a playthrough do not replace editable source.
If this is still shaky. Use an open room and C-PATROL before C-GUARD-AI. Do not expect direct steering to navigate a maze. Start with the first assigned task and re-run its acceptance test before adding the remaining feature.
Optional extra. Add a visible state label or icon and compare how it changes player understanding. Document one changed parameter, your prediction, and the observed effect; optional, not required for completion.
Turn in typed notes and a JPEG, PNG, or WebP screenshot. One https address is stored as text and is not opened, extracted, or run. An MP3, WAV, M4A, MP4, or WebM file up to 4 MB can show a demo. The demo does not replace editable source. Zip and code files are refused. If there is no private link, keep the project on the family computer and review the code in person.
- Your first 2D game
Free public tutorial; no account; external asset download is optional, not used by this course.
· Opening this link does not submit the lesson.
This is a preview. A parent assigns the lesson and chooses the due date after the student is enrolled. The answer key is not on this page.