GameMaker Lesson 3: Collisions & Bounce

🎮 Overview

Topic: Physics — Collisions & Bounce
Goal: Detect collision with walls/obstacles and respond by stopping or bouncing.
Duration: 45–60 minutes • Level: High School

Learning Objectives

1. Create solid wall/obstacle objects.
2. Detect collisions between the player and walls.
3. Respond to collisions using stop or bounce logic.
4. Debug stuck/overlap issues at contact edges.
5. Implement a reset or game-over flow (extension).

Key Concepts & Vocabulary

Step-by-Step Build

1) Create a Wall Object

1. Right-click Objects → Create Object → name it obj_wall.
2. Assign a wall/brick sprite (or create a simple colored sprite).
3. Check the “Solid” box so the player can’t overlap it.

2) Place Walls in the Room

1. Open your room in the Room Editor.
2. Place obj_wall instances around borders and as interior obstacles.
3. Save the room.

3) Add a Collision Event to the Player

1. Open obj_player.
2. Add Event → Collision → select obj_wall.
3. In the code block for this event, add one of the behaviors below (bounce or stop).

GML Code Snippets

A) Bounce off walls

// Collision Event in obj_player with obj_wall
// Make the player bounce off a solid wall (no "precise" angle calc)
move_bounce_solid(false);

B) Stop on collision (simple)

// Collision Event in obj_player with obj_wall
// Stop all motion when hitting a wall
speed = 0;

C) Stop and separate (common platformer/top-down approach)

// Example if you use hsp (horizontal speed) and vsp (vertical speed)
// 1) Horizontal step (in Step event):
//   x += hsp;
//   if (place_meeting(x, y, obj_wall)) {
//       while (!place_meeting(x - sign(hsp), y, obj_wall)) {
//           x -= sign(hsp);
//       }
//       hsp = 0;
//   }
//
// 2) Vertical step:
//   y += vsp;
//   if (place_meeting(x, y, obj_wall)) {
//       while (!place_meeting(x, y - sign(vsp), obj_wall)) {
//           y -= sign(vsp);
//       }
//       vsp = 0;
//   }
//
// Collision Event version (when present) can zero speeds too:
hsp = 0;
vsp = 0;

D) Extension: Reset/Game-Over on collision

// Option 1: Quick reset of the room with a message
show_message("Game Over!");
room_restart();

// Option 2: Reset player to a spawn point (store spawn_x/spawn_y in Create)
x = spawn_x;
y = spawn_y;

Debugging Stuck Movement

• Ensure obj_wall is marked Solid.
• If using continuous input, make sure you’re not forcing the player into the wall every step (zero out speed on collision).
• Prefer moving one axis at a time (separate horizontal/vertical resolution).
• Check sprite collision masks: tight masks reduce “phantom” hits; use a rectangle for simplicity while learning.

Pseudocode Summary

IF player collides with wall THEN
    EITHER:
        bounce off wall
    OR:
        stop movement and separate from wall
    IF extension enabled THEN
        show "Game Over"
        restart the room OR reset player position
END IF

Assessment Rubric (100 pts)

Reflection Questions

1. How does a Solid object change collision behavior compared to a non-Solid?
2. When would you choose bounce vs. stop in your game’s design?
3. What steps help prevent the player from getting stuck in walls?

Extension Challenge

Build a mini-maze. If the player touches a “danger wall,” trigger a message and reset or end the run. Add a timer and track the best time to complete the maze.