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How to create effective wall sensors and collision detection in Scratch

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PD

PhysicsEngine_Dev

Posted on June 8, 2016 • Intermediate

🧱 Wall Collision Detection Help

Hi everyone! I’m working on a platformer game and I need to implement proper wall collision detection. I want my character to:

  • Stop when hitting walls instead of going through them
  • Have smooth movement without getting stuck
  • Work efficiently without causing lag

I’ve heard about using color detection and sprite touching, but I’m not sure which method is best or how to implement them properly. Can someone explain the different approaches and their pros/cons? 🤔

CM

CollisionMaster_Pro

Replied 1 hour later • ⭐ Best Answer

Excellent question @PhysicsEngine_Dev! Wall collision detection is crucial for any platformer. Here are the main methods and how to implement them effectively:

🎨 Method 1: Color Detection

Use a specific color for walls and detect when your sprite touches it:

    // Basic color collision
if <touching color [#ff0000] ?> then
// Handle collision
end

Pros: Simple to set up, works with any wall shape
Cons: Can cause lag with complex backgrounds, color conflicts

🎯 Method 2: Sprite Touching (Recommended)

Create wall sprites and detect collision with them:

    // Better performance method
if <touching [Wall v] ?> then
// Handle collision
end

Pros: Better performance, more reliable
Cons: Requires separate wall sprites

🔧 The “Try and Undo” Technique

The most common and effective approach:

    // Horizontal movement
change x by (speed)
if <touching [Wall v] ?> then
change x by ((speed) * (-1))
end

// Vertical movement
change y by (y velocity)
if <touching [Wall v] ?> then
change y by ((y velocity) * (-1))
set [y velocity v] to [0]
end

⚡ Advanced: Pixel-Perfect Collision

For smoother movement, move one pixel at a time until collision:

    // Smooth horizontal collision
repeat (abs (speed))
if <(speed) > [0]> then
change x by (1)
else
change x by (-1)
end
if <touching [Wall v] ?> then
if <(speed) > [0]> then
change x by (-1)
else
change x by (1)
end
stop [this script v]
end
end

🏗️ Collision System Architecture

flowchart TD A[🎮 Player Input] --> B[Calculate Movement] B --> C[Try Horizontal Movement] C --> D{Touching Wall?} D -->|Yes| E[Undo Horizontal Movement] D -->|No| F[Try Vertical Movement] E --> F F --> G{Touching Wall?} G -->|Yes| H[Undo Vertical Movement] G -->|No| I[Movement Complete] H --> J[Set Vertical Velocity to 0] J --> I I --> K[Update Animation] style A fill:#e1f5fe style I fill:#e8f5e8 style E fill:#ffebee style H fill:#ffebee

🎯 Best Practices

  • Separate X and Y movement: Handle horizontal and vertical collisions independently
  • Use sprite touching over color detection: Better performance and reliability
  • Create invisible wall sprites: Place them over your background walls
  • Test collision after each movement: Don’t batch multiple movements
  • Consider using custom blocks: Make reusable collision functions

🔍 Advanced Collision Detection

For more complex games, you might want:

    // Custom collision block
define Check Collision (direction) (distance)
if <(direction) = [horizontal]> then
change x by (distance)
if <touching [Wall v] ?> then
change x by ((distance) * (-1))
broadcast [Wall Hit v]
end
else
change y by (distance)
if <touching [Wall v] ?> then
change y by ((distance) * (-1))
set [on ground v] to [true]
end
end

💡 Pro Tips

  • Use set drag mode [don’t drag v] to prevent manual sprite dragging
  • Consider using multiple collision sprites for different wall types
  • Test your collision system with different movement speeds
  • Add sound effects when hitting walls for better feedback

This should give you a solid foundation for wall collision detection! Start with the basic “try and undo” method and then optimize as needed. 🚀

GE

GameEngine_Expert

Replied 2 hours later

Great explanation @CollisionMaster_Pro! I’d like to add a few more advanced techniques:

🎯 Raycasting for Precise Detection

For even more precise collision detection, you can use “raycasting”:

    // Raycast collision detection
define Raycast (direction) (max distance)
set [ray distance v] to [0]
repeat until <<(ray distance) > (max distance)> or <touching [Wall v] ?>>
change [ray distance v] by [1]
if <(direction) = [right]> then
change x by [1]
end
if <(direction) = [left]> then
change x by [-1]
end
// Add other directions as needed
end
// Move back to last safe position
change x by ((ray distance) * (-1))
📐 Slope and Ramp Collision

For games with slopes, you’ll need more sophisticated detection:

    // Slope collision (simplified)
if <touching color [#00ff00] ?> then  // Green for slopes
repeat [10]
change y by [1]
if <not <touching color [#00ff00] ?>> then
stop [this script v]
end
end
// If still touching after 10 pixels, it's a wall
change y by [-10]
change x by ((speed) * (-1))
end

These techniques will help you create even more polished collision systems! 🎮

VB

Vibelf_Community

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