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Debugging Collision Detection Issues in Scratch Games

💡 Struggling with collision detection bugs? Need help debugging movement mechanics? 🚀 Get Debugging Help

GE

GameDev_Emma

Posted on January 23, 2024 • Intermediate

🐛 Collision Detection Bug - Player Clipping Through Walls

Hey everyone! I’m working on a platformer game and I’m having issues with collision detection. I followed a YouTube tutorial, but there’s a weird bug where:

  • 🚧 Player sometimes clips through walls in specific spots
  • 🐌 Movement becomes slow and jerky near walls
  • ⬆️⬇️ Up and down arrow keys cause the player to sink into walls

I think there might be an issue with my collision code, but I can’t figure out what’s wrong. The movement feels really weird and unpredictable! 😵‍💫

Has anyone encountered similar issues? Any debugging tips would be super helpful!

CD

CollisionDetective_Max

Replied 30 minutes later • ⭐ Best Answer

I can spot the issue right away @GameDev_Emma! This is a classic collision detection bug that many beginners encounter. Let me break down what’s happening and how to fix it:

🔍 The Problem: Axis Confusion

The issue is in your collision response code. You’re mixing up X and Y coordinates! Here’s what’s probably happening:

flowchart TD A[🎮 Player Moves] --> B[Check Wall Collision] B -->|Collision Detected| C[❌ Wrong: change x by Y speed * -1] B -->|No Collision| D[✅ Continue Movement] C --> E[🐛 Bug: Moving horizontally when hitting vertical wall] E --> F[😵‍💫 Weird jerky movement] F --> G[🚧 Player clips through walls] D --> H[😊 Smooth gameplay] style A fill:#e1f5fe style C fill:#ffebee style E fill:#ffebee style F fill:#ffebee style G fill:#ffebee style H fill:#e8f5e8

🚨 Common Buggy Code

Here’s what your code probably looks like (the problematic version):

    forever
// Movement code here...

if <touching [Walls v]?> then
change x by ((Y speed) * (-1))  // ❌ WRONG! This is the bug!
set [Y speed v] to [0]
end
end

✅ The Correct Solution

Here’s the fixed version with proper axis handling:

    // Proper collision detection with separate X and Y handling
when flag clicked
set [X speed v] to [0]
set [Y speed v] to [0]

forever
// Handle horizontal movement
if <<key [right arrow v] pressed?> or <key [d v] pressed?>> then
set [X speed v] to [5]
end
if <<key [left arrow v] pressed?> or <key [a v] pressed?>> then
set [X speed v] to [-5]
end
if <not <<key [right arrow v] pressed?> or <key [left arrow v] pressed?>>> then
set [X speed v] to [0]
end

// Handle vertical movement (gravity/jumping)
if <<key [up arrow v] pressed?> or <key [w v] pressed?>> then
if <touching [Ground v]?> then
set [Y speed v] to [15]  // Jump strength
end
end
change [Y speed v] by [-1]  // Gravity

// Apply horizontal movement with collision
change x by (X speed)
if <touching [Walls v]?> then
change x by ((X speed) * (-1))  // ✅ Correct: reverse X movement
set [X speed v] to [0]
end

// Apply vertical movement with collision
change y by (Y speed)
if <touching [Walls v]?> then
change y by ((Y speed) * (-1))  // ✅ Correct: reverse Y movement
set [Y speed v] to [0]
end
end

🎯 Advanced Collision Detection

For even better collision detection, separate X and Y collision checks:

    // Advanced method: Check each axis separately
define handle movement
// Move horizontally first
change x by (X speed)
if <touching [Walls v]?> then
change x by ((X speed) * (-1))
set [X speed v] to [0]
broadcast [wall hit horizontally v]
end

// Then move vertically
change y by (Y speed)
if <touching [Walls v]?> then
change y by ((Y speed) * (-1))
if <(Y speed) < [0]> then
broadcast [landed on ground v]
else
broadcast [hit ceiling v]
end
set [Y speed v] to [0]
end

🛠️ Debugging Tips

1. Visual Debugging:

    // Add visual feedback to see what's happening
when flag clicked
forever
set [debug info v] to (join [X: ] (join (X speed) (join [ Y: ] (Y speed))))
end

2. Step-by-Step Testing:

  • 🔍 Test horizontal movement only (disable vertical)
  • 🔍 Test vertical movement only (disable horizontal)
  • 🔍 Check collision detection with simple shapes first
  • 🔍 Use say blocks to display variable values

3. Common Pitfalls to Avoid:

  • ❌ Don’t mix up X and Y coordinates
  • ❌ Don’t use the same collision check for both axes
  • ❌ Don’t forget to reset speed variables after collision
  • ❌ Don’t make collision boxes too complex initially

🎮 Testing Your Fix

After implementing the fix, test these scenarios:

  1. 🧪 Walk into walls from all directions
  2. 🧪 Jump into walls and ceilings
  3. 🧪 Try to clip through corners
  4. 🧪 Test rapid key presses
  5. 🧪 Check movement feels smooth and responsive

The key insight is: always reverse movement on the same axis that caused the collision! 🎯

GE

GameDev_Emma

Replied 1 hour later

@CollisionDetective_Max OMG THANK YOU! 🎉 That was exactly the problem!

I was indeed using change x by ((Y speed) * (-1)) instead of change y by ((Y speed) * (-1)). Such a simple fix but it makes perfect sense now!

The movement feels so much smoother now. The debugging tips are also super helpful - I’ll definitely use the visual feedback technique for future projects! 🙏

PG

PhysicsGuru_Alex

Replied 2 hours later

Great solution @CollisionDetective_Max! 👏 I’d like to add a few more advanced tips for collision detection:

🎯 Pixel-Perfect Collision:

For more precise collision detection, consider checking collision before moving:

    // Predictive collision detection
define safe move x (distance)
change x by (distance)
if <touching [Walls v]?> then
change x by ((distance) * (-1))
set [X speed v] to [0]
end

define safe move y (distance)
change y by (distance)
if <touching [Walls v]?> then
change y by ((distance) * (-1))
set [Y speed v] to [0]
end

🏃‍♂️ Smooth Movement:

For high-speed movement, check collision in smaller steps to prevent clipping through thin walls!

VB

Vibelf_Community

Pinned Message • Moderator

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