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How to fix collision detection issues in Scratch games

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CF

CollisionFixer_Sam

Posted on July 26, 2025 • Intermediate

🔧 Collision detection not working properly

I’m working on a school project where the player needs to avoid obstacles and collect stars. The obstacle collisions work perfectly, but the star collision is really inconsistent - especially when the player touches the star from the side.

The game mechanics are:

  • Player character moves around the screen
  • Stars fall from the top and rotate
  • Player should collect stars for points
  • Obstacle collision works fine
  • Star collision only works sometimes

Any help debugging this would be amazing! Thanks in advance! 🙏

CD

CollisionDebug_Expert

Replied 4 hours later • ⭐ Best Answer

Great question @CollisionFixer_Sam! This is a very common collision detection issue. Let me break down the problem and provide several solutions:

🔍 Understanding the Problem

The issue occurs because of how collision detection timing works in your loop:

flowchart TD A[🌟 Star Moving] --> B[Check: Touching Player?] B -->|No| C[Move Star Down] C --> D[Rotate Star] D --> E[Check: Reached Bottom?] E -->|No| B E -->|Yes| F[Hide Star] G[👤 Player Moving] --> H[Player Touches Star] H --> I{When does check happen?} I -->|During star's turn| J[✅ Collision Detected] I -->|Between star's turns| K[❌ Collision Missed] style A fill:#e1f5fe style J fill:#e8f5e8 style K fill:#ffebee

🚫 The Problematic Code Pattern

Here’s what’s likely causing your issue:

    // PROBLEMATIC - Only checks during star's movement
repeat until <<(y position) < (-240)> or <touching [Player v]?>>
change y by (-3)
turn right (3) degrees
if <touching [Player v]?> then
broadcast [collect star v]
end
end
hide

Why this fails: If the player moves into the star between the star’s movement cycles, the collision isn’t detected!

✅ Solution 1: Fix the Loop Structure

Move the collision check outside the movement condition:

    // FIXED - Always check collision after loop
repeat until <<(y position) < (-240)> or <touching [Player v]?>>
change y by (-3)
turn right (3) degrees
end

// Check collision regardless of why loop ended
if <touching [Player v]?> then
broadcast [collect star v]
play sound [collect v]
change [Score v] by (10)
end
hide

✅ Solution 2: Continuous Collision Checking

Use a separate script that constantly monitors collisions:

    // Main star movement script
when flag clicked
forever
go to x: (pick random (-240) to (240)) y: (180)
show
set [star active v] to [true]
repeat until <(y position) < (-240)>
change y by (-3)
turn right (3) degrees
end
set [star active v] to [false]
hide
wait (1) seconds
end

// Separate collision detection script
when flag clicked
forever
if <<(star active) = [true]> and <touching [Player v]?>> then
broadcast [collect star v]
set [star active v] to [false]
hide
end
end

✅ Solution 3: Hitbox Method

Create invisible circular hitboxes for more reliable collision:

    // Create a circular hitbox sprite
when flag clicked
set [ghost v] effect to (100) // Make invisible
set size to (80) % // Adjust hitbox size
forever
go to [Star v] // Follow the star
if <touching [Player v]?> then
broadcast [collect star v]
tell [Star v] to [hide v]
end
end

🎯 Solution 4: Distance-Based Detection

Use mathematical distance for pixel-perfect detection:

    // More precise collision detection
define check collision
set [distance v] to (sqrt (((x position) - ([x position v] of [Player v])) * ((x position) - ([x position v] of [Player v])) + ((y position) - ([y position v] of [Player v])) * ((y position) - ([y position v] of [Player v]))))

if <(distance) < [30]> then
broadcast [collect star v]
end

// Call this in your main loop
repeat until <(y position) < (-240)>
change y by (-3)
turn right (3) degrees
check collision
end

🔧 Solution 5: Player-Side Detection

Let the player sprite handle star collection:

    // In Player sprite
when flag clicked
forever
if <touching [Star v]?> then
broadcast [collect star v]
tell [Star v] to [hide v]
play sound [collect v]
change [Score v] by (10)
wait (0.1) seconds // Prevent multiple triggers
end
end

🚀 Advanced: Multi-Frame Collision Buffer

For ultra-reliable detection, check collisions across multiple frames:

    // Advanced collision buffering
define advanced collision check
set [collision buffer v] to [0]
repeat (3) // Check for 3 frames
if <touching [Player v]?> then
change [collision buffer v] by (1)
end
wait (0.01) seconds
end

if <(collision buffer) > [0]> then
broadcast [collect star v]
end

💡 Pro Tips for Collision Detection

  • Use consistent timing: Always check collisions at the same rate
  • Avoid complex shapes: Stars and irregular shapes can have detection gaps
  • Test edge cases: Try approaching from all angles and speeds
  • Add visual feedback: Temporarily change color when collision occurs for debugging
  • Consider sprite size: Larger sprites are easier to collide with

🐛 Debugging Your Collisions

Add this temporary debugging code to see what’s happening:

    // Debugging collision detection
when flag clicked
forever
if <touching [Player v]?> then
say [COLLISION!] for (0.5) seconds
set [color v] effect to (50)
wait (0.5) seconds
clear graphic effects
end
end

Try Solution 1 first - it’s the simplest fix and should solve your problem! The key is making sure collision detection happens regardless of what caused your movement loop to end. 🎯

CF

CollisionFixer_Sam

Replied 2 hours later

@CollisionDebug_Expert This is incredible! Thank you so much! 🎉

I tried Solution 1 and it works perfectly now! The collision detection is 100% reliable. I never realized the timing issue with the loop structure. This explanation really helped me understand what was going wrong.

HB

HitboxHelper_Pro

Replied 1 hour later

@CollisionFixer_Sam Just wanted to add that for star shapes specifically, the hitbox method (Solution 3) is often the most reliable because star points can create “dead zones” where collision detection fails.

Pro tip: Make your hitbox slightly smaller than the visual star so players need to get close to the center rather than just touching a point! 🌟

VB

Vibelf_Community

Pinned Message • Moderator

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