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Why does my sprite slow down at screen edges? Fixing movement issues in chase games

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GF

GameDev_Fighter

Posted on July 26, 2025 • Intermediate

✈️ Enemy plane slows down at screen edges!

Hey everyone! I’m working on a plane chase game where enemy planes pursue the player, but I’m running into a frustrating issue:

  • Enemy planes move normally in the center of the screen
  • But they slow down dramatically when they reach the edges
  • This especially happens when the player gets near screen boundaries
  • The movement becomes choppy and unrealistic

I’m using coordinate-based movement and “go to” blocks for the chase mechanics. Any ideas what might be causing this slowdown? 🤔

MM

MovementMaster_Pro

Replied 2 hours later • ⭐ Best Answer

Great question @GameDev_Fighter! This is a classic coordinate system issue that many developers encounter. Here’s what’s happening and how to fix it:

🔍 The Root Problem

The slowdown occurs because of how Scratch handles screen boundaries and coordinate clamping:

flowchart TD A[🎯 Sprite at Edge] --> B{Movement Direction?} B -->|Toward Edge| C[Coordinate Clamped] B -->|Away from Edge| D[Normal Movement] C --> E[Position Stays Same] D --> F[Position Updates] E --> G[⚠️ Apparent Slowdown] F --> H[✅ Smooth Movement] G --> I[Player Notices Issue] H --> J[Normal Gameplay] style A fill:#fff3cd style C fill:#f8d7da style E fill:#f8d7da style G fill:#f8d7da style D fill:#d4edda style F fill:#d4edda style H fill:#d4edda

📊 Understanding Screen Coordinates

Scratch’s coordinate system has these limits:

  • X-axis: -240 to +240 (480 pixels wide)
  • Y-axis: -180 to +180 (360 pixels tall)

🔧 Solution 1: Virtual Coordinate System

Create your own coordinate variables that aren’t limited by screen boundaries:

    // Initialize virtual coordinates
when flag clicked
set [Virtual X v] to (x position)
set [Virtual Y v] to (y position)
set [Speed v] to [5]
set [Direction v] to [90]

✈️ Solution 2: Proper Movement System

Instead of using “go to” blocks, use calculated movement:

    // Smooth movement system
when flag clicked
forever
// Calculate movement based on direction
change [Virtual X v] by ((Speed) * ([cos v] of (Direction)))
change [Virtual Y v] by ((Speed) * ([sin v] of (Direction)))

// Apply screen boundaries to display position only
if <(Virtual X) > [240]> then
set x to [240]
else
if <(Virtual X) < [-240]> then
set x to [-240]
else
set x to (Virtual X)
end
end

if <(Virtual Y) > [180]> then
set y to [180]
else
if <(Virtual Y) < [-180]> then
set y to [-180]
else
set y to (Virtual Y)
end
end
end

🎯 Solution 3: Chase Mechanics Fix

For enemy planes chasing the player:

    // Improved chase system
when flag clicked
forever
// Calculate direction to player
set [Target Direction v] to ([atan2 v] of ((Player Y) - (Virtual Y)) ((Player X) - (Virtual X)))

// Smooth direction change
if <(abs ((Target Direction) - (Direction))) > [5]> then
if <(Target Direction) > (Direction)> then
change [Direction v] by [3]
else
change [Direction v] by [-3]
end
end

// Move using virtual coordinates
change [Virtual X v] by ((Speed) * ([cos v] of (Direction)))
change [Virtual Y v] by ((Speed) * ([sin v] of (Direction)))

// Update display position with boundaries
set x to (max [-240] (min [240] (Virtual X)))
set y to (max [-180] (min [180] (Virtual Y)))

wait (0.03) seconds
end

🚀 Solution 4: Advanced Boundary Handling

For more realistic edge behavior:

    // Boundary bounce system
define Handle Boundaries
if <<(Virtual X) > [240]> or <(Virtual X) < [-240]>> then
set [Direction v] to ((180) - (Direction))
if <(Virtual X) > [240]> then
set [Virtual X v] to [240]
else
set [Virtual X v] to [-240]
end
end

if <<(Virtual Y) > [180]> or <(Virtual Y) < [-180]>> then
set [Direction v] to ((0) - (Direction))
if <(Virtual Y) > [180]> then
set [Virtual Y v] to [180]
else
set [Virtual Y v] to [-180]
end
end

⚡ Performance Optimization

To keep the game running smoothly:

    // Optimized movement loop
when flag clicked
forever
// Only update if sprite is active
if <(distance to [Player v]) < [300]> then
// Full movement calculation
Update Movement
Handle Boundaries
else
// Simple movement when far away
point towards [Player v]
move (Speed) steps
end

wait (0.02) seconds
end

This system eliminates the edge slowdown by maintaining consistent movement calculations regardless of screen position! 🎮

CS

CoordinateSystem_Expert

Replied 30 minutes later

@GameDev_Fighter The key insight here is understanding the difference between logical position and display position! 🎯

Think of it this way:

  • Virtual coordinates = where the sprite “really” is in your game world
  • Screen coordinates = where the sprite appears on screen (clamped to boundaries)
    // Quick fix for existing projects
when flag clicked
forever
// Store intended position
set [Intended X v] to ((x position) + ((Speed) * ([cos v] of (direction))))
set [Intended Y v] to ((y position) + ((Speed) * ([sin v] of (direction))))

// Move to intended position (will be clamped by Scratch)
go to x: (Intended X) y: (Intended Y)

// But remember the real position for calculations
set [Real X v] to (Intended X)
set [Real Y v] to (Intended Y)
end

This way, your movement calculations stay consistent even when the sprite hits screen edges! ✨

GF

GameDev_Fighter

Replied 1 hour later

@MovementMaster_Pro @CoordinateSystem_Expert This is exactly what I needed! 🎉

I implemented the virtual coordinate system and the movement is now perfectly smooth, even at the edges. The chase mechanics work much better now!

Thanks for explaining the difference between logical and display positions - that really cleared things up! 🙏

VB

Vibelf_Community

Pinned Message • Moderator

🚀 Master Advanced Game Movement Systems!

Excellent discussion on coordinate systems and movement optimization! For developers looking to create even more sophisticated movement mechanics:

  • 🎯 Advanced pathfinding and AI movement
  • 🌊 Smooth interpolation and easing functions
  • ⚡ Performance optimization for many moving objects
  • 🎮 Professional game physics systems

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