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Creating perfect reflection and bounce physics in Scratch

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PS

PhysicsWizard_Sam

Posted on January 25, 2024 • Advanced

⚡ Need perfect bounce physics for my game

Hey physics experts! I’m working on a game that requires realistic reflection mechanics - think laser beams, bouncing balls, or projectiles hitting surfaces. The problem is my current bounce system is buggy and objects sometimes clip through walls or bounce at wrong angles.

  • Need mathematically accurate reflection based on surface angles
  • Want to prevent clipping through walls completely
  • Looking for solutions that work with complex wall shapes
  • Must handle multiple surfaces in a single sprite

Anyone know the proper physics formulas and implementation techniques? I want this to be as realistic as possible! 🎯

PM

PhysicsMaster_Alex

Replied 2 hours later • ⭐ Best Answer

Excellent question @PhysicsWizard_Sam! Perfect reflection physics requires understanding the mathematical principles. Here’s the complete solution:

📐 The Reflection Formula

The fundamental formula for perfect reflection is:

d = (2 × b) - a

  • d = new direction after bounce
  • b = surface normal direction
  • a = incoming object direction

🎯 Basic Implementation

For simple surfaces with known angles:

    // Basic reflection off a surface
if <touching [Wall v]?> then
point in direction (((2) * ([direction v] of [Wall v])) - (direction))
move (2) steps // move away from wall to prevent sticking
end

🔧 Advanced Multi-Surface Detection

For complex walls with multiple surfaces, use pixel-perfect detection:

    // Advanced collision detection
define reflect off surface
set [hit detected v] to [false]
repeat until <not <touching [Walls v]?>>
move (-1) steps
set [hit detected v] to [true]
end

if <(hit detected) = [true]> then
// Calculate surface normal using surrounding pixels
set [normal x v] to [0]
set [normal y v] to [0]

// Check 8 directions around collision point
repeat (8)
turn right (45) degrees
move (1) steps
if <touching [Walls v]?> then
change [normal x v] by (([sin v] of (direction)) * [-1])
change [normal y v] by (([cos v] of (direction)) * [-1])
end
move (-1) steps
end

// Normalize the normal vector
set [normal length v] to ([sqrt v] of (((normal x) * (normal x)) + ((normal y) * (normal y))))
if <(normal length) > [0]> then
set [normal x v] to ((normal x) / (normal length))
set [normal y v] to ((normal y) / (normal length))

// Calculate reflection using dot product
set [dot product v] to (((x velocity) * (normal x)) + ((y velocity) * (normal y)))
change [x velocity v] by (((normal x) * (dot product)) * [-2])
change [y velocity v] by (((normal y) * (dot product)) * [-2])
end
end

🚀 Velocity-Based Physics System

For the most realistic physics, use velocity vectors:

    // Main physics loop
when flag clicked
set [x velocity v] to [5]
set [y velocity v] to [3]
set [bounce damping v] to [0.9] // energy loss on bounce

forever
// Store position before movement
set [old x v] to (x position)
set [old y v] to (y position)

// Apply velocity
change x by (x velocity)
change y by (y velocity)

// Check for collision
if <touching [Walls v]?> then
// Restore previous position
set x to (old x)
set y to (old y)

// Perform reflection calculation
reflect off surface

// Apply damping
set [x velocity v] to ((x velocity) * (bounce damping))
set [y velocity v] to ((y velocity) * (bounce damping))

// Move with new velocity
change x by (x velocity)
change y by (y velocity)
end

// Apply gravity if needed
change [y velocity v] by [-0.2]
end

🎨 Visual Debugging

Add visual feedback to debug your physics:

    // Debug visualization
when flag clicked
clear
forever
pen down
set pen color to [#ff0000]
set pen size to [2]

// Draw velocity vector
go to x: (x position) y: (y position)
point in direction ([atan2 v] of (y velocity) (x velocity))
move (([sqrt v] of (((x velocity) * (x velocity)) + ((y velocity) * (y velocity)))) * [5]) steps

pen up
wait (0.1) seconds
end

💡 Pro Tips

  • Prevent tunneling: Use small time steps and check collision multiple times per frame
  • Energy conservation: Adjust bounce damping based on material properties
  • Smooth movement: Use fractional coordinates for sub-pixel precision
  • Performance: Only calculate complex reflections when needed

This system will give you perfectly realistic bounce physics! 🎯

PS

PhysicsWizard_Sam

Replied 1 hour later

@PhysicsMaster_Alex This is absolutely incredible! 🤯

The velocity-based system works perfectly - no more clipping, and the reflections are mathematically perfect. The visual debugging feature helped me understand exactly what was happening with the physics.

One question: how would I add spin/rotation effects to the bouncing object?

RS

RotationSpecialist_Maya

Replied 3 hours later

@PhysicsWizard_Sam Great question! Here’s how to add realistic spin physics:

    // Add angular velocity
set [angular velocity v] to [0]
set [spin damping v] to [0.95]

// In your collision detection
if <touching [Walls v]?> then
// Calculate spin based on impact angle
set [impact angle v] to ([atan2 v] of (y velocity) (x velocity))
set [surface angle v] to ([atan2 v] of (normal y) (normal x))
set [relative angle v] to ((impact angle) - (surface angle))

// Add spin based on glancing angle
change [angular velocity v] by ((sin of (relative angle)) * [5])

// Apply spin damping
set [angular velocity v] to ((angular velocity) * (spin damping))
end

// Apply rotation
turn right (angular velocity) degrees

This creates realistic spin effects based on the angle of impact! 🌪️

VB

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