+3 votes

I apologize for asking the same thing here. The forum becomes depopulated and I 'm not an informatician, so I suck at maths.

It's about a ball new direction after bouncing.
The ball is a kinematic body 2D. I want the new vector direction to be symmetric to the previous direction for the sake of predictability. The axis of symmetry is the collision normal.
Like a breakout but with a formula who works for every collision normal.

I'm looking for the pink vector

in Engine by (516 points)
recategorized by
I'll move it to the Engine category as finally the best answer is Godot-specific :)

3 Answers

+6 votes
Best answer

To find a reflection vector, you can use Vector2D/3D built-in function Vector2D reflect(Vector2D normal). I'm pretty sure the input vector is the reflection normal.

by (830 points)
selected by
That's it ! I I tried this function before asking here, but I used the wrong order. The doc description is a bit confusing, it talks about sliding.
Anyway, I guess using a built-in function is faster at runtime than write it "manually" in GDScript. Thanks.
–1 vote

This isn't really a Godot question and it can be solved super easily by googling "reflection vector".

First result

by (372 points)
+4 votes

What you are looking for is Vector2.reflect(). In order to give you a better idea of how you implement this, a full example of a BALL script (taken from this video).

Example of use:

Code in ball.gd that is attached to a KinematicBody2D.

Ball Scene Hierarchy:
KinematicBody2D [script attached here]
- Sprite
- CollisionShape2D
- VisibilityNotifier2D


extends KinematicBody2D

# Gravity
var _gravity = 0

# Movement
var _movement = Vector2()

# Bounce reduction
var bounce = 0.6

# Initialize shot from another script after adding it to the scene
func shoot(directional_force, gravity):
    _movement = directional_force
    _gravity = gravity

# Processing - 60 ticks per second
func _fixed_process(delta):
    # Simulate gravity
    _movement.y += delta * _gravity

    # Check if we have collided
        # Get node that we are colliding with
        var entity = get_collider()

        # Apply physics
        var normal = get_collision_normal()

        # Apply bounce physics
        # _movement = (_movement - 2 * _movement.dot(normal) * normal) * bounce
        _movement = normal.reflect(_movement) * bounce

    # Move

# On screen exit
func _on_visibility_notifier_exit_screen():

Note that in order to reflect the movement, you will need to assign it to the movement var that you want to "move" to.
movement = normal.reflect(_movement)*

In a nutshell:
NewMovementVector2 = NormalVector2.reflect(CurrentMovementVector2)

(I know this have technically been answered, but I figure an example of how it could be implemented would be very useful to majority of people) :)

by (495 points)
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