HOW TO MAKE A RIGIDBODY STICK ON A ROTATING KINEMATICBODY AT THE POINT OF COLLISION OR CONTACT

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:bust_in_silhouette: Asked By ExceptionalWorks

AM CREATING A GAME WHERE RIGID-BODY FIRED SHOULD STICK AT THE POINT OF COLLISION OR CONTACT WITH A KINEMATIC-BODY ROTATING IN THE CENTER WITH THE RIGID-BODY ROTATING WITH THE KINEMATIC-BODY

SO FAR THE CODES I HAVE HERE MAKES THE RIGID-BODY STICK AT THE CENTER OF THE ROTATING KINEMATIC-BODY INSTEAD OF THE POINT OF COLLISION OR CONTACT

Get the rigid body on which the ball will stick

body_on_which_sticked = body_state.get_contact_collider_object(0)

Some transforms (tr) at the collision instant (ci)…

from the world coordinate system to the ball coordinate system

var tr_ci_world_to_ball = get_global_transform()

from the world cs to the collider cs

var tr_ci_world_to_collider = body_on_which_sticked.get_global_transform()

compute the transform form the collider to the ball.

collider->ball = collider->world then world->ball = inverse(world->collider) then world->ball

tr_ci_collider_to_ball = tr_ci_world_to_collider.inverse() * tr_ci_world_to_ball

We take the last transform of the moving collider, and we “add” the same relative position of the ball to the collider it had at the collision instant.

In other words: “world->collider (at latest news), and then, collider->ball (like at the collision instant)”.

global_transform = body_on_which_sticked.get_global_transform() * tr_ci_collider_to_ball

extends RigidBody2D

var is_sticking = false

rigid body on which the ball will stick

var body_on_which_sticked

(used when sticked) Transform (tr) at the collision instant (ci) from the collider to the ball

var tr_ci_collider_to_ball = Transform2D()

func _ready():
# Enable the logging of 5 collisions.
set_contact_monitor(true)
set_max_contacts_reported(5)

# Apply Godot physics at first
set_use_custom_integrator(false)

func _integrate_forces( body_state ):

# We turn on the "sticking mode" as soon as the ball collides
if is_sticking == false && body_state.get_contact_count() == 1 :
    is_sticking = true

    # Ignore Godot physics once the ball sticks
    set_use_custom_integrator(true) 

    # Get the rigid body on which the ball will stick
    body_on_which_sticked = body_state.get_contact_collider_object(0)

    # For debug, check on which we are sticking
    print("The ball is sticking on a ", body_on_which_sticked.get_name())

    # Some transforms (tr) at the collision instant (ci)

    var tr_ci_world_to_ball = get_global_transform() # from the world coordinate system to the ball coordinate system

    var tr_ci_world_to_collider = body_on_which_sticked.get_global_transform() # from the world cs to the collider cs

    tr_ci_collider_to_ball = tr_ci_world_to_collider.inverse() * tr_ci_world_to_ball

Because: collider->ball = collider->world then world->ball = inverse(world->collider) then world->ball

# Behavior when sticking

if is_sticking :

    # We take the last transform of the moving collider, and we keep the same relative position of the ball to the collider it had at the collision instant.

    # In other words: "world->collider (at latest news), and then, collider->ball (like at the collision instant)".

    global_transform = body_on_which_sticked.get_global_transform() * tr_ci_collider_to_ball

Could you reformat your question to make it readable?

SteveSmith | 2022-10-03 15:08