[Box2d] One dynamic body bounces and the other dynamic body not

Krakatoa

Imagine you are programming a breakout game. You got a paddle and a ball.

-The paddle moves in four directions not two.
-The ball bounces in the paddle but the paddle keeps moving with no bounce
-Both objects are dynamic and moves around the world

I thought a way to solve this issue: disable the contact before it occurs using the PRE_SOLVE callback. Now the bounce must be calculed manually obtaining the vector of the bounce and applying it as a linear velocity to the ball.

I read "colllision-anatomy" tutorial from iforce2d. About the normal vector that gives the manifold says: the collision normal does not give you the angle that these fixtures collided at .... it only gives the shortest direction to move the fixtures so that they don't overlap.

That means that the normal can not be used to calcule the bounce manually. I don't know how to calcule the bounce manually.

How would you solve this? Do you know an easier solution that it doesn't need to calcule forces manually?

hgvyas123

Hi

i think if you set the paddle to kinematic instead of dynamic it will also solve your problem. just use linearvelocity to move the paddle

Krakatoa

The problem of kinematic it's that doesn't detect collision between kinematics and statics bodies.

hgvyas123

ok then other solution.

1) apply very high amount of linear damping to the paddle

2) create two body for paddle one kinematic and one dynamic and use weld join on both of this body

Krakatoa

I chose the second solution and works. What I have made is to make the dynamic body smaller than the kinematic and in the update box2d function i check if there is a kinematic body attached to get it and refresh the position and angle as the dynamic body.

I doubt if when i make the weldjoint and I refresh the body position in the updatebox2d function the other body should be updated also. I'm doing it manually like this.

function GameState:updateBox2d()
world:step(1/60, 10, 5)

for body, sprite in pairs(actors) do
if(body.name == "Block") then
sprite:setPosition(body:getPosition())
body.object:getKinematicShadow():setPosition(body:getPosition())
sprite:setRotation((body:getAngle() * 180) / math.pi)
body.object:getKinematicShadow():setAngle(body:getAngle())
else
sprite:setPosition(body:getPosition())
sprite:setRotation((body:getAngle() * 180) / math.pi)
end
end
end



Thank you