[Bas]

hi I want to speed an object over time.

in this case it is a ball and a gametype like pong.

i want to move the ball quicker over time, but with all the other physics in tact like friction etc.

how can i achieve this?

[AlexFelgo Team]

Hi Bas,

the force property of the colliders will be the way to go, which is used to apply a force to a body every frame.

Cheers,
Alex

[Bas]

thanx Alex,

 

but how do i aply this to a ball in a pong like game?

when i apply this, i get a drunken ball 🙂

[Bas]

EntityBase {
    id: entity
    entityType: "ball"

    property int speed: 400

    CircleCollider {
        id: circleCollider
        property alias entity:entity
        radius: 172>>1
        // Entity does not have width and height, therefore we center the CircleCollider in the entity.
//        anchors.centerIn: parent
        // set friction between 0 and 1
        fixture.friction: 0.3
        // restitution is bounciness
        fixture.restitution: 1.0
        fixture.density: 0.3
//        body.angularDamping: 1
//        body.angularVelocity: 0
        force: Qt.point(1000000.0,1000000.0)
        // The ball is a fast and small object therefore, we set the bullet flag to true to get better physic results
        // during collision detection.
        bullet: true
//        body.fixedRotation: true
    }

 

[AlexFelgo Team]

What’s wrong about a drunken ball? 😉

Well if you take the current velocity into calculation you could achieve a good result? I didn’t test it yet but using something like

Qt.point(circleCollider.linearVelocity.x/10,circleCollider.linearVelocity.y/10)

could make sense? The idea would be to accelerate the ball by a 10th of its own speed every frame.

Cheers,
Alex

[Bas]

hahaha, drunken ball is cool!

hmm i am new to physics here so is it an initial force which can be set in the collider?

i use the code of the pong tutorial online.

but when i apply the force the ball is not going any faster.

i use friction as well for the bouce rotation.

 

[AlexFelgo Team]

Oh well, the pong tutorial is outdated and not yet part of the official documentation at the moment.

The problem that you are facing in that old tutorial is, the ball is not really bouncing with pure physics, but instead on every collision it is stopped and the new velocity is calculated and applied to the ball. So you won’t see the effect of this small acceleration, because the next collision is happening and again the ball is stopped and the new velocity is calculated, always with the speed variable that is set to a fixed value of 400. I added a small project which shows the acceleration I suggested.

import Felgo 3.0
import QtQuick 2.0

GameWindow {
  id: gameWindow
  activeScene: scene
  width: 960
  height: 640

  EntityManager {
    entityContainer: scene
  }

  Component.onCompleted: {
    ball.collider.applyLinearImpulse(Qt.point(70,34));
  }

  Scene {
    id: scene
    width: 480
    height: 320

    PhysicsWorld {

    }

    BoxCollider {
      width: 10
      height: parent.height
      bodyType: Body.Static
      anchors.right: parent.left
    }
    BoxCollider {
      width: 10
      height: parent.height
      bodyType: Body.Static
      anchors.left: parent.right
    }
    BoxCollider {
      width: parent.width
      height: 10
      bodyType: Body.Static
      anchors.bottom: parent.top
    }
    BoxCollider {
      width: parent.width
      height: 10
      bodyType: Body.Static
      anchors.top: parent.bottom
    }

    EntityBase {
      id: ball
      x: 50
      y: 50
      property alias collider: collider
      Rectangle {
        color: "#f00"
        width: 10
        height: 10
        radius: 5
        anchors.centerIn: parent
      }

      CircleCollider {
        id: collider
        radius: 5
        restitution: 1
        friction: 0
        bullet: true
        force: Qt.point(linearVelocity.x/10,linearVelocity.y/10)
        anchors.centerIn: parent
      }
    }
  }
}

In the pong tutorial, you could increase the value of the speed variable on every collision and this would make the ball faster.

Cheers,
Alex

• This reply was modified 8 years, 5 months ago by  Alex.

[Bas]

hi Alex,

 

thanx for that, but i need rotation of the ball (in fact it isn’t really a ball) so i turned friction on.

and there is only an initial force applied to the ball but when i alter some params i get the drunken ball again 🙂

 

import Felgo 3.0
import QtQuick 2.0

GameWindow {
  id: gameWindow
  activeScene: scene
  width: 960
  height: 640

  EntityManager {
    entityContainer: scene
  }

  Component.onCompleted: {
    ball.collider.applyLinearImpulse(Qt.point(20,14));
  }

  Scene {
    id: scene
    width: 480
    height: 320

    PhysicsWorld {

    }

    BoxCollider {
      width: 10
      height: parent.height
      bodyType: Body.Static
      anchors.right: parent.left
    }
    BoxCollider {
      width: 10
      height: parent.height
      bodyType: Body.Static
      anchors.left: parent.right
    }
    BoxCollider {
      width: parent.width
      height: 10
      bodyType: Body.Static
      anchors.bottom: parent.top
    }
    BoxCollider {
      width: parent.width
      height: 10
      bodyType: Body.Static
      anchors.top: parent.bottom
    }

    EntityBase {
      id: ball
      x: 50
      y: 50
      property alias collider: collider
      Rectangle {
        color: "#f00"
        width: 10
        height: 10
        radius: 5
        anchors.centerIn: parent
      }

      CircleCollider {
        id: collider
        radius: 5
        restitution: 1
        friction: 0.2
        density: 0.03
        bullet: true
        force: Qt.point(linearVelocity.x/10,linearVelocity.y/10)
        anchors.centerIn: parent
      }
    }
  }
}

 

[AlexFelgo Team]

Hi Bas,

well that will get more tricky, as you will have to take the rotation of the ball into account then when calculating forces. Also remember, if you have friction, you won’t have perfect bounces, so for a pong game friction is maybe not very suitable. Do you need the rotation to be 100% accurate or is it just a visual thing?

Cheers,
Alex

[Bas]

hi Alex,

it is more a visual thing,

but would be nice that it is correct in a way that the rotation will be reversed when bouncing in oposite direction.

the rotation does not need to affect speed after bounce.

 

[AlexFelgo Team]

So I added a suggestion for the rotation, in this case it’s just a property binding on the velocity property of a MovementAnimation that is constantly rotating the rectangle of the entity, so it rotates in the direction it is traveling, and also rotates faster if the ball travels faster (at least in x direction):

import Felgo 3.0
import QtQuick 2.0

GameWindow {
  id: gameWindow
  activeScene: scene
  width: 960
  height: 640

  EntityManager {
    id: entityManager
    entityContainer: scene
  }

  Component.onCompleted: {
    ball.collider.applyLinearImpulse(Qt.point(50,50), ball.collider.body.getWorldCenter());
  }

  Scene {
    id: scene
    width: 480
    height: 320

    PhysicsWorld {

    }

    BoxCollider {
      width: 10
      height: parent.height
      bodyType: Body.Static
      anchors.right: parent.left
    }
    BoxCollider {
      width: 10
      height: parent.height
      bodyType: Body.Static
      anchors.left: parent.right
    }
    BoxCollider {
      width: parent.width
      height: 10
      bodyType: Body.Static
      anchors.bottom: parent.top
    }
    BoxCollider {
      width: parent.width
      height: 10
      bodyType: Body.Static
      anchors.top: parent.bottom
    }

    EntityBase {
      id: ball
      x: 50
      y: 50
      property alias collider: collider
      Rectangle {
        id: rect
        color: "#f00"
        width: 10
        height: 10
      }

      MovementAnimation {
        target: rect
        property: "rotation"
        velocity: collider.linearVelocity.x // this is the property binding to alter the rotation
        running: true
      }

      CircleCollider {
        id: collider
        radius: 5
        restitution: 1
        friction: 0
        bullet: true
        force: Qt.point(linearVelocity.x/10,linearVelocity.y/10)
        fixture.onEndContact: {
          // you could also calculate the rotation velocity here, after a contact, based on any values you prefer
          // this would give you even more control
        }
      }
    }
  }
}

Cheers,
Alex

[Bas]

hi Alex,

i aplied the force to a ball like object and i noticed that after a while the speed is not increasing anymore.

is it limited?

 

[AlexFelgo Team]

Hi,

no there is no limit, just the fact that the ball is pretty much traveling the whole scene in a single frame at some point makes accurate acceleration not possible anymore at some point.

If I add

onLinearVelocityChanged: {
  console.debug("velocity " + linearVelocity.x + " | " + linearVelocity.y)
}

to the collider, I see that at a linearVelocity of around 1400 – 1600 pixels/second it starts to swing up and down. That’s probably because it is traveling so fast, that the force is applied in one direction before the property binding is updated with the direction change, which then actually slows the ball in that particular frame. But that only happens at a very high speed, which I don’t expect to be reached in a game anway?

Cheers,
Alex

[Bas]

i added this also and got 1880 when boucing vertically with les horizontal velocity.

The game is running at 1920 x 1080 resolution and the ball sprite is 180 pixels square.

when the game speed is at its max, it is still quite easy to handle (arkanoid variant game)

can i adjust other parameters so that it wil go faster?

 

[AlexFelgo Team]

Hi,

and you are also using 1920 x 1080 as the size of your scene? Why are you not keeping the scene at a smaller logical size and use the scene scaling mechanism that Felgo has integrated instead? Box2D is made for physical simulations at a smaller scale, see this link to an example of a similar problem.

import Felgo 3.0
import QtQuick 2.0

GameWindow {
  id: gameWindow
  activeScene: scene
  width: 1920
  height: 1080

  EntityManager {
    id: entityManager
    entityContainer: scene
  }

  Component.onCompleted: {
    ball.collider.applyLinearImpulse(Qt.point(200,200), ball.collider.body.getWorldCenter());
  }

  Scene {
    id: scene
    width: 480 // this is a fourth of the gameWindow width
    height: 270 // this is a fourth of the gameWindow height
    // the scene will be scaled to fit into the gameWindow

    PhysicsWorld {

    }

    EntityBase {
      id: leftWall
      width: 10
      height: parent.height
      anchors.right: parent.left
      BoxCollider {
        anchors.fill: parent
        bodyType: Body.Static
      }
    }
    EntityBase {
      id: rightWall
      width: 10
      height: parent.height
      anchors.left: parent.right
      BoxCollider {
        anchors.fill: parent
        bodyType: Body.Static
      }
    }
    EntityBase {
      id: topWall
      width: parent.width
      height: 10
      anchors.bottom: parent.top
      BoxCollider {
        anchors.fill: parent
        bodyType: Body.Static
      }
    }
    EntityBase {
      id: bottomWall
      width: parent.width
      height: 10
      anchors.top: parent.bottom
      BoxCollider {
        anchors.fill: parent
        bodyType: Body.Static
      }
    }

    EntityBase {
      id: ball
      x: 50
      y: 50
      property alias collider: collider
      Rectangle {
        id: rect
        color: "#f00"
        width: 10
        height: 10
      }

      MovementAnimation {
        target: rect
        property: "rotation"
        velocity: collider.linearVelocity.x // this is the property binding to alter the rotation
        running: true
      }

      CircleCollider {
        id: collider
        radius: 5
        restitution: 1
        friction: 0
        bullet: true
        force: Qt.point(linearVelocity.x/10,linearVelocity.y/10)
        onLinearVelocityChanged: {
          console.debug("velocity " + linearVelocity.x + " | " + linearVelocity.y)
        }
      }
    }
  }
}

Cheers,
Alex