[Rob]

Hi,

I’m creating a Car like this

EntityBase {
    id: car
    entityType: "car"

    MultiResolutionImage{
        id: carpic
        source: pickCar(1,7) //this function just picks a diffrent sprite
        anchors.fill: parent
    }

    DropShadow{
        anchors.fill: carpic
        horizontalOffset: 5
        verticalOffset: 10
        radius: 3
        color: "#aa000000"
        source: carpic
    }

    MovementAnimation {
      id: animationrightleft
      target: parent
      property: "x"
      velocity: generateRandomValueBetween(-50,-100)
      minPropertyValue: 0 
      onLimitReached: {
          car.removeEntity()
      }
    }

    BoxCollider{
        anchors.centerIn: carpic
        categories: Box.Category1
        collidesWith: Box.Category2 // Cars can't collide with eachother let only collide with ball
    }
}

and im creating them dynamically in my Scene. Problem is the Boxcolliders just appear on top of the screen. They are moving with the cars but on the wrong position. How can i center them correctly on the cars?

 

Thanks in advance

Rob

[GüntherFelgo Team]

Hi Rob!

There’s a small difference between the BoxCollider and the Car entity item:

• The actual physics collider that you see at the wrong position registers with and is part of the PhysicsWorld in your project (positioned relative to the PhysicsWorld)
• The QML entity item that is linked with the collider is treated as a normal QML item (positioned relative to the parent QML item)

This means, that the position of the Car is also affected by the position of all parent items in your QML hierarchy, while the collider is not. If put in other works, you can say they are in different coordinate systems. (PhysicsWorld v Entity Parent Item)

To avoid this problem you can

• Remove the additional position offset of the entity (e.g. by avoiding position modifications in parent items of the entity or by placing the entity directly into your Scene besides the PhysicsWorld)
• Take the relative position of the Car’s parents into account for positioning the collider

You can also have a look at Item::mapFromItem and Item::mapToItem position mapping functions to calculate position of items in different coordinate systems.

Hope this helps!

Best,
Günther

 

[Rob]

Thanks with this insight I could solve the problem

 

Best

Rob

[Barb]

Can you please give an example how to map coordinates from PhysicsWorld to parent-relative coordinates with the suggested functions?
In my case I have a BoxCollider (Body.Dynamic) that shall move to the position of a moving Image collectorPlayer. It works for my default resolution but as soon as the scene is scaled to a different screen ratio there is a misplacement of the collider by exactly the sum of the borders added to the scene to fit the game window.

 

PhysicsWorld does not have gravity or anything else set, setting its x, y, width, height to those of the scene did not work as adaption of the coordinates system.

I need to calculate Image positions independent from PhysicsWorld, so I see the only solution in replacement of the BoxCollider PhysicsWorld coordinates to parent-relative coordinates like x: collectorPlayer.x, width: collectorPlayer.width etc. instead of using anchors, this was my attempt 3 of the attached snippets. Any solution adapting one of my 3 attempts would be great. Thanks.

 

//original attempt
        BoxCollider {
          categories: Box.Category2
          collidesWith: Box.Category1
          anchors.fill: collectorPlayer
          bodyType: Body.Dynamic
          collisionTestingOnlyMode: true
          fixture.onBeginContact: { 
              //...
          }
        }

//second attempt 
BoxCollider {
          id: playerCollider
          categories: Box.Category2
          //may collide with the fruit but not with the ground
          collidesWith: Box.Category1
          x: collectorPlayer.x
          y: collectorPlayer.y + (gameWindow.height-scene.height)
          width: collectorPlayer.width
          height: collectorPlayer.height
          bodyType: Body.Dynamic
          collisionTestingOnlyMode: true
....

 

//3rd attempt - probably the best idea
 BoxCollider {
          id: playerCollider
          categories: Box.Category2
          //may collide with the fruit but not with the ground
          collidesWith: Box.Category1
          x: collectorPlayer.x
          y: playerCollider.mapToItem(scene, collectorPlayer.x, collectorPlayer.y).y
          width: collectorPlayer.width
          height: collectorPlayer.height
          bodyType: Body.Dynamic
          collisionTestingOnlyMode: true
....

 

[GüntherFelgo Team]

Hi Barb!

I guess you have the PhysicsWorld placed within the GameWindow, and the Entity with your BoxCollider placed within your Scene?
This would explain the collider position mismatch when the GameWindow size changes.

Is there a reason why you do not add the PhysicsWorld as well as the Entity to your Scene, so you won’t have to worry about the position offset?
That would be the easiest solution and my first suggestion.

Best,
Günther

 

[Barb]

thank you for the reply. I also tried moving the PhysicsWorld to within the scene as it also made sense to me according to your reply from February. didn’t change anything though. rescaling works for resolution changes as long as screen ratio is the same as for the original scene. should I set anything within PhysicsWorld like anchors?

 

[GüntherFelgo Team]

If both the PhysicsWorld and the Entity are within your Scene, the entity position and collider position should match up.

You can have a look at the FlappyBird demo for example:

• Open GameScene.qml
• Set debugDrawVisible: true for PhysicsWorld { } to see the colliders
• Run the game and resize the window, the bird collider always matches the position of the player bird entity

If you compare the demo source code with your Scene and Entity implementation, do you see any difference that might cause your position mismatch?

Best,
Günther from Felgo