Get coordinates of pivot point in ModelEntity

Hi,

as far as I know there is currently no way to retrieve the pivot point directly. What you can do though is to query the bounding box and then calculate a Y-Offset based on the min and max positions of the bounding boxes corners and set that as the position of the entity. Now when you wrap it into another empty parent entity, you should have the desired effect.

public extension Entity {

    enum PivotPosition {
        case top
        case center
        case bottom
    }

    func wrapEntityAndSetPivotPosition(to targetPosition: PivotPosition) -> Entity {
        setPivotPosition(to: targetPosition, animated: false)

        let entity = Entity()
        entity.addChild(self)
        return entity
    }

    func setPivotPosition(to targetPosition: PivotPosition, animated: Bool = false) {
        let boundingBox = visualBounds(relativeTo: nil)
        let min = boundingBox.min
        let max = boundingBox.max

        let yTranslation: Float

        switch targetPosition {
        case .top:
            yTranslation = -max.y
        case .center:
            yTranslation = -(min.y + (max.y - min.y) / 2)
        case .bottom:
            yTranslation = -min.y
        }

        let targetPosition = simd_float3(
            x: boundingBox.center.x * -1,
            y: yTranslation,
            z: boundingBox.center.z * -1
        )

        guard animated else {
            position = targetPosition
            return
        }

        guard isAnchored, parent != nil else {
            print("Warning: to set the Entities pivot position animated make sure it is already anchored and has a parent set.")
            return
        }

        var translationTransform = transform
        translationTransform.translation = targetPosition
        move(to: translationTransform, relativeTo: parent, duration: 0.3, timingFunction: .easeOut)
    }
}

And the whole thing in action:

        let boxModel = ModelEntity(mesh: .generateBox(size: 0.3))
        let wrappedBoxEntity = boxModel.wrapEntityAndSetPivotPosition(to: .bottom)
        let boxAnchor = AnchorEntity(plane: .horizontal)
        boxAnchor.addChild(wrappedBoxEntity)

        arView.scene.anchors.append(boxAnchor)

I agree though that there should be a dedicated API for this – as we have in SceneKit. I filed feedback a while ago.