CODEWITHSUNDEEP
iosswiftscenekit
JP Silvashy
JP Silvashy

Reputation: 48495

SceneKit matrix transformation to match camera angle

I'm building a UIPanGestureRecognizer so I can move nodes in 3D space.

Currently, I have something that works, but only when the camera is exactly perpendicular to the plane, my UIPanGestureRecognizer looks like this:

@objc func handlePan(_ sender:UIPanGestureRecognizer) {
  let projectedOrigin = self.sceneView!.projectPoint(SCNVector3Zero)

  let viewCenter = CGPoint(
    x: self.view!.bounds.midX,
    y: self.view!.bounds.midY
  )

  let touchlocation = sender.translation(in: self.view!)

  let moveLoc = CGPoint(
    x: CGFloat(touchlocation.x + viewCenter.x),
    y: CGFloat(touchlocation.y + viewCenter.y)
  )

  let touchVector = SCNVector3(x: Float(moveLoc.x), y: Float(moveLoc.y), z: Float(projectedOrigin.z))
  let worldPoint = self.sceneView!.unprojectPoint(touchVector)
  let loc = SCNVector3( x: worldPoint.x, y: 0, z: worldPoint.z )

  worldHandle?.position = loc
}

The problem happens when the camera is rotated, and the coordinates are effected by the perspective change. Here is you can see the touch position drifting:

enter image description here

Related SO post for which I used to get to this position: How to use iOS (Swift) SceneKit SCNSceneRenderer unprojectPoint properly

It referenced these great slides: http://www.terathon.com/gdc07_lengyel.pdf

Upvotes: 0

Views: 1404

Answers (1)

Xartec
Xartec

Reputation: 2415

The tricky part of going from 2D touch position to 3D space is obviously the z-coordinate. Instead of trying to convert the touch position to an imaginary 3D space, map the 2D touch to a 2D plane in that 3D space using a hittest. Especially when movement is required only in two direction, for example like chess pieces on a board, this approach works very well. Regardless of the orientation of the plane and the camera settings (as long as the camera doesn't look at the plane from the side obviously) this will map the touch position to a 3D position directly under the finger of the touch and follow consistently.

I modified the Game template from Xcode with an example. https://github.com/Xartec/PrecisePan/

enter image description here

The main parts are:

  1. the pan gesture code:

    // retrieve the SCNView
    let scnView = self.view as! SCNView
    // check what nodes are tapped
    let p = gestureRecognize.location(in: scnView)
    let hitResults = scnView.hitTest(p, options: [SCNHitTestOption.searchMode: 1, SCNHitTestOption.ignoreHiddenNodes: false])

    if hitResults.count > 0 {
        // check if the XZPlane is in the hitresults
        for result in hitResults {
            if result.node.name == "XZPlane" {
                //NSLog("Local Coordinates on XZPlane %f, %f, %f", result.localCoordinates.x, result.localCoordinates.y, result.localCoordinates.z)

                //NSLog("World Coordinates on XZPlane %f, %f, %f", result.worldCoordinates.x, result.worldCoordinates.y, result.worldCoordinates.z)
                ship.position = result.worldCoordinates
                ship.position.y += 1.5
                return;
            }
        }
    }

  2. The addition of a XZ plane node in viewDidload:

    let XZPlaneGeo = SCNPlane(width: 100, height: 100)
let XZPlaneNode = SCNNode(geometry: XZPlaneGeo)
XZPlaneNode.geometry?.firstMaterial?.diffuse.contents = UIImage(named: "grid")
XZPlaneNode.name = "XZPlane"
XZPlaneNode.rotation = SCNVector4(-1, 0, 0, Float.pi / 2)
//XZPlaneNode.isHidden = true
scene.rootNode.addChildNode(XZPlaneNode)

Uncomment the isHidden line to hide the helper plane and it will still work. The plane obviously needs to be large enough to fill the screen or at least the portion where the user is allowed to pan.

By setting a global var to hold a startWorldPosition of the pan (in state .began) and comparing it to the hit worldPosition in the state .change you can determine the delta/translation in world space and translate other objects accordingly.

Upvotes: 3

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