Scene Kit - Dragging an Object - macos

Question

I’m trying to drag a chess piece with the mouse/trackpad using Scene Kit. All objects (board and pieces) are children of the root node, loaded from a Collada file.
I found a helpful description of the process elsewhere on Stack Overflow. Using that description I wrote the initial version of the code below. My problem is the disparity between the click coordinates and the piece node position — their coordinates are different orders of magnitude. I remain unclear on how to match them up — put them in the “same universe”. I’ve tried a number of suggestions from the Apple forums along with flights of fancy of my own.
Here’s my current attempt, mostly reverted back to the original version based on the link above, along with logged coordinate values along the way. The result is that dragging a chess piece causes it to abruptly jump off screen:
- (NSPoint)
viewPointForEvent: (NSEvent *) event_
{
NSPoint windowPoint = [event_ locationInWindow];
NSPoint viewPoint = [self.view convertPoint: windowPoint
fromView: nil];
return viewPoint;
}
- (SCNHitTestResult *)
hitTestResultForEvent: (NSEvent *) event_
{
NSPoint viewPoint = [self viewPointForEvent: event_];
CGPoint cgPoint = CGPointMake (viewPoint.x, viewPoint.y);
NSArray * points = [(SCNView *) self.view hitTest: cgPoint
options: #{}];
return points.firstObject;
}
- (void)
mouseDown: (NSEvent *) theEvent
{
SCNHitTestResult * result = [self hitTestResultForEvent: theEvent];
SCNVector3 clickWorldCoordinates = result.worldCoordinates;
log output: clickWorldCoordinates x 208.124578, y -12827.223365, z 3163.659073
SCNVector3 screenCoordinates = [(SCNView *) self.view projectPoint: clickWorldCoordinates];
log output: screenCoordinates x 245.128906, y 149.335938, z 0.985565
// save the z coordinate for use in mouseDragged
mouseDownClickOnObjectZCoordinate = screenCoordinates.z;
selectedPiece = result.node; // save selected piece for use in mouseDragged
SCNVector3 piecePosition = selectedPiece.position;
log output: piecePosition x -18.200000, y 6.483060, z 2.350000
offsetOfMouseClickFromPiece.x = clickWorldCoordinates.x - piecePosition.x;
offsetOfMouseClickFromPiece.y = clickWorldCoordinates.y - piecePosition.y;
offsetOfMouseClickFromPiece.z = clickWorldCoordinates.z - piecePosition.z;
log output: offsetOfMouseClickFromPiece x 226.324578, y -12833.706425, z 3161.309073
}
- (void)
mouseDragged: (NSEvent *) theEvent;
{
NSPoint viewClickPoint = [self viewPointForEvent: theEvent];
SCNVector3 clickCoordinates;
clickCoordinates.x = viewClickPoint.x;
clickCoordinates.y = viewClickPoint.y;
clickCoordinates.z = mouseDownClickOnObjectZCoordinate;
log output: clickCoordinates x 246.128906, y 0.000000, z 0.985565
log output: pieceWorldTransform =
m11 = 242.15889219510001, m12 = -0.000045609300002524833, m13 = -0.00000721691076126, m14 = 0,
m21 = 0.0000072168760805499971, m22 = -0.000039452697396149999, m23 = 242.15890446329999, m24 = 0,
m31 = -0.000045609300002524833, m32 = -242.15889219510001, m33 = -0.000039452676995750002, m34 = 0,
m41 = -4268.2349924762348, m42 = -12724.050221935429, m43 = 4852.6652710104272, m44 = 1)
SCNVector3 newPiecePosition;
newPiecePosition.x = offsetOfMouseClickFromPiece.x + clickCoordinates.x;
newPiecePosition.y = offsetOfMouseClickFromPiece.y + clickCoordinates.y;
newPiecePosition.z = offsetOfMouseClickFromPiece.z + clickCoordinates.z;
log output: newPiecePosition x 472.453484, y -12833.706425, z 3162.294639
selectedPiece.position = newPiecePosition;
}
Up to this point, I’ve gotten a lot of interesting and useful comments and advice. But I’ve realized that to move forward, I’m probably going to need a working code sample which shows the secret sauce which allows clicks and vectors to exist in the “same universe”.

Answer 1

You don't need to "play around" to find the origin. You can do code like this:
let originZ = sceneView.projectPoint(SCNVector3Zero).z
let viewLocation = /* Location in SCNView coordinate system */
let viewLocationWithOriginZ = SCNVector3(
x: Float(viewLocation.x), // X & Y are in view coordinate system
y: Float(viewLocation.y),
z: originZ // Z is in scene coordinate system
)
var position = sceneView.unprojectPoint(viewLocationWithOriginZ)
/* "position" is in the scene's coordinate system with z of 0 */

Answer 2

You have to use the method unprojectPoint. The key is to play around with z-values to find the correct depth of your scene relative to your click point (at least that's how I understood it). I had a similar problem trying to do touch-dragging on iOS. This is what I did to solve it:
let convertedTranslation = scnView.unprojectPoint(SCNVector3Make(Float(translation.x), Float(translation.y), 1.0))
Notice that the z-value for the method is 1. From the documentation, you can see that it can be a value between -1 and 1. -1 and 0 did not work for me, but 1 did.