Aligning SCNText with SCNBox in iOS SceneKit with boundingBox?

Question

REVISED PROBLEM:

I don't understand why the white node is centered when it's a box or sphere but not when it's text. You can comment/uncomment the whiteGeometry variable to see how each different geometry is displayed. I was originally thinking that I had to manually center the text by determining the width of the box and the text and calculating the position of the text. Do I need to do that? Why is the text behaving differently?

import SceneKit
import PlaygroundSupport

let scene = SCNScene()
let sceneView = SCNView(frame: CGRect(x: 0, y: 0, width: 500, height: 500))
sceneView.scene = scene
sceneView.backgroundColor = .darkGray
sceneView.autoenablesDefaultLighting = true
sceneView.allowsCameraControl = true
PlaygroundPage.current.liveView = sceneView

// Camera
let cameraNode = SCNNode()
cameraNode.camera = SCNCamera()
scene.rootNode.addChildNode(cameraNode)
cameraNode.position = SCNVector3(x: 0, y: 0, z: 25)
sceneView.pointOfView = cameraNode

let blackGeometry = SCNBox(width: 10.0, height: 10.0, length: 10.0, chamferRadius: 0)
blackGeometry.firstMaterial?.diffuse.contents = UIColor.black
print("blackGeometry min=\(blackGeometry.boundingBox.min)")
print("blackGeometry max=\(blackGeometry.boundingBox.max)")
let blackNode = SCNNode(geometry: blackGeometry)
blackNode.position = SCNVector3(x: 0, y: 0, z: 0)
scene.rootNode.addChildNode(blackNode)

// let whiteGeometry = SCNBox(width: 3.0, height: 3.0, length: 3.0, chamferRadius: 0)
let whiteGeometry = SCNText(string: "L", extrusionDepth: 0)
whiteGeometry.alignmentMode =  kCAAlignmentLeft
whiteGeometry.font = UIFont.systemFont(ofSize: 8.0)
// let whiteGeometry = SCNSphere(radius: 3.0)
whiteGeometry.firstMaterial?.diffuse.contents = UIColor.white
print("whiteGeometry min=\(whiteGeometry.boundingBox.min)")
print("whiteGeometry max=\(whiteGeometry.boundingBox.max)")
let whiteNode = SCNNode(geometry: whiteGeometry)

let boxWidth = blackGeometry.boundingBox.max.x - blackGeometry.boundingBox.min.x
let boxHeight = blackGeometry.boundingBox.max.y - blackGeometry.boundingBox.min.y
print("boxWidth=\(boxWidth)")
print("boxHeight=\(boxHeight)")

let txtWidth = whiteGeometry.boundingBox.max.x - whiteGeometry.boundingBox.min.x
let txtHeight = whiteGeometry.boundingBox.max.y - whiteGeometry.boundingBox.min.y
print("txtWidth=\(txtWidth)")
print("txtHeight=\(txtHeight)")

whiteNode.position = SCNVector3(x: -5.0, y: -5.0, z: 10)
scene.rootNode.addChildNode(whiteNode)
//blackNode.addChildNode(whiteNode)

print("done")

ORIGINAL PROBLEM (OLD):

Let's say I have two SCNBox nodes (I'm simplifying this to make it clear BUT the solution must work for other geometries). A large black box and a small white box. I want to center the white box in front of the black box.

To do this, I need to determine the width and height of the two nodes. Remember that the node could be something other than a box like a sphere or text. From what I can tell, the only way to determine width/height is via the boundingBox property on the geometry. It has a min and max value that is NOT clearly and fully described in Apple's reference manual. To get the height, it seems like I would calculate it based on boundingBox.max.y and boundingBox.min.y. So looking at the example below of a 10x10x10 box, I can't see how I can get 10.0 as the height because max.y 5.20507812

e.g.

let box = SCNBox(width: 10.0, height: 10.0, length: 10.0, chamferRadius: 0)
print("box min=\(box.boundingBox.min)")
print("box max=\(box.boundingBox.max)")

yields:

box min=SCNVector3(x: -5.0, y: -5.0, z: -5.0)
box max=SCNVector3(x: 5.0, y: 5.20507812, z: 5.0)

Why is max.y=5.20507812? How should I determine the height/width?

See also: SCNBoundingVolume

Answer 1

I think I understand now.

It looks like you're trying to put some text in front of an object, with the text centered on the object relative to the camera. And this question isn't about bounding boxes per se. Right?

You see different behavior with the white geometry being a sphere or SCNText because SCNText's uses a different origin convention than the other concrete SCNGeometry classes. Other geometries put the origin at the center. SCNText puts it at the lower left of the text. If I remember correctly, "lower" means the bottom of the font (lowest descender), not the baseline.

So you'll need to compute the text's bounding box, and account for that when you position the text node, to get it centered. You don't need to do that for the other geometry types. If the covered object or the camera is moving, you'll need to compute the ray from the camera to the covered object in the render loop, and update the text's position.

Swift Scenekit - Centering SCNText - the getBoundingBoxMin:Max issue is relevant.

If you just want overlay text, you might find it easier to put the text in an SKScene overlay. An example of that is at https://github.com/halmueller/ImmersiveInterfaces/tree/master/Tracking%20Overlay (warning, bit rot may have set in, I haven't tried that code in a while).