How to make a transparent object that casts a shadow in SceneKit?

Question

I need to make a transparent object in SceneKit that casts a shadow. I tried going through all the texture settings, but I didn't get a positive result. If you completely hide the textures of the object, the shadow disappears.

I need to get a shadow without displaying an object and I don't understand how to do it.

Answer 1

Shadow catcher

To make an invisible object that casts a shadow in SceneKit, use .shadowOnly lighting model.

import SceneKit
import Cocoa

class ViewController: NSViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let sceneView = self.view as! SCNView
        sceneView.scene = SCNScene()
        sceneView.backgroundColor = .darkGray
        sceneView.allowsCameraControl = true
        
        let lightNode = SCNNode()
        lightNode.light = SCNLight()
        lightNode.light?.type = .directional
        lightNode.light?.castsShadow = true
        lightNode.eulerAngles.x = -.pi/4
        sceneView.scene?.rootNode.addChildNode(lightNode)
        
        let sphere = SCNNode(geometry: SCNSphere(radius: 2))
        sphere.geometry?.firstMaterial?.lightingModel = .shadowOnly    // 1
        sphere.geometry?.firstMaterial?.colorBufferWriteMask = []      // 2
        sphere.geometry?.firstMaterial?.transparencyMode = .rgbZero    // 3
        sceneView.scene?.rootNode.addChildNode(sphere)
        
        let plane = SCNNode(geometry: SCNPlane(width: 10, height: 10))
        plane.position.y = -2
        plane.eulerAngles.x = -.pi/2
        sceneView.scene?.rootNode.addChildNode(plane)
    }
}

If you need just a shadow catcher (i.e. transparent plane) add the following line to your code:

plane.geometry?.firstMaterial?.lightingModel = .shadowOnly

To control shadow's transparency use this property:

lightNode.light?.shadowColor = NSColor(white: 0, alpha: 0.5)