Make an SCNNode transparent so you see other nodes through it

How do I make the Cylinders transparent so you always see the green wall through them, regardless of the green's alpha value?

Problem description

(full code at the end)

I have an SCNNode() defined as two squares (forming two sides of a cube) with a material defined as

var meshMaterial: SCNMaterial = {
    let mat = SCNMaterial()
    mat.diffuse.contents = UIColor.green
    mat.lightingModel = .constant
    return mat
}()

I place two cylinders on each wall, where the cylinder's opacity is set to 0.3. You can see the green walls through the cylinders.

Issue

When I set the meshMaterial's alpha to anything other than 1.0

var meshMaterial: SCNMaterial = {
    let mat = SCNMaterial()
    mat.diffuse.contents = UIColor.green.withAlphaComponent(0.5)
    mat.lightingModel = .constant
    return mat
}()

You can no longer see the green walls through the cylinders. And instead you see the black background through the clyinders

At certain rotation angles you can see the green through the cylinder (the right-wall cylinder in this example)

Playground

Here is the full playground code to replicate the issue.

import UIKit
import SceneKit
import PlaygroundSupport
import SceneKit.ModelIO

// MARK: Set up Scene
let scene = SCNScene()

var sceneView: SCNView = {
    let sv = SCNView(frame: CGRect(x: 0, y: 0, width: 800, height: 800))
    sv.autoenablesDefaultLighting = false
    sv.allowsCameraControl = true
    sv.backgroundColor = .black
    sv.scene = scene
    return sv
}()

var cameraNode: SCNNode = {
    let n = SCNNode()
    n.camera = SCNCamera()
    n.camera?.contrast = 0.0
    n.camera?.wantsHDR = false
    n.position = SCNVector3(x: 0, y: 0, z: 10)
    //n.addChildNode(omniLightNode)
    return n
}()

var ambientLightNode: SCNNode = {
    let n = SCNNode()
    n.light = SCNLight()
    n.light!.type = SCNLight.LightType.ambient
    n.light!.intensity = 100
    return n
}()

private var omniLightNode: SCNNode = {
    let n = SCNNode()
    n.light = SCNLight()
    n.light!.type = SCNLight.LightType.directional
    n.light!.color = UIColor.white
    n.position = SCNVector3(x: 0, y: 10, z: 0)
    n.light!.intensity = 800
    return n
}()

scene.rootNode.addChildNode(omniLightNode)
scene.rootNode.addChildNode(ambientLightNode)
scene.rootNode.addChildNode(cameraNode)

PlaygroundPage.current.liveView = sceneView

// MARK: - Mesh
let vertices: [SCNVector3] = [
    // back square
    SCNVector3(-1, -1,  0),
    SCNVector3( 1, -1,  0),
    SCNVector3( 1,  1,  0),
    SCNVector3(-1,  1,  0),
    
    // right wall
    SCNVector3(1,  -1,  0),
    SCNVector3(1,  -1,  2),
    SCNVector3(1,   1,  2),
    SCNVector3(1,   1,  0)
]
let vertexSource = SCNGeometrySource(vertices: vertices)

let indices: [Int32] = [
    0,1,2,
    0,2,3,
    
    4,5,6,
    4,6,7
]
let indexElement = SCNGeometryElement(
    indices: indices,
    primitiveType: .triangles
)

let textures: [CGPoint] = [
    CGPoint(x: 0, y: 0),
    CGPoint(x: 0, y: 1),
    CGPoint(x: 1, y: 1),
    CGPoint(x: 1, y: 0),
    
    CGPoint(x: 0, y: 0),
    CGPoint(x: 0, y: 1),
    CGPoint(x: 1, y: 1),
    CGPoint(x: 1, y: 0)
]
let textureSource = SCNGeometrySource(textureCoordinates: textures)
let meshGeometry = SCNGeometry(
    sources: [vertexSource, textureSource],
    elements: [indexElement]
)


var meshMaterial: SCNMaterial = {
    let mat = SCNMaterial()
    mat.diffuse.contents = UIColor.green.withAlphaComponent(0.5) // <-- set alpha to 1.0 
    mat.lightingModel = .constant
    return mat
}()

meshGeometry.materials = [meshMaterial]
let meshNode = SCNNode(geometry: meshGeometry)
meshNode.position = SCNVector3(0, 0, 0)

// MARK: Cylinders

// Define a Node to hold the cylinders
let shapeNode = SCNNode()
shapeNode.position = SCNVector3(0, 0, 0)

let cylinderBack = SCNNode(
    geometry: SCNCylinder(
        radius: 0.1,
        height: 1.0
    )
)

let cylinderRight = SCNNode(
    geometry: SCNCylinder(
        radius: 0.1,
        height: 1.0
    )
)

let cylinderMaterial: SCNMaterial = {
    let mat = SCNMaterial()
    mat.lightingModel = .physicallyBased
    mat.diffuse.contents = UIColor.red
    mat.roughness.contents = 0.8
    mat.metalness.contents = 0.4
    return mat
}()

cylinderBack.geometry?.materials = [cylinderMaterial]
cylinderBack.position = SCNVector3(0, 0, 0.05)

cylinderRight.geometry?.materials = [cylinderMaterial]
cylinderRight.position = SCNVector3(1, 0, 1)

// Set Cylinders opacity
shapeNode.opacity = 0.3


sceneView.scene?.rootNode.addChildNode(meshNode)
sceneView.scene?.rootNode.addChildNode(shapeNode)

shapeNode.addChildNode(cylinderBack)
shapeNode.addChildNode(cylinderRight)

The issue is in the order of rendering which is very important to achieve correct transparency. There are lot of ways to solve this, one easy option is to design your geometry to not overlap each other which makes it hard for the ordering. Or set manual order of rendering using renderingOrder

There is a post that speaks about this topic in detail. https://stackoverflow.com/a/60840898/409315