How to use SCNTechnique to create a "masked" portal effect in SceneKit / ARKit?

Question

I'm trying to wrap my head around how to use SCNTechnique with Scenekit.

The effect I'm trying to create can easily be achieved in Unity by adding a custom shader to two materials, making objects with the "filtered" shader to only be visible when it is seen through the other shader (the portal). I've followed this tutorial to create this in Unity: https://www.youtube.com/watch?v=b9xUO0zHNXw

I'm trying to achieve the same effect in SceneKit, but I'm not sure how to apply the passes. I'm looking at SCNTechnique because it has options for stencilStates, but I'm not sure if this will work.

There are very little resources and tutorials on SCNTechnique

Answer 1

As mentioned in the comments above, I am not sure how to achieve an occlusion effect using Stencil Tests which as we know are fairly (and I use that term very loosely) easy to do in Unity.

Having said this, we can achieve a similar effect in Swift using transparency, and rendering order.

Rendering Order simply refers to:

The order the node’s content is drawn in relative to that of other nodes.

Whereby SCNNodes with a larger rendering order are rendered last and vice versa.

In order to make an object virtually invisible to the naked eye we would need to set the transparency value of an SCNMaterial to a value such as 0.0000001.

So how would we go about this?

In this very basic example which is simply a portal door, and two walls we can do something like this (which is easy enough to adapt into something more substantial and aesthetically pleasing):

The example is fully commented so it should be easy to understand what we are doing.

/// Generates A Portal Door And Walls Which Can Only Be Seen From Behind e.g. When Walking Through The Portsal
///
/// - Returns: SCNNode
func portalNode() -> SCNNode{

    //1. Create An SCNNode To Hold Our Portal
    let portal = SCNNode()

    //2. Create The Portal Door
    let doorFrame = SCNNode()

    //a. Create The Top Of The Door Frame
    let doorFrameTop = SCNNode()
    let doorFrameTopGeometry = SCNPlane(width: 0.55, height: 0.1)
    doorFrameTopGeometry.firstMaterial?.diffuse.contents = UIColor.black
    doorFrameTopGeometry.firstMaterial?.isDoubleSided = true
    doorFrameTop.geometry = doorFrameTopGeometry
    doorFrame.addChildNode(doorFrameTop)
    doorFrameTop.position = SCNVector3(0, 0.45, 0)

    //b. Create The Left Side Of The Door Frame
    let doorFrameLeft = SCNNode()
    let doorFrameLeftGeometry = SCNPlane(width: 0.1, height: 1)
    doorFrameLeftGeometry.firstMaterial?.diffuse.contents = UIColor.black
    doorFrameLeftGeometry.firstMaterial?.isDoubleSided = true
    doorFrameLeft.geometry = doorFrameLeftGeometry
    doorFrame.addChildNode(doorFrameLeft)
    doorFrameLeft.position = SCNVector3(-0.25, 0, 0)

    //c. Create The Right Side Of The Door Frame
    let doorFrameRight = SCNNode()
    let doorFrameRightGeometry = SCNPlane(width: 0.1, height: 1)
    doorFrameRightGeometry.firstMaterial?.diffuse.contents = UIColor.black
    doorFrameRightGeometry.firstMaterial?.isDoubleSided = true
    doorFrameRight.geometry = doorFrameRightGeometry
    doorFrame.addChildNode(doorFrameRight)
    doorFrameRight.position = SCNVector3(0.25, 0, 0)

    //d. Add The Portal Door To The Portal And Set Its Rendering Order To 200 Meaning It Will Be Rendered After Our Masks
    portal.addChildNode(doorFrame)
    doorFrame.renderingOrder = 200
    doorFrame.position = SCNVector3(0, 0, -1)

    //3. Create The Left Wall Enclosure To Hold Our Wall And The Occlusion Node
    let leftWallEnclosure = SCNNode()

    //a. Create The Left Wall And Set Its Rendering Order To 200 Meaning It Will Be Rendered After Our Masks
    let leftWallNode = SCNNode()
    let leftWallMainGeometry = SCNPlane(width: 0.5, height: 1)
    leftWallNode.geometry = leftWallMainGeometry
    leftWallMainGeometry.firstMaterial?.diffuse.contents = UIColor.red
    leftWallMainGeometry.firstMaterial?.isDoubleSided = true
    leftWallNode.renderingOrder = 200

    //b. Create The Left Wall Mask And Set Its Rendering Order To 10 Meaning It Will Be Rendered Before Our Walls
    let leftWallMaskNode = SCNNode()
    let leftWallMaskGeometry = SCNPlane(width: 0.5, height: 1)
    leftWallMaskNode.geometry = leftWallMaskGeometry
    leftWallMaskGeometry.firstMaterial?.diffuse.contents = UIColor.blue
    leftWallMaskGeometry.firstMaterial?.isDoubleSided = true
    leftWallMaskGeometry.firstMaterial?.transparency = 0.0000001
    leftWallMaskNode.renderingOrder = 10
    leftWallMaskNode.position = SCNVector3(0, 0, 0.001)

    //c. Add Our Wall And Mask To The Wall Enclosure
    leftWallEnclosure.addChildNode(leftWallMaskNode)
    leftWallEnclosure.addChildNode(leftWallNode)

    //4. Add The Left Wall Enclosure To Our Portal
    portal.addChildNode(leftWallEnclosure)
    leftWallEnclosure.position = SCNVector3(-0.55, 0, -1)

    //5. Create The Left Wall Enclosure
    let rightWallEnclosure = SCNNode()

    //a. Create The Right Wall And Set Its Rendering Order To 200 Meaning It Will Be Rendered After Our Masks
    let rightWallNode = SCNNode()
    let rightWallMainGeometry = SCNPlane(width: 0.5, height: 1)
    rightWallNode.geometry = rightWallMainGeometry
    rightWallMainGeometry.firstMaterial?.diffuse.contents = UIColor.red
    rightWallMainGeometry.firstMaterial?.isDoubleSided = true
    rightWallNode.renderingOrder = 200

    //b. Create The Right Wall Mask And Set Its Rendering Order To 10 Meaning It Will Be Rendered Before Our Walls
    let rightWallMaskNode = SCNNode()
    let rightWallMaskGeometry = SCNPlane(width: 0.5, height: 1)
    rightWallMaskNode.geometry = rightWallMaskGeometry
    rightWallMaskGeometry.firstMaterial?.diffuse.contents = UIColor.blue
    rightWallMaskGeometry.firstMaterial?.isDoubleSided = true
    rightWallMaskGeometry.firstMaterial?.transparency = 0.0000001
    rightWallMaskNode.renderingOrder = 10
    rightWallMaskNode.position = SCNVector3(0, 0, 0.001)

    //c. Add Our Wall And Mask To The Wall Enclosure
    rightWallEnclosure.addChildNode(rightWallMaskNode)
    rightWallEnclosure.addChildNode(rightWallNode)

    //6. Add The Left Wall Enclosure To Our Portal
    portal.addChildNode(rightWallEnclosure)
    rightWallEnclosure.position = SCNVector3(0.55, 0, -1)

    return portal
}

Which can be tested like so:

 let portal = portalNode()
 portal.position = SCNVector3(0, 0, -1.5)
 self.sceneView.scene.rootNode.addChildNode(portal)

Hope it points you in the right direction...

FYI there is a good tutorial here from Ray Wenderlich which will also be of use to you...