Ggtcf

So I'm using SceneKit with iOS 12 (Swift 4.2), and I want to add a twirl/bump distortion to the camera. I found something similar here (Fish Eye Wide-angle with a Scene Kit Camera: Possible?) that supposedly creates a barrel distortion. But when I tried adding it to my project the scene just turns black and I get an error in the console

2019-03-07 13:35:14.982232+0000 TestingSCN[551:66202] [framework] CUIThemeStore: No theme registered with id=0

2019-03-07 13:35:15.064859+0000 TestingSCN[551:66202] [framework] CUIThemeStore: No theme registered with id=0

2019-03-07 13:35:15.097517+0000 TestingSCN[551:66270] [framework] CUIThemeStore: No theme registered with id=0

2019-03-07 13:35:15.118445+0000 TestingSCN[551:66270] [SceneKit] Error: can not render without programs, using default

Now the method is basically using a JSON dictionary file to define a technique and GLSL vertex and fragment shader files. Then in my main swift file, I add that technique to the camera. This is the code I used:
barrel.json (located in art.scnassets)

 "passes" : 
 "barrel" : 
 "outputs" : 
 "color" : "COLOR"
 ,
 "inputs" : 
 "colorSampler" : "COLOR",
 "noiseSampler" : "noiseSymbol",
 "a_position" : "a_position-symbol"
 ,
 "program" : "art.scnassets/barrel",
 "draw" : "DRAW_QUAD"
 
 ,
 "sequence" : [
 "barrel"
 ],
 "symbols" : 
 "a_position-symbol" : 
 "semantic" : "vertex"
 ,
 "noiseSymbol" : 
 "image" : "noise.png",
 "type" : "sampler2D"
 ,
 "barrelPower" : 
 "type" : "float"
 
 

barrel.fsh

uniform sampler2D colorSampler;
const float PI = 3.1415926535;
uniform float barrelPower;
varying vec2 uv;
vec2 Distort(vec2 p)

 float theta = atan(p.y, p.x);
 float radius = length(p);
 radius = pow(radius, barrelPower);
 p.x = radius * cos(theta);
 p.y = radius * sin(theta);
 return 0.5 * (p + 1.0);

void main() 
 vec2 rg = 2.0 * uv.xy - 1.0;
 vec2 uv2;
 float d = length(xy);
 if (d < 1.0)
 uv2 = Distort(xy);
 else 
 uv2 = uv.xy;
 
 gl_FragColor = texture2D(colorSampler, uv2);

barrel.vsh

attribute vec4 a_position;
varying vec2 uv;

void main() 
 gl_Position = a_position;
 uv = a_position.xy;

GameViewController.swift (in viewDidLoad)

let url: URL = Bundle.main.url(forResource: "art.scnassets/barrel", withExtension: "json")!

do 
 let jsonData = try Data(contentsOf: url)
 let jsonObject = try JSONSerialization.jsonObject(with: jsonData, options:JSONSerialization.ReadingOptions(rawValue: 0))
 guard let dictionary = jsonObject as? Dictionary<String, Any> else 
 print("Not a Dictionary")
 return
 
 var technique: SCNTechnique? = nil
 technique = SCNTechnique(dictionary: dictionary)
 technique?.setValue(NSNumber(value: 0.5), forKey: "barrelPower")
 cameraNode.camera?.technique = technique

catch let error as NSError 
 print("Found an error - (error)")

I'm not really an expert in shaders, and I know that it's probably better to write an SCNProgram or something but I have not clue where to even begin with that.
Any help is appreciated :)

You may try

 let scnView = self.view as! SCNView
 scnView.technique = technique

as you are using a ("draw" : "DRAW_QUAD") which is rendering a texture of render objects.

Also there is a compiler error :

 float d = length(xy); // `xy` is not defined