Swift / iOS silhouette shader modifier

OutlineModifier.fragment

// Fragment Shader Modifier

// Implicit struct to use:
/*
 struct SCNShaderOutput {
    vec4 color;
 } _output;
 */

varying vec3 varyingNormal;
uniform vec3 uniformFragToCamVector;

#pragma transparent
#pragma body

// Testing out outline code from here: https://stackoverflow.com/a/37260904/1751073

float cameraFacingPercentage = dot(normalize(vec4(-uniformFragToCamVector, 1.0)), (vec4(varyingNormal, 1.0) * u_inverseModelTransform));
vec4 outerColor = vec4(1.0, 0.0, 0.0, 1.0);
vec4 innerColor = vec4(0.0, 0.0, 0.0, 0.0);
_output.color = innerColor * cameraFacingPercentage + outerColor * (1.0 - cameraFacingPercentage);

OutlineModifier.vertex

// Vertex Shader Modifier

// Implicit struct to use:
 
// struct SCNShaderGeometry {
//     vec3 position;
//     vec3 normal;
//     vec4 tangent;
//     vec2 texcoords[kSCNTexcoordCount];
// } _geometry;
//
// // Implicit uniforms:
//    u_time float - The current system time (in seconds) since SceneKit started rendering with the shader.
//    u_boundingBox mat32 - The bounding box of the geometry being rendered, in model space.
//    u_modelTransform mat4 - The transform matrices used for converting vertex positions and normals between model, world, view, and clip coordinate spaces.
//    u_inverseModelTransform mat4 - The inverse matrices corresponding to each transform.
//    u_diffuseTexture sampler2D or samplerCube - The texture contents of the corresponding material property. Declared only if the material property’s contents object provides a texture image.

varying vec3 varyingNormal;

#pragma transparent
#pragma body
varyingNormal = _geometry.normal;

ViewController.swift

    private func handleShader(turn on: Bool) {
        if on {
            // attach shader snippets from text files
            do {
                let shaderFileName = "OutlineModifier"
                model.geometry?.shaderModifiers = [
                    // geometry
                    SCNShaderModifierEntryPoint.geometry: try String(contentsOfFile: Bundle.main.path(forResource: shaderFileName, ofType: "vertex")!, encoding: String.Encoding.utf8),
                    // fragment
                    SCNShaderModifierEntryPoint.fragment: try String(contentsOfFile: Bundle.main.path(forResource: shaderFileName, ofType: "fragment")!, encoding: String.Encoding.utf8),
                ]
                
                let sub = model.worldPosition - currentCamera!.worldPosition
                let nsVal = NSValue(scnVector3: sub)
                model.geometry?.setValue(nsVal, forKey: "uniformFragToCamVector")
            } catch {
                assert(false, "could not turn shaders into strings for .shaderModifiers dictionary...")
                return
            }
        } else {
            model.geometry?.shaderModifiers = nil
        }
    }