SparkAR greyscale black and white shader

script.js

const D = require('Diagnostics');
const Scene = require('Scene');
const Time = require('Time');
const R = require('Reactive');
const S = require('Shaders');
const Mat = require('Materials');
const Tex = require('Textures');
const Patches = require('Patches');

//
// Get materials
//
const matBackground = Mat.get('background');

//
// Get textures
//
const texSigCamera = Tex.get('camera').signal;

//
// Get scene objects
//
const objBackground = Scene.root.find('background');

//
// Define constants
// 
// const screenScale = Patches.getScalarValue('screenScale');
// const screenSize = R.pack2(Patches.getPoint2DValue('screenSize').x, Patches.getPoint2DValue('screenSize').y);
// const screenSizeScaled = R.div(screenSize, screenScale);
const uv = S.vertexAttribute({'variableName': S.VertexAttribute.TEX_COORDS});

//
// Varies
//
// let FRAME = Time.ms.div(1000);

//
// B&W shader 
//
function make_bw(tex, texcoord) {
    const color = S.textureSampler(tex, texcoord);
    const gray = R.dot(R.pack3(color.x, color.y, color.z), R.pack3(0.299, 0.587, 0.114));

    return R.pack4(gray, gray, gray, color.w);
}

let texBW = make_bw(texSigCamera, uv);
matBackground.setTexture(texBW, {textureSlotName: 'diffuseTexture'});