text: import {
      shader, toFloatString, parseParamNumber
  } from '../util'
  
  /*
   * @param {Number} amount  -1..1  ,  value < 0  is darken, value > 0 is brighten 
   */
  export default function brightness (amount = 1) {
      const C = toFloatString( parseParamNumber(amount) );
  
      return shader(`
          outColor = pixelColor + (${C});
      `);
  }