FluidTYPO3\Vhs\Utility\FrontendSimulationUtility::resetFrontendEnvironment PHP Method

    public static function resetFrontendEnvironment($tsfeBackup)
    {
        if ('BE' !== TYPO3_MODE) {
            return;
        }
        $GLOBALS['TSFE'] = $tsfeBackup;
    }

 /**
  * used to attach srcset variants of a given image to the specified tag
  *
  * @param \TYPO3\CMS\Fluid\Core\ViewHelper\TagBuilder $tag the tag to add the srcset as argument
  * @param string $src image path to render srcsets for
  * @return array
  */
 public function addSourceSet($tag, $src)
 {
     $srcsets = $this->getSourceSetWidths();
     if ('BE' === TYPO3_MODE) {
         FrontendSimulationUtility::simulateFrontendEnvironment();
     }
     $format = $this->arguments['format'];
     $quality = $this->arguments['quality'];
     $treatIdAsReference = (bool) $this->arguments['treatIdAsReference'];
     if (true === $treatIdAsReference) {
         $src = $this->arguments['src'];
     }
     $imageSources = [];
     $srcsetVariants = [];
     foreach ($srcsets as $key => $width) {
         $srcsetVariant = $this->getImgResource($src, $width, $format, $quality, $treatIdAsReference);
         $srcsetVariantSrc = rawurldecode($srcsetVariant[3]);
         $srcsetVariantSrc = $this->preprocessSourceUri(GeneralUtility::rawUrlEncodeFP($srcsetVariantSrc));
         $imageSources[$srcsetVariant[0]] = ['src' => $srcsetVariantSrc, 'width' => $srcsetVariant[0], 'height' => $srcsetVariant[1]];
         $srcsetVariants[$srcsetVariant[0]] = $srcsetVariantSrc . ' ' . $srcsetVariant[0] . 'w';
     }
     $tag->addAttribute('srcset', implode(',', $srcsetVariants));
     if ('BE' === TYPO3_MODE) {
         FrontendSimulationUtility::resetFrontendEnvironment();
     }
     return $imageSources;
 }