Pimcore\Model\Asset\Image\Thumbnail::generate PHP Method

    public function generate($deferredAllowed = true)
    {
        $errorImage = PIMCORE_PATH . '/static6/img/filetype-not-supported.png';
        $deferred = false;
        $generated = false;
        if (!$this->asset) {
            $this->filesystemPath = $errorImage;
        } elseif (!$this->filesystemPath) {
            // if no correct thumbnail config is given use the original image as thumbnail
            if (!$this->config) {
                $this->filesystemPath = $this->asset->getRealFullPath();
            } else {
                try {
                    $deferred = $deferredAllowed && $this->deferred ? true : false;
                    $this->filesystemPath = Thumbnail\Processor::process($this->asset, $this->config, null, $deferred, true, $generated);
                } catch (\Exception $e) {
                    $this->filesystemPath = $errorImage;
                    Logger::error("Couldn't create thumbnail of image " . $this->asset->getRealFullPath());
                    Logger::error($e);
                }
            }
        }
        \Pimcore::getEventManager()->trigger("asset.image.thumbnail", $this, ["deferred" => $deferred, "generated" => $generated]);
    }