code: public function checkImportantPart(Contao\DataContainer $dc)
  	{
  		if (!$dc->id)
  		{
  			return;
  		}
  
  		$rootDir = Contao\System::getContainer()->getParameter('kernel.project_dir');
  
  		if (is_dir($rootDir . '/' . $dc->id) || !\in_array(strtolower(substr($dc->id, strrpos($dc->id, '.') + 1)), Contao\StringUtil::trimsplit(',', strtolower(Contao\Config::get('validImageTypes')))))
  		{
  			$GLOBALS['TL_DCA'][$dc->table]['palettes'] = str_replace(',importantPartX,importantPartY,importantPartWidth,importantPartHeight', '', $GLOBALS['TL_DCA'][$dc->table]['palettes']);
  		}
  	}
docs: Only show the important part fields for images
  
  @param Contao\DataContainer $dc