core-bundle/src/Resources/contao/dca/tl_files.php
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']);
		}
	}
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']);
		}
	}
Only show the important part fields for images

@param Contao\DataContainer $dc