开源17.7无法显示登录验证码

php日志如下：

[15-Dec-2022 12:31:29 Asia/Shanghai] PHP Fatal error:  Uncaught Error: Call to undefined function imagettfbbox() in /opt/zbox/app/zentao/lib/captcha/captcha.class.php:340

Stack trace:
#0 /opt/zbox/app/zentao/lib/captcha/captcha.class.php(454): captcha->writePhrase(Resource id #71, 'ptcUy', '/opt/zbox/app/z...', 150, 40)
#1 /opt/zbox/app/zentao/module/misc/control.php(239): captcha->build()
#2 /opt/zbox/app/zentao/framework/base/router.class.php(2174): misc->captcha('captcha', '')
#3 /opt/zbox/app/zentao/www/index.php(74): baseRouter->loadModule()
#4 {main}