function not found

[laughmaker]

hi, i run the code below:

$qrCode = new QrCode();
$qrCode -> setText('hzd')
-> setSize(300)
-> setPadding(10)
-> setErrorCorrection('high')
-> setForegroundColor(array('r'=>0, 'g'=>0, 'b'=>0, 'a'=>0))
-> setBackgroundColor(array('r'=>255, 'g'=>255, 'b'=>255, 'a'=>0))
-> render();

i got a error:
PHP Fatal error: Call to undefined function Endroid\QrCode\imagecreatefrompng() in /usr/local/var/www/QrCode/src/Endroid/QrCode/QrCode.php on line 1101

thanks.