为什么GD库显示黑页

Question

我尝试制作水印，但GD库显示了黑色背景

function addWatermark($image)
{
      $image = imagecreatefrompng($image);  
      $textcolor = imagecolorallocate($image, 255, 255,255);  
      imagettftext($image, 14, 0, imagesx($image)-125, imagesy($image)-20, $textcolor, 'arial.ttf', "Matin");  
      header("Content-type: image/png");  
      imagepng($image);  
      imagedestroy($image); 
}

<img src="<?php echo addWatermark("download.png") ?>" width="300" title="mm" alt="">