Transparent text with imagemagick

gistfile1.php

<?php
$position = 'waist';
$size = 'normal';
$letters = 'SRT';
$color = "000000";
$font = 'arial';

$positions = array(
	'waist'=>array(358,440,-6)
);
$sizes	= array(
	'normal'=>array(415,503),
	'zoom'=>array(415,503),	
);

$image	= new Imagick();
$draw	= new ImagickDraw();
$pixel	= new ImagickPixel("rgba(255,255,255,0)");
$image->newImage($sizes[$size][0], $sizes[$size][1], $pixel);
$draw->setFillColor($color);
$draw->setFont($font.'.ttf');
$draw->setFontSize(8);
$image->annotateImage($draw, $positions[$position][0], $positions[$position][1], $positions[$position][2], $letters);
$image->setImageFormat('png');
header('Content-type: image/png');
echo $image;