Redimensionar PNG com fundo transparente

Bom dia Galera,

Estou com uma pequena dúvida. Estou usando uma classe para fazer redimensionamento de imagens. Ela está funcionando perfeitamente, o meu problema é que quanto redimensiono uma imagem .png, a nova imagem fica com fundo PRETO. Alguém sabe dizer como faço para ficar com fundo transparente?

Segue a classe:

<?php

 # ========================================================================#
 #
 # Author: Jarrod Oberto
 # Version: 1.0
 # Date: 17-Jan-10
 # Purpose: Resizes and saves image
 # Requires : Requires PHP5, GD library.
 # Usage Example:
 # include("classes/resize_class.php");
 # $resizeObj = new resize('images/cars/large/input.jpg');
 # $resizeObj -> resizeImage(150, 100, 0);
 # $resizeObj -> saveImage('images/cars/large/output.jpg', 100);
 #
 #
 # ========================================================================#

Class resize
{
// *** Class variables

private $image;
 private $width;
 private $height;
private $imageResized;

function __construct($fileName)

{
// *** Open up the file

$this->image = $this->openImage($fileName);

 // *** Get width and height
 $this->width = imagesx($this->image);
 $this->height = imagesy($this->image);
}

--------------------------------------------------------

private function openImage($file)
{
// *** Get extension
$extension = strtolower(strrchr($file, '.'));

switch($extension)
{
case '.jpg':
case '.jpeg':

$img = @imagecreatefromjpeg($file);
break;

case '.gif':

$img = @imagecreatefromgif($file);
break;

case '.png':

$img = @imagecreatefrompng($file);
break;

default:

$img = false;
break;
}
return $img;
}

--------------------------------------------------------

public function resizeImage($newWidth, $newHeight, $option="auto")
{
// *** Get optimal width and height - based on $option

$optionArray = $this->getDimensions($newWidth, $newHeight, $option);

$optimalWidth = $optionArray['optimalWidth'];
$optimalHeight = $optionArray['optimalHeight'];

// *** Resample - create image canvas of x, y size

$this->imageResized = imagecreatetruecolor($optimalWidth, $optimalHeight);
imagecopyresampled($this->imageResized, $this->image, 0, 0, 0, 0, $optimalWidth, $optimalHeight, $this->width, $this->height);

// *** if option is 'crop', then crop too

if ($option == 'crop') {
$this->crop($optimalWidth, $optimalHeight, $newWidth, $newHeight);
}
}

--------------------------------------------------------

private function getDimensions($newWidth, $newHeight, $option)
{

switch ($option)
{
case 'exact':

$optimalWidth = $newWidth;
$optimalHeight= $newHeight;
break;

case 'portrait':

$optimalWidth = $this->getSizeByFixedHeight($newHeight);
$optimalHeight= $newHeight;
break;

case 'landscape':

$optimalWidth = $newWidth;
$optimalHeight= $this->getSizeByFixedWidth($newWidth);
break;

case 'auto':

$optionArray = $this->getSizeByAuto($newWidth, $newHeight);
$optimalWidth = $optionArray['optimalWidth'];
$optimalHeight = $optionArray['optimalHeight'];
break;

case 'crop':

$optionArray = $this->getOptimalCrop($newWidth, $newHeight);
$optimalWidth = $optionArray['optimalWidth'];
$optimalHeight = $optionArray['optimalHeight'];
break;
}
return array('optimalWidth' => $optimalWidth, 'optimalHeight' => $optimalHeight);
}

--------------------------------------------------------

private function getSizeByFixedHeight($newHeight)
{

$ratio = $this->width / $this->height;
$newWidth = $newHeight * $ratio;
return $newWidth;
}

private function getSizeByFixedWidth($newWidth)

{

$ratio = $this->height / $this->width;
$newHeight = $newWidth * $ratio;
return $newHeight;
}

private function getSizeByAuto($newWidth, $newHeight)

{
if ($this->height < $this->width)
// *** Image to be resized is wider (landscape)
{

$optimalWidth = $newWidth;
$optimalHeight= $this->getSizeByFixedWidth($newWidth);
}
elseif ($this->height > $this->width)

// *** Image to be resized is taller (portrait)
{

$optimalWidth = $this->getSizeByFixedHeight($newHeight);
$optimalHeight= $newHeight;
}

else
// *** Image to be resizerd is a square
{

if ($newHeight < $newWidth) {
$optimalWidth = $newWidth;
$optimalHeight= $this->getSizeByFixedWidth($newWidth);
} else if ($newHeight > $newWidth) {
$optimalWidth = $this->getSizeByFixedHeight($newHeight);
$optimalHeight= $newHeight;

} else {
// *** Sqaure being resized to a square

$optimalWidth = $newWidth;
$optimalHeight= $newHeight;
}
}

return array('optimalWidth' => $optimalWidth, 'optimalHeight' => $optimalHeight);
}

--------------------------------------------------------

private function getOptimalCrop($newWidth, $newHeight)
{

$heightRatio = $this->height / $newHeight;
$widthRatio = $this->width / $newWidth;

if ($heightRatio < $widthRatio) {
$optimalRatio = $heightRatio;

} else {

$optimalRatio = $widthRatio;
}

$optimalHeight = $this->height / $optimalRatio;
$optimalWidth = $this->width / $optimalRatio;

return array('optimalWidth' => $optimalWidth, 'optimalHeight' => $optimalHeight);
}

--------------------------------------------------------

private function crop($optimalWidth, $optimalHeight, $newWidth, $newHeight)
{
// *** Find center - this will be used for the crop

$cropStartX = ( $optimalWidth / 2) - ( $newWidth /2 );
$cropStartY = ( $optimalHeight/ 2) - ( $newHeight/2 );

$crop = $this->imageResized;
//imagedestroy($this->imageResized);

// *** Now crop from center to exact requested size

$this->imageResized = imagecreatetruecolor($newWidth , $newHeight);
imagecopyresampled($this->imageResized, $crop, 0, 0, $cropStartX, $cropStartY, $newWidth, $newHeight , $newWidth, $newHeight);
}

--------------------------------------------------------

public function saveImage($savePath, $imageQuality="100")
{
// *** Get extension

 $extension = strrchr($savePath, '.');
 $extension = strtolower($extension);

switch($extension)
{
case '.jpg':
case '.jpeg':
if (imagetypes() & IMG_JPG) {

imagejpeg($this->imageResized, $savePath, $imageQuality);
}
break;

case '.gif':
if (imagetypes() & IMG_GIF) {

imagegif($this->imageResized, $savePath);
}
break;

case '.png':
// *** Scale quality from 0-100 to 0-9
$scaleQuality = round(($imageQuality/100) * 9);

// *** Invert quality setting as 0 is best, not 9
$invertScaleQuality = 9 - $scaleQuality;

if (imagetypes() & IMG_PNG) {

imagepng($this->imageResized, $savePath, $invertScaleQuality);
}
break;

// ... etc

default:
// *** No extension - No save.

break;
}

imagedestroy($this->imageResized);
}

--------------------------------------------------------

}
?>