Retaining Transparency with PHP Image Resizing

Jul
05

Following a huge amount of emails asking how to retain transparency when resizing GIF and PNG images we’ve finally had a chance to update our SimpleImage class with this very feature.

The PHP functions in the GD library such as imagecopyresampled() by default will replace transparency within an image with a black background. The new code involves a modification to our resize function as per the below:

function resize($width,$height) {
	$new_image = imagecreatetruecolor($width, $height);
	if( $this->image_type == IMAGETYPE_GIF || $this->image_type == IMAGETYPE_PNG ) {
		$current_transparent = imagecolortransparent($this->image);
		if($current_transparent != -1) {
			$transparent_color = imagecolorsforindex($this->image, $current_transparent);
			$current_transparent = imagecolorallocate($new_image, $transparent_color['red'], $transparent_color['green'], $transparent_color['blue']);
			imagefill($new_image, 0, 0, $current_transparent);
			imagecolortransparent($new_image, $current_transparent);
		} elseif( $this->image_type == IMAGETYPE_PNG) {
			imagealphablending($new_image, false);
			$color = imagecolorallocatealpha($new_image, 0, 0, 0, 127);
			imagefill($new_image, 0, 0, $color);
			imagesavealpha($new_image, true);
		}
	}
	imagecopyresampled($new_image, $this->image, 0, 0, 0, 0, $width, $height, $this->getWidth(), $this->getHeight());
	$this->image = $new_image;	
}

This has worked for us in the somewhat limited amount of tests we’ve performed so far but please do let us know your thoughts and if this works for you