Irregular image outline with GD

[Nameless One]

I have a PNG image that depicts a single item of irregular shape, much like the phpfreaks logo. I would like to add a 1px-2px thick white outline around that shape with GD. Is there any way to do this? ImageMagick is not an option.

[Nameless One]

Here's the code I came up with:

 

function _create_slideshow_image($image, $src_width, $src_height, $outline_ratio, $outline_transparency = 0) {
	$width = 300;
	$height = 400;
	$outline_width = round($width*($outline_ratio + 1));
	$outline_height = round($height*($outline_ratio + 1));
	$slideshow_image_temp = $this->_resize_image($image, $src_width, $src_height, $outline_width, $outline_height);
	$outline_width = imagesx($slideshow_image_temp);
	$outline_height = imagesy($slideshow_image_temp);
	$slideshow_image_outline = imagecreatetruecolor($outline_width, $outline_height);
	imageantialias($slideshow_image_outline, TRUE);
	imagealphablending($slideshow_image_outline, FALSE);
	imagesavealpha($slideshow_image_outline, TRUE);
	for ($i=1; $i<=$outline_width; $i++)
		for ($j=1; $j<=$outline_height; $j++) {
			$color = imagecolorat($slideshow_image_temp, $i, $j);
			$transparency = ($color & 0x7F000000) >> 24;
			$white = imagecolorallocatealpha($slideshow_image_outline, 255, 255, 255, $outline_transparency);
			$transparent = imagecolorallocatealpha($slideshow_image_outline, 0, 0, 0, 127);
			if ($transparency > 100)
				imagesetpixel($slideshow_image_outline, $i, $j, $transparent);
			else
				imagesetpixel($slideshow_image_outline, $i, $j, $white);
		}
	imagecolortransparent($slideshow_image_outline, $transparent);
	$slideshow_image_temp_reduced = $this->_resize_image($image, $src_width, $src_height, $width, $height);
	$width = imagesx($slideshow_image_temp_reduced);
	$height = imagesy($slideshow_image_temp_reduced);
	$reduction_x = round(($outline_width - $width)/2);
	$reduction_y = round(($outline_height - $height)/2);
	imagealphablending($slideshow_image_outline, TRUE);
	imagealphablending($slideshow_image_temp_reduced, TRUE);
	imagecopy($slideshow_image_outline, $slideshow_image_temp_reduced, $reduction_x, $reduction_y, 0, 0, $width, $height);
	return $slideshow_image_outline;
}

 

It creates a white outline by creating a white silhouette of the item on the source image which is slightly larger than the target size of the item, then pastes the items itself (resized to target size) over the silhouette. However, my pixel by pixel colour setting leaves much to be desired in the terms of quality. The edges are jagged, as you can see in the example I attached, and applying filters with imageconvolution() breaks the transparency since it probably processes the transparent parts of the image as well.

 

Any suggestions for improving this piece of code?

 

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