Invert color and preserve transparency with PHP - php

Question

I'm trying to create a function that receives a PNG image and returns it with inverted colors. However, I'm having trouble with the transparency/alpha channel: basically, transparent pixels are coming back as either black or white, and thus transparency is not retained.
This is an example of a source image (which will of course vary), a PNG with a transparent background:
I would like to invert those colors while preserving transparency/alpha channel, like a CTRL+I in Photoshop. I'm using GD imagefilter function with the IMG_FILTER_NEGATE parameter
With the code:
$im = imagecreatefrompng($image_file);
imagefilter($im, IMG_FILTER_NEGATE);
header('image/png');
imagepng($im,NULL,0);
But it yields:
As you can see, transparent pixels turned black.
I then tried adding the alpha channel to the function imagecreatefrompng (like this):
$im = imagecreatefrompng($image_file);
imagealphablending($im, false);
imagesavealpha($im, true);
imagefilter($im, IMG_FILTER_NEGATE);
header('image/png');
imagepng($im,NULL,0);
But now I get white instead of black:
Now the issue seems to happen after the imagefilter function is applied. For example, if I run this code:
$im = imagecreatefrompng($image_file);
imagealphablending($im, false);
imagesavealpha($im, true);
//imagefilter($im, IMG_FILTER_NEGATE);
header('image/png');
imagepng($im,NULL,0);
The output image retains its transparent background, while remaining identical to the original image.
How can I invert colors of transparent PNGs without losing the transparent background?
NOTE: this is not the same question as imagecreatefrompng() Makes a black background instead of transparent? - the inversion step is the tricky part here

Answer 1

Here's what worked for me, dealing with transparent background and inverting image. Note that I'm using base64.
$im = imagecreatefromstring(base64_decode(str_replace('data:image/png;base64,', '', $base64)));
$width = imagesx($im);
$height = imagesy($im);
$dest_image = imagecreatetruecolor($width, $height);
imagealphablending($dest_image, FALSE);
imagesavealpha($dest_image, TRUE);
imagefilter($im, IMG_FILTER_NEGATE);
imagecopyresampled($dest_image, $im, 0, 0, 0, 0, $width, $height, $width, $height);
//optional output back to base64 or use imagepng with destination
ob_start();
imagepng($dest_image);
$contents = ob_get_contents();
ob_end_clean();
$base64 = 'data:image/png;base64,'.base64_encode($contents);