Merge two PNG images with PHP. Output image CHANGES and wrong opacity - php

Question

I need to merge the following images, however my solution is not working.
My code is the following (I pass the URL of the images to the script via GET parameter)
<?php
$dest = imagecreatefrompng($_GET['img1']);
$src = imagecreatefrompng($_GET['img2']);
imagecopymerge($dest, $src, 0, 0, 0, 0, 1500, 1500, 50);
$white = imagecolorallocate($dest, 255, 255, 255);
imagecolortransparent($dest, $white);
header('Content-Type: image/png');
imagepng($dest);
?>
Both image are PNG with transparent background and both are 1500x1500.
First image:
Second image:
What I get:
Why can't I make the final image have the right opacity? I tried changing the last value of imagecopymerge() to 0 or 100 but in those cases I only get one image or another. I need them both, exactly overlapped on one another!
Also, if you look carefully around the gem in the final image, there is some extra blue color... how is this possible?

Answer 1

This is caused by setting the resulting image to use image transparency rather than per-pixel alpha-channel transparency by your call to imagecolortransparent which sets any white (#ffffff) pixels to 100% transparent.
The artefacts ('extra blue colour' and edge pixels) on the resulting image are caused by the alpha-channel transparency from the original images not being saved and the pixels (which were 100% transparent but not white) showing their original colour.
The 'washed out' look of your result comes from telling imagecopymerge to merge the two images with 50% transparency.
The solution is to use the correct alpha settings:
<?php
$dest = imagecreatefrompng($_GET['img1']);
$src = imagecreatefrompng($_GET['img2']);
imagesavealpha($dest, true);
imagecopy($dest, $src, 0, 0, 0, 0, 1500, 1500);
header('Content-Type: image/png');
imagepng($dest);