Adding some opacity on an image with imagecopymerge in PHP - php

Question

Here my problem:
I want to change the opacity of an image, by copying it on another transparent image.
My code:
$opacity = 50;
$transparentImage = imagecreatetruecolor($width, $height);
imagesavealpha($transparentImage, true);
$transColour = imagecolorallocatealpha($transparentImage , 0, 0, 0, 127);
imagefill($transparentImage , 0, 0, $transColour);
imagecopymerge($transparentImage, $image, 0, 0, 0, 0, $width, $height, $opacity);
$image = $transparentImage;
header('Content-type: image/png');
imagepng($image);
By doing this, when I use imagecopymerge, $transparentImage loses its transparency... So $image is merged on a black image... and not on a transparent image !
However, when I show $transparentImage before calling imagecopymerge, the image is transparent in my navigator !
Is there a solution to change opacity of my image, without adding it on a colored background ?

Answer 1

It seems that imagecopymerge does not support the alpha (transparency) channel on images. Fortunately you can use a workaround with imagecopy to do it correctly. Here's a function to do this, taken from the comments on php.net:
function imagecopymerge_alpha($dst_im, $src_im, $dst_x, $dst_y, $src_x, $src_y, $src_w, $src_h, $pct){
// creating a cut resource
$cut = imagecreatetruecolor($src_w, $src_h);
// copying relevant section from background to the cut resource
imagecopy($cut, $dst_im, 0, 0, $dst_x, $dst_y, $src_w, $src_h);
// copying relevant section from watermark to the cut resource
imagecopy($cut, $src_im, 0, 0, $src_x, $src_y, $src_w, $src_h);
// insert cut resource to destination image
imagecopymerge($dst_im, $cut, $dst_x, $dst_y, 0, 0, $src_w, $src_h, $pct);
}