Adding alpha channels to PHP GD Images - php

Question

I want to create an png image with transparency using the GD functions of PHP.
Specifically, I'm have text with different opacity levels (for anti-aliasing).
Using the following code I am able to create an alpha in the main part of the background:
//Create image
$image = imagecreatetruecolor($width, $height);
//Set background to opaque
imagecolortransparent($image, imagecolorallocate($image, 0, 0, 0));
Though it does properly create an alpha, in area the image has opacity levels that are not 0% or 100%, it makes black.
How can I correctly create the opacity levels of these areas in the image?

Answer 1

Specifically, I'm have text with different opacity levels (for
anti-aliasing)
Using a different opacity level for text doesn't anti-alias it. And there's no reason to do this since GD outputs anti-aliased text anyway.
Example 1: This creates an image with an opaque black background with white text at various opacity levels
// create image resource.
$img = imagecreatetruecolor(250, 200);
// create image colours.
$black = imagecolorallocate($img, 0, 0, 0);
$white_0 = imagecolorallocatealpha($img, 255, 255, 255, 0);
$white_25 = imagecolorallocatealpha($img, 255, 255, 255, 32);
$white_50 = imagecolorallocatealpha($img, 255, 255, 255, 64);
$white_75 = imagecolorallocatealpha($img, 255, 255, 255, 96);
$white_100 = imagecolorallocatealpha($img, 255, 255, 255, 127);
// set background to opaque black.
imagefill($img, 0, 0, $black);
// output text strings.
imagettftext($img, 20, 0, 10, 30, $white_0, 'arial.ttf', '0% transparent');
imagettftext($img, 20, 0, 10, 60, $white_25, 'arial.ttf', '25% transparent');
imagettftext($img, 20, 0, 10, 90, $white_50, 'arial.ttf', '50% transparent');
imagettftext($img, 20, 0, 10, 120, $white_75, 'arial.ttf', '75% transparent');
imagettftext($img, 20, 0, 10, 150, $white_100, 'arial.ttf', '100% transparent'); // not visible!
header('Content-type: image/png');
imagepng($img);
imagedestroy($img);
Result 1:
Example 2: If what you want is a transparent background:
// create image resource.
$img = imagecreatetruecolor(250, 200);
// save alpha channel information (for transparent background).
imagealphablending($img, false);
imagesavealpha($img, true);
// create image colours.
$transparent = imagecolorallocatealpha($img, 0, 0, 0, 127);
$black_0 = imagecolorallocatealpha($img, 0, 0, 0, 0);
$black_25 = imagecolorallocatealpha($img, 0, 0, 0, 32);
$black_50 = imagecolorallocatealpha($img, 0, 0, 0, 64);
$black_75 = imagecolorallocatealpha($img, 0, 0, 0, 96);
// set background to transparent.
imagefill($img, 0, 0, $transparent);
// output text strings.
imagettftext($img, 20, 0, 10, 30, $black_0, 'arial.ttf', '0% transparent');
imagettftext($img, 20, 0, 10, 60, $black_25, 'arial.ttf', '25% transparent');
imagettftext($img, 20, 0, 10, 90, $black_50, 'arial.ttf', '50% transparent');
imagettftext($img, 20, 0, 10, 120, $black_75, 'arial.ttf', '75% transparent');
imagettftext($img, 20, 0, 10, 150, $transparent, 'arial.ttf', '100% transparent'); // not visible!
header('Content-type: image/png');
imagepng($img);
imagedestroy($img);
Result 2: