Warn users on older versions of Gd, that it looses the alpha channel

[rosell-dk]

Old versions of Gd does not handle transparency (in some PNGs)
Experienced in GD Version: 2.1.1-dev.

Also discussed here: rosell-dk/webp-express#374

[rosell-dk]

There is hope. Perhaps the solution is here or here.

[rosell-dk]

The following code demonstrates that old versions of Gd looses the alpha channel when saving WebP, but is able to retain it when saving PNG:
The version I see it fail on is: GD Version: 2.1.1-dev, PHP version: 5.5.9-1ubuntu4.29.

if (!extension_loaded('gd')) {
  echo 'Gd extension is not available. The test cannot be run.'; exit;
}
if (!function_exists('imagewebp')) {
    echo 'Gd has been compiled without webp support and the test cannot be run.'; exit;
}
if (!function_exists('imagecreatefrompng')) {
    echo 'Gd has been compiled without PNG support and the test cannot be run.'; exit;
};

echo 'PHP version: ' . PHP_VERSION . '<br>';
echo 'GD Version: ' . gd_info()["GD Version"] . '<br>';

// Create 50x50 true color
$new = imagecreatetruecolor(50, 50);
$color = imagecolorallocatealpha($new, 0, 0, 0, 70);
imagefill($new, 0, 0, $color);

// Set flag to retain full alpha channel when saving PNG
// Works for PNG, but seems to have no effect for saving WebP
if (imagesavealpha($new, true) === false) {
    echo 'imagesavealpha() failed!';
}

$destinationWebp = __DIR__ . '/test.webp';
imagewebp($new, $destinationWebp);
if (filesize($destinationWebp) % 2 == 1) {
    file_put_contents($destinationWebp, "\0", FILE_APPEND);
}

$destinationPng = __DIR__ . '/test.png';
imagepng($new, $destinationPng);

On newer Gd, both the PNG and the WebP are semitransparent. On older Gd, the WebP is completely black.

It seems that imagesavealpha() has no effect for webp - but only applies to png. It does not have any affect in newer Gd either (setting to false does not make it black). This is in accordance with the documentation, which says that it applies to PNG. There is no equivalent function for webp, so this flag cannot be controlled for webp. Older Gd versions simply discards alpha channel while newer versions simply retains it.

It would seem that all hope is lost for retaining alpha channel for webp on old Gd.

[rosell-dk]

Here is a link to knowledge on the PNG format. http://www.libpng.org/pub/png/book/chapter08.html

[rosell-dk]

Gd 2.2.5 has no such problems. I do not know about the versions between 2.1.1-dev and 2.2.5. Please contribute!

[rosell-dk]

Also reported on 2.1.1.

[rosell-dk]

For those that experiences this, here are your options:

• Disable PNG. Gd isn't very good at converting PNG anyway, as Gd cannot create lossless webp, only lossy webp
• Ask your host to upgrade Gd
• Ask your host to install imagick, vips or cwebp
• Use a cloud converter (ewww or connect to a remote webp-express / wpc)

[ve3]

To be more specific, this GD's problem occur on prior PHP 7.0. It means PHP 5.6 or older.
Tested on Windows x86, x64.