Why Do PNG Thumbnails Lose Transparency When Using imagecreatefrompng()?

PNG Transparency Loss in Image Thumbnails

When creating thumbnails with the GD library using the imagecreatefrompng() function, users have encountered an issue where the PNG's transparent background is replaced with a solid black color. To address this, let's examine the code and identify a solution.

The provided code effectively resizes an image and saves it as a JPEG. However, the problem lies in the initialization of the destination image ($dimg) using the imagecreatetruecolor() function. To preserve transparency in PNG images, additional steps are required. Here's a modified version of the code:

<code class="php">$dimg = imagecreatetruecolor($width_new, $height_new);

// Start changes for PNG transparency
switch ($stype) {

    case 'gif':
    case 'png':
        // Define black as a color
        $background = imagecolorallocate($dimg, 0, 0, 0);
        // Make black transparent
        imagecolortransparent($dimg, $background);
        // Disable blending to avoid mixing black with the image
        imagealphablending($dimg, false);
        // Enable alpha channel preservation
        imagesavealpha($dimg, true);
        break;

    default:
        break;
}
// End changes

$wm = $w/$nw;
$hm = $h/$nh;</code>

By adding these steps, we ensure that:

• A transparent background color is defined.
• The transparent color is removed from the destination image.
• Alpha blending is disabled to prevent the transparent color from mixing with the image.
• Alpha channel information is preserved, maintaining transparency in the resulting thumbnail.

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