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English
When processing images with PHP, the GD library offers many powerful functions, and imagecolorallocatealpha() is a function used to allocate colors with transparency. Many developers have noticed that even though they assign a transparent color, the saved PNG or GIF file still appears fully opaque. Why does this happen? The key lies in another function that is easily overlooked: imagesavealpha().
imagecolorallocatealpha() is typically used like this:
$image = imagecreatetruecolor(200, 200);
$transparentColor = imagecolorallocatealpha($image, 255, 0, 0, 127); // Red, fully transparent
Its parameters include red, green, blue, and transparency (0-127, where 0 is fully opaque and 127 is fully transparent). With this function, you can draw elements with transparency on the image, such as semi-transparent rectangles, text, and so on.
However, this function only allocates the color and does not automatically instruct the GD library to “preserve the alpha channel when outputting the image.”
This is the role of imagesavealpha():
imagesavealpha($image, true);
When you call this function, the GD library will preserve the alpha channel data when outputting a PNG (or another format that supports transparency). If it’s not enabled, GD will fill the transparent areas with a solid background (usually black or white), causing the transparent color you assigned to be ineffective, and the saved image will appear fully opaque.
In other words:
? imagecolorallocatealpha() → defines the transparent part of the color
?? but doesn’t automatically preserve the transparency information in the file → requires manually calling imagesavealpha()
Here’s a simple example:
<?php
// Create a canvas
$image = imagecreatetruecolor(200, 200);
<p>// Enable alpha channel saving<br>
imagesavealpha($image, true);</p>
<p>// Create a fully transparent background color<br>
$transparent = imagecolorallocatealpha($image, 0, 0, 0, 127);</p>
<p>// Fill the background<br>
imagefill($image, 0, 0, $transparent);</p>
<p>// Draw a semi-transparent red rectangle<br>
$red = imagecolorallocatealpha($image, 255, 0, 0, 63);<br>
imagefilledrectangle($image, 50, 50, 150, 150, $red);</p>
<p>// Output the image<br>
header('Content-Type: image/png');<br>
imagepng($image);</p>
<p>// Free memory<br>
imagedestroy($image);<br>
?><br>
If you remove the line imagesavealpha($image, true), the saved PNG file will lose its transparent area and will appear as a red rectangle on a black background.
If you want to save the image to a file instead of outputting it to the browser, you can do this:
