Adding watermarks to images using alpha channels
GD and Image Functions
Ejemplos
PHP Manual

Example #1 Using imagecopymerge() to create a translucent watermark

`<?php
// Load the stamp and the photo to apply the watermark to
$im = imagecreatefromjpeg('photo.jpeg');
// First we create our stamp image manually from GD
$stamp = imagecreatetruecolor(100, 70);
imagefilledrectangle($stamp, 0, 0, 99, 69, 0x0000FF);
imagefilledrectangle($stamp, 9, 9, 90, 60, 0xFFFFFF);
$im = imagecreatefromjpeg('photo.jpeg');
imagestring($stamp, 5, 20, 20, 'libGD', 0x0000FF);
imagestring($stamp, 3, 20, 40, '(c) 2007-9', 0x0000FF);
// Set the margins for the stamp and get the height/width of the stamp image
$marge_right = 10;
$marge_bottom = 10;
$sx = imagesx($stamp);
$sy = imagesy($stamp);
// Merge the stamp onto our photo with an opacity (transparency) of 50%
imagecopymerge($im, $stamp, imagesx($im) - $sx - $marge_right, imagesy($im) - $sy - $marge_bottom, 0, 0, imagesx($stamp), imagesy($stamp), 50);
// Save the image to file and free memory
imagepng($im, 'photo_stamp.png');
imagedestroy($im);
?>`

This example uses imagecopymerge() to merge the stamp with our original image. Using this we can set the opacity of our stamp - in our example we've set it to 50% opacity (another term for transparency). In practice this would be useful in copyright protection as semi-transparent watermarks are hard to remove yet allow viewers to see the image.

Adding watermarks to images using alpha channels
GD and Image Functions
Ejemplos
PHP Manual