Creating Transparent PNG Images in GD

13:07.51 - Tuesday 27th March 2007   (Link to This Entry)

For a project at work I had to create a true colour PNG from scratch using GD, copy a load of image files to it from various locations and then display it on-screen.

The code to create a transparent PNG turned out to be a litle complicated:

<?php
 // create a true colour, transparent image
 // turn blending OFF and draw a background rectangle in our transparent colour  $image=imagecreatetruecolor($iwidth,$iheight);
 imagealphablending($image,false);
 $col=imagecolorallocatealpha($image,255,255,255,127);
 imagefilledrectangle($image,0,0,$iwidth,$iheight,$col);
 imagealphablending($image,true);

 // ^^ Alpha blanding is back on.
 // insert image manipulation stuff in here

 // output the results...
 header("Content-Type: image/png;");
 imagealphablending($image,false);
 imagesavealpha($image,true);
 imagepng($image);
?>

Important Note: If you output the image direct into the document stream as this example does, the PNG fix that uses bahaviours won't work as it's not a true PNG. For that, you'll have to drop the header and give imagepng() a filename and show that file. Seems to work fine in other browsers, however.