How to check if an image has transparency using GD?

Question

How do I check if an image has transparent pixels with php's GD library?

Answer 1

I know this is old, but I just found this on the comments of the PHP docs. (link)

Here is the function which determines whether the PNG image contains alpha or not:

<?php
    function is_alpha_png($fn){
      return (ord(@file_get_contents($fn, NULL, NULL, 25, 1)) == 6);
    }
?>

The color type of PNG image is stored at byte offset 25. Possible values of that 25'th byte is:

• 0 - greyscale
• 2 - RGB
• 3 - RGB with palette
• 4 - greyscale + alpha
• 6 - RGB + alpha

Only works for PNG images though.