How to check if a GIF has transparency using GD?

Question

I found the question How to check if an image has transparency using GD? but the the answers are all for PNG files. Is there a solution for checking if a GIF image has transparency in PHP using the GD extension?

Answer 1

I am assuming that:

• all GIFs are palettised,
• the alpha component will be non-zero (probably 127) for any palette entry which is transparent,
• encoders do not add transparent palette entries unnecessarily.

On that basis, the following code will load a GIF and check that no palette entry contains transparency - rather than checking every single pixel in a very slow double loop over height and width of an image:

<?php

function GIFcontainstransparency($fname){

   // Load up the image
   $src=imagecreatefromgif($fname);

   // Check image is palettised
   if(imageistruecolor($src)){
      fwrite(STDERR,"ERROR: Unexpectedly got a truecolour (non-palettised) GIF!");
   }

   // Get number of colours - i.e. number of entries in palette
   $ncolours=imagecolorstotal($src);

   // Check palette for any transparent colours rather than all pixels - to speed it up
   for($index=0;$index<$ncolours;$index++){
      $rgba = imagecolorsforindex($src,$index);
      if($rgba['alpha']>0){
         return true;
      }
   }
   return false;
}

////////////////////////////////////////////////////////////////////////////////
// main
////////////////////////////////////////////////////////////////////////////////

   if(GIFcontainstransparency("image.gif")){
      echo "Contains transparency";
   } else {
      echo "Is fully opaque";
   }
?>