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

Improved cronoklee's function. Removed unnecessary bit shifting for each pixel, reduced false negatives count, added explanation in function description.

/**
 * Estimates, if image has pixels with transparency. It shrinks image to 64 times smaller
 * size, if necessary, and searches for the first pixel with non-zero alpha byte.
 * If image has 1% opacity, it will be detected. If any block of 8x8 pixels has at least
 * one semi-opaque pixel, the block will trigger positive result. There are still cases,
 * where image with hardly noticeable transparency will be reported as non-transparent,
 * but it's almost always safe to fill such image with monotonic background.
 *
 * Icons with size <= 64x64 (or having square <= 4096 pixels) are fully scanned with
 * absolutely reliable result.
 *
 * @param  resource $image
 * @return bool
 */
function hasTransparency ($image): bool {
  if (!is_resource($image)) {
    throw new \InvalidArgumentException("Image resource expected. Got: " . gettype($image));
  }

  $shrinkFactor      = 64.0;
  $minSquareToShrink = 64.0 * 64.0;

  $width  = imagesx($image);
  $height = imagesy($image);
  $square = $width * $height;

  if ($square <= $minSquareToShrink) {
    [$thumb, $thumbWidth, $thumbHeight] = [$image, $width, $height];
  } else {
    $thumbSquare = $square / $shrinkFactor;
    $thumbWidth  = (int) round($width / sqrt($shrinkFactor));
    $thumbWidth < 1 and $thumbWidth = 1;
    $thumbHeight = (int) round($thumbSquare / $thumbWidth);
    $thumb       = imagecreatetruecolor($thumbWidth, $thumbHeight);
    imagealphablending($thumb, false);
    imagecopyresized($thumb, $image, 0, 0, 0, 0, $thumbWidth, $thumbHeight, $width, $height);
  }

  for ($i = 0; $i < $thumbWidth; $i++) { 
    for ($j = 0; $j < $thumbHeight; $j++) {
      if (imagecolorat($thumb, $i, $j) & 0x7F000000) {
        return true;
      }
    }
  }

  return false;
}

Usage:

hasTransparency( imagecreatefrompng("myfile.png") );   //returns true if img has transparency

Answer 2

I know this is an old thread, but in my opinion it needs improvement since walking through a huge png by checking all pixels only to find out it is not transparent is a waste of time. So after some googleing I found Jon Fox's Blog and I improved his code with the help of the W3C PNG Specification further to be reliable, fast and have a minimum on memory imprint:

function IsTransparentPng($File){
    //32-bit pngs
    //4 checks for greyscale + alpha and RGB + alpha
    if ((ord(file_get_contents($File, false, null, 25, 1)) & 4)>0){
        return true;
    }
    //8 bit pngs
    $fd=fopen($File, 'r');
    $continue=true;
    $plte=false;
    $trns=false;
    $idat=false;
    while($continue===true){
        $continue=false;
        $line=fread($fd, 1024);
        if ($plte===false){
            $plte=(stripos($line, 'PLTE')!==false);
        }
        if ($trns===false){
            $trns=(stripos($line, 'tRNS')!==false);
        }
        if ($idat===false){
            $idat=(stripos($line, 'IDAT')!==false);
        }
        if ($idat===false and !($plte===true and $trns===true)){
            $continue=true;
        }
    }
    fclose($fd);
    return ($plte===true and $trns===true);
}