Mike42\Escpos\EscposImage::isGdLoaded PHP Method

    public static function isGdLoaded()
    {
        return extension_loaded('gd');
    }

 /**
  * Load actual image pixels from GD resource.
  *
  * @param resource $im GD resource to use
  * @throws Exception Where the image can't be read.
  */
 public function readImageFromGdResource($im)
 {
     if (!is_resource($im)) {
         throw new Exception("Failed to load image.");
     } elseif (!EscposImage::isGdLoaded()) {
         throw new Exception(__FUNCTION__ . " requires 'gd' extension.");
     }
     /* Make a string of 1's and 0's */
     $imgHeight = imagesy($im);
     $imgWidth = imagesx($im);
     $imgData = str_repeat("", $imgHeight * $imgWidth);
     for ($y = 0; $y < $imgHeight; $y++) {
         for ($x = 0; $x < $imgWidth; $x++) {
             /* Faster to average channels, blend alpha and negate the image here than via filters (tested!) */
             $cols = imagecolorsforindex($im, imagecolorat($im, $x, $y));
             // 1 for white, 0 for black, ignoring transparency
             $greyness = (int) (($cols['red'] + $cols['green'] + $cols['blue']) / 3) >> 7;
             // 1 for black, 0 for white, taking into account transparency
             $black = 1 - $greyness >> ($cols['alpha'] >> 6);
             $imgData[$y * $imgWidth + $x] = $black;
         }
     }
     $this->setImgWidth($imgWidth);
     $this->setImgHeight($imgHeight);
     $this->setImgData($imgData);
 }