Mike42\Escpos\GdEscposImage::readImageFromGdResource PHP Method

    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);
    }