Grafika\Gd\Editor::opacity PHP Method

    public function opacity(&$image, $opacity)
    {
        if ($image->isAnimated()) {
            // Ignore animated GIF for now
            return $this;
        }
        // Bounds checks
        $opacity = $opacity > 1 ? 1 : $opacity;
        $opacity = $opacity < 0 ? 0 : $opacity;
        for ($y = 0; $y < $image->getHeight(); $y++) {
            for ($x = 0; $x < $image->getWidth(); $x++) {
                $rgb = imagecolorat($image->getCore(), $x, $y);
                $alpha = $rgb >> 24 & 0x7f;
                // 127 in hex. These are binary operations.
                $r = $rgb >> 16 & 0xff;
                $g = $rgb >> 8 & 0xff;
                $b = $rgb & 0xff;
                // Reverse alpha values from 127-0 (transparent to opaque) to 0-127 for easy math
                // Previously: 0 = opaque, 127 = transparent.
                // Now: 0 = transparent, 127 = opaque
                $reverse = 127 - $alpha;
                $reverse = round($reverse * $opacity);
                if ($alpha < 127) {
                    // Process non transparent pixels only
                    imagesetpixel($image->getCore(), $x, $y, imagecolorallocatealpha($image->getCore(), $r, $g, $b, 127 - $reverse));
                }
            }
        }
        return $this;
    }