Creating ‘mask layers’ using PHP GD

This morning I came across a question on stackoverflow that puzzled me a bit:

Is there a reasonably straightforward way to copy a circular area from one image resource to another? Something like imagecopymerge except with circles or ovals etc?
If possible, I want to avoid having to use pre-created image files (any oval shape should be possible), and if there’s transparency colours involved they should naturally leave the rest of the image alone.

Reason I’m asking, I have a few classes that allow to apply image operations inside a “selected area” of an image, which works by first deleting that area from a copy of the image, then overlaying the copy back on the original. But what if you want to select a rectangle, and then inside that deselect a circle, and have the operations only affect the area that’s left?

I’ve done it a thousand times in Photoshop, but never using PHP. So I set about coming up with a solution (reformatted from my original post on stackoverflow.):

1. Start with original image – $img
   $img = imagecreatefromjpeg("./original.jpg");
$img_magicpink = imagecolorallocatealpha($img, 255, 0, 255, 127);
// (Get its dimensions for copying)
list($w, $h) = getimagesize("./original.jpg");
2. Copy that image to a png – $copy
   $copy = imagecreatefromjpeg("./original.jpg");
imagealphablending($copy, true);
$copy_magicpink = imagecolorallocate($copy, 255, 0, 255);
imagecolortransparent($copy, $copy_magicpink);
3. Create a mask png image of the area you want in the circle/ellipse (a ‘magicpink’ image with a black shape on it, with black set to the colour of alpha transparency) – $mask
   $mask = imagecreatetruecolor($w, $h);
   imagealphablending($mask, true);

   // Set the masking colours
   $mask_black = imagecolorallocate($mask, 0, 0, 0);
   $mask_magicpink = imagecolorallocate($mask, 255, 0, 255);
   imagecolortransparent($mask, $mask_black);
   imagefill($mask, 0, 0, $mask_magicpink);

   // Draw the circle for the mask
$circle_x = $w/2;
$circle_y = $h/2;
$circle_w = 150;
$circle_h = 150;
imagefilledellipse($mask, $circle_x, $circle_y, $circle_w, $circle_h, $mask_black);

4. Copy $mask over the top of $copy maintaining the Alpha transparency
   imagecopymerge($copy, $mask, 0, 0, 0, 0, $w, $h, 100);
5. Change what you need to on $copy
   // My example is turning the original image gray and leaving the masked area as colour
$x = imagesx($img);
$y = imagesy($img);
$gray = imagecreatetruecolor($x, $y);
imagecolorallocate($gray, 0, 0, 0);
for ($i = 0; $i < $x; $i++) {
for ($j = 0; $j < $y; $j++) {
$rgb = imagecolorat($img, $i, $j);
$r = ( $rgb >> 16 ) & 0xFF;
$g = ( $rgb >> 8 ) & 0xFF;
$b = $rgb & 0xFF;
//for gray mode $r = $g = $b
$color = max(array($r, $g, $b));
$gray_color = imagecolorexact($img, $color, $color, $color);
imagesetpixel($gray, $i, $j, $gray_color);
}
}
6. Copy $copy back over $img maintaining the Alpha transparency
   imagecopymergegray($gray, $copy, 0, 0, 0, 0, $w, $h, 100);
   imagealphablending($gray, true);
   imagecolortransparent($gray, $mask_magicpink);

   header('Content-Type: image/png');
imagepng($gray);
imagedestroy($gray);