Converta automaticamente imagens carregadas em tons de cinza no WordPress

add_filter('wp_generate_attachment_metadata','rb_bw_filter');

function rb_bw_filter($meta) {

$path
= wp_upload_dir(); // get upload directory
$file
= $path['basedir'].'/'.$meta['file']; // Get full size image

$files
[] = $file; // Set up an array of image size urls

foreach ($meta['sizes'] as $size) {
$files
[] = $path['path'].'/'.$size['file'];
}

foreach ($files as $file) { // iterate through each image size

// Convert image to grayscale credit to http://ottopress.com/2011/customizing-wordpress-images/

list
($orig_w, $orig_h, $orig_type) = @getimagesize($file);
$image
= wp_load_image($file);
imagefilter
($image, IMG_FILTER_GRAYSCALE);
switch ($orig_type) {
case IMAGETYPE_GIF:
imagegif
( $image, $file );
break;
case IMAGETYPE_PNG:
imagepng
( $image, $file );
break;
case IMAGETYPE_JPEG:
imagejpeg
( $image, $file );
break;
}
}
return $meta;
}