How to use WPThumb to create grayscale images.

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<?php

/**
 *	Use WPThumb to create grayscale images.
 */

// Register a new image size. 
// Note: We have could just passed a custom arg to wpthumb.
add_image_size( 'medium:grayscale', 100, 100, true );

/**
 * Filter the image
 *
 * @param object $thumb the GDThumb object
 * @param array $args
 * @return object $thumb
 */
add_filter( 'wpthumb_image_filter', function(  $thumb, $args ) {

	if( 'medium:grayscale' == $args['image_size'] ) {

		$thumb->setWorkingImage( $thumb->getOldImage() );
		
		imagefilter( $thumb->getWorkingImage(), IMG_FILTER_GRAYSCALE );
	
	}
	
	return $thumb;	
	
} , 10, 2 );

?>