Convert images to grayscale in WordPress

class-grayscale-image-editor.php

<?php
/**
 * Grayscale Image Editor
 *
 * Expands GD Image Editor class by adding a grayscale filter.
 */

require_once ABSPATH . 'wp-includes/class-wp-image-editor.php';
require_once ABSPATH . 'wp-includes/class-wp-image-editor-gd.php';

class Grayscale_Image_Editor extends WP_Image_Editor_GD {
	/**
	 * Convert image to grayscale.
	 *
	 * @return boolean|WP_Error
	 */
	public function make_grayscale() {
		// Create copy of image.
		$w = $this->size['width'];
		$h = $this->size['height'];
		$dest = wp_imagecreatetruecolor( $w, $h );
		imagecopy( $dest, $this->image, 0, 0, 0, 0, $w, $h );

		if ( is_resource( $dest ) ) {
			// Apply grayscale filter to copy.
			if ( imagefilter( $dest, IMG_FILTER_GRAYSCALE ) && imagegammacorrect( $dest, 1.0, 1.1 ) ) {
				// Make the image progressive as a performance enhancement.
				imageinterlace( $dest, 1 );

				imagedestroy( $this->image );
				$this->image = $dest;
				return TRUE;
			}
		}
		return new WP_Error( 'image_grayscale_error', __( 'Image grayscale conversion failed.' ), $this->file );
	}
}

functions.php

<?php
/**
 * Convert images to grayscale.
 */
function image_grayscale_filter( $metadata, $attachment_id ) {
	require_once 'class-grayscale-image-editor.php';
	$path = wp_upload_dir();
	$basedir = trailingslashit( $path['basedir'] );
	$subdir = trailingslashit( dirname( $metadata['file'] ) );
	$image_sizes = get_intermediate_image_sizes();
	// Gather a list of all generated image files.
	$files = array();
	foreach ( $image_sizes as $size_name ) {
		if ( isset( $metadata['sizes'][$size_name]['file'] ) ) {
			$files[] = $basedir . $subdir . $metadata['sizes'][$size_name]['file'];
		}
	}
	// Reduce duplicates
	$files = array_unique( $files );
	// Make sure not to include the original file.
	$files = array_diff( $files, array( $basedir . $metadata['file'] ) );
	// Apply grayscale to each image.
	foreach ( $files as $file ) {
		$image = new Grayscale_Image_Editor( $file );
		$image->load();
		if ( $image->make_grayscale() ) {
			$image->save( $file );
		}
	}
	return $metadata;
}
add_filter( 'wp_generate_attachment_metadata', 'image_grayscale_filter', 10, 2 );