wp_retina_support_code.php

<?php
/*
Just copy the following code in functions.php file into your theme.
*/
add_action('after_setup_theme','window_mag_retina_script');
function window_mag_retina_script(){
	wp_enqueue_script( 'retina-js',  'https://cdnjs.cloudflare.com/ajax/libs/retina.js/2.1.3/retina.min.js', array( 'jquery' ), '1.0', true );
}

/**
 * Retina images
 *
 * This function is attached to the 'wp_generate_attachment_metadata' filter hook.
 */
add_filter( 'wp_generate_attachment_metadata', 'window_mag_retina_support_attachment_meta', 10, 2 );
function window_mag_retina_support_attachment_meta( $metadata, $attachment_id ) {
	
		foreach ( $metadata as $key => $value ) {
			if ( is_array( $value ) ) {
				foreach ( $value as $image => $attr ) {
					if ( is_array( $attr ) && isset ( $attr['width'] ) && isset ( $attr['height'] ) ) {
						window_mag_retina_support_create_images( get_attached_file( $attachment_id ), $attr['width'], $attr['height'], true );
					}
				}
			}
		}

		return $metadata;
	
}

/**
 * Create retina-ready images
 *
 * Referenced via retina_support_attachment_meta().
 */
function window_mag_retina_support_create_images( $file, $width, $height, $crop = false ) {
	if ( $width || $height ) {
		$resized_file = wp_get_image_editor( $file );
		if ( ! is_wp_error( $resized_file ) ) {
			$filename = $resized_file->generate_filename( $width . 'x' . $height . '@2x' );

			$resized_file->resize( $width * 2, $height * 2, $crop );
			$resized_file->save( $filename );

			$info = $resized_file->get_size();

			return array(
				'file'   => wp_basename( $filename ),
				'width'  => $info['width'],
				'height' => $info['height'],
			);
		}
	}

	return false;
}

add_filter( 'delete_attachment', 'window_mag_delete_retina_support_images' );
/**
 * Delete retina-ready images
 *
 * This function is attached to the 'delete_attachment' filter hook.
 */
function window_mag_delete_retina_support_images( $attachment_id ) {
	
		$meta       = wp_get_attachment_metadata( $attachment_id );
		$upload_dir = wp_upload_dir();
		if ( isset( $meta['file'] ) ) {
			$path = pathinfo( $meta['file'] );
			foreach ( $meta as $key => $value ) {
				if ( 'sizes' === $key ) {
					foreach ( $value as $sizes => $size ) {
						$original_filename = $upload_dir['basedir'] . '/' . $path['dirname'] . '/' . $size['file'];
						$retina_filename   = substr_replace( $original_filename, '@2x.', strrpos( $original_filename, '.' ), strlen( '.' ) );
						if ( file_exists( $retina_filename ) ) {
							unlink( $retina_filename );
						}
					}
				}
			}
		
	}
}