Creates a custom image size with the provided dimensions

endpoint.php

add_action( 'rest_api_init', function () {
	register_rest_route(
		'wp/v2',
		'/media/(?P<id>[\d]+)/custom',
		array(
			'methods'             => 'GET',
			'args'                => array(
				'id'     => array(
					'validate_callback' => function ( $id ) {
						if ( ! is_numeric( $id ) ) {
							return false;
						}

						$mime_type        = get_post_mime_type( $id );
						$valid_mime_types = array(
							'image/jpeg' => true,
							'image/jpg'  => true,
							'image/webp' => true,
						);

						return isset( $valid_mime_types[ $mime_type ] );
					},
				),
				'width'  => array(
					'description' => __( 'Image width.', 'performance-lab' ),
					'type'        => 'integer',
				),
				'height' => array(
					'type'        => 'string',
					'description' => __( 'Image height.', 'performance-lab' ),
				),
				'crop'   => array(
					'description' => __( 'If the image should be crop to the provided dimensions', 'performance-lab' ),
					'type'        => 'boolean',
					'default'     => false,
				),
				'crop_x' => array(
					'description' => __( '', 'performance-lab' ),
					'type'        => 'string',
					'enum'        => array(
						'left',
						'center',
						'right',
					),
				),
				'crop_y' => array(
					'description' => __( '', 'performance-lab' ),
					'type'        => 'string',
					'enum'        => array(
						'top',
						'center',
						'bottom',
					),
				),
				'format' => array(
					'description'       => __( 'A valid mime type for an image.', 'performance-lab' ),
					'type'              => 'string',
					'default'           => 'jpeg',
					'enum'              => array(
						'jpeg',
						'jpg',
						'webp',
					),
					'sanitize_callback' => function ( $image_mime ) {
						return $image_mime === 'jpg' ? 'jpeg' : sanitize_text_field( $image_mime );
					},
				),
				'scale'  => array(
					'description' => __( 'A valid number where to scale the dimensions.' ),
					'type'        => 'integer',
					'default'     => 1,
				),
			),
			'callback'            => function ( WP_REST_Request $request ) {
				if ( headers_sent() ) {
					return;
				}

				$image_file = find_the_appropiate_image_url( $request );

				if ( ! file_exists( $image_file ) ) {
					// TODO: Return 404.
					return;
				}

				$modified_date = get_the_modified_date( DATE_RFC2822, $request->get_param( 'id' ) );

				//	header( 'Accept-Ranges: bytes' );
				header( 'Content-type: image/' . $request->get_param( 'mime' ) );
				header( 'Content-Length: ' . filesize( $image_file ) );
				header( 'Cache-Control: public, max-age=' . HOUR_IN_SECONDS );
				header( 'Content-Disposition: filename=' . wp_basename( $image_file ) );
				header( 'Last-Modified: ' . $modified_date );

				readfile( $image_file );
			},
			'permission_callback' => '__return_true',
		)
	);
}
);

function find_the_appropiate_image_url( WP_REST_Request $request ) {
	$attachment_id = (int) $request->get_param( 'id' );

	$attached_file = get_attached_file( $attachment_id );
	if ( ! $request->has_param( 'width' ) && ! $request->has_param( 'height' ) ) {
		return $attached_file;
	}

	$scale    = (int) $request->get_param( 'scale' );
	$width    = (int) $request->get_param( 'width' ) * $scale;
	$height   = (int) $request->get_param( 'height' ) * $scale;
	$metadata = wp_get_attachment_metadata( $attachment_id );

	$directory = pathinfo( $attached_file, PATHINFO_DIRNAME );
	$mime_type = 'image/' . $request->get_param( 'format' );
	if ( $metadata['width'] === $width && $metadata['height'] === $height ) {
		if ( isset( $metadata['sources'][ $mime_type ]['file'] ) ) {
			return path_join( $directory, $metadata['sources'][ $mime_type ]['file'] );
		}

		return $attached_file;
	}

	$size_name = "{$width}_{$height}";
	$crop = $request->get_param( 'crop' );
	if ( $request->has_param( 'crop' ) || ( $request->has_param( 'crop_x' ) && $request->has_param( 'crop_y' ) ) ) {
		$size_name .= "_cropped";
		if ( $request->has_param( 'crop_x' ) && $request->has_param( 'crop_y' ) ) {
			$crop = array( $request->get_param( 'crop_x' ), $request->get_param( 'crop_y' ) );
			$size_name .= '_x_' . $request->get_param('crop_x') . '_y_' . $request->get_param('crop_y');
		}
	}

	if ( isset( $metadata['sizes'][ $size_name ]['sources'][ $mime_type ]['file'] ) ) {
		return path_join( $directory, $metadata['sizes'][ $size_name ]['sources'][ $mime_type ]['file'] );
	}

	foreach ( $metadata['sizes'] as $name => $properties ) {
		if ( $properties['width'] === $width && $properties['height'] === $height ) {
			if ( isset( $properties['sources'][ $mime_type ]['file'] ) ) {
				return path_join( $directory, $properties['sources'][ $mime_type ]['file'] );
			}

			$editor = wp_get_image_editor( wp_get_original_image_path( $attachment_id ), array( 'mime_type' => $mime_type ) );
			$editor->resize( $width, $height, $crop );
			$result = $editor->save();

			$properties['sources'][ $mime_type ] = array(
				'file'     => $result['file'],
				'filesize' => filesize( $result['path'] ),
			);
			$metadata['sizes'][ $name ]          = $properties;
			wp_update_attachment_metadata( $attachment_id, $metadata['sizes'] );
		}
	}

	$editor = wp_get_image_editor( wp_get_original_image_path( $attachment_id ), array( 'mime_type' => $mime_type ) );
	$editor->resize( $width, $height, $crop );
	$result = $editor->save( null, $mime_type );

	if ( is_wp_error( $result ) ) {
		return $attached_file;
	}

	if ( isset( $metadata['sizes'][ $size_name ] ) ) {
		if ( ! isset( $metadata['sizes'][ $size_name ]['sources'] ) ) {
			$metadata['sizes'][ $size_name ]['sources'] = array();
		}
		$metadata['sizes'][ $size_name ]['sources'][ $mime_type ] = array(
			'file'     => $result['file'],
			'filesize' => filesize( $result['path'] ),
		);
	} else {
		$metadata['sizes'][ $size_name ] = array(
			'file'      => $result['file'],
			'width'     => $result['width'],
			'height'    => $result['height'],
			'mime-type' => $result['mime-type'],
			'sources'   => array(
				$mime_type => array(
					'file'     => $result['file'],
					'filesize' => filesize( $result['path'] ),
				),
			),
		);
	}

	wp_update_attachment_metadata( $attachment_id, $metadata );

	return $result['path'];
}