Source

File: wp-includes/class-wp-image-editor-gd.php

protected function update_size( $width = false, $height = false ) {
		if ( ! $width ) {
			$width = imagesx( $this->image );
		}

		if ( ! $height ) {
			$height = imagesy( $this->image );
		}

		return parent::update_size( $width, $height );
	}