magento generate invoice order

$orderId = 2;
$order = Mage::getModel('sales/order')->load($orderId);

$invoice->register();
$invoice->getOrder()->setCustomerNoteNotify(false);
$invoice->getOrder()->setIsInProcess(true);
$order->addStatusHistoryComment('invoice by Aditya.', false);
$transactionSave = Mage::getModel('core/resource_transaction')
                    ->addObject($invoice)
                    ->addObject($invoice->getOrder());
                $transactionSave->save();