jQuery AJAX succeeded, but $_POST is empty

jQuery AJAX succeeded, but $_POST is empty

index.php:

<script> var image = canvas.toDataURL('image/png'); $.ajax({ type: 'POST', data: image, success: function() { alert('Ok.'); } }); </script> <?php var_dump($_POST); ?>

This returns "Ok." alert and array(0) { }. How can I get jQuery image variable?