The โbestโ way to do this is to create a PAM module. Here's how features like fingerprint identification are implemented. This will allow you to keep the same login screen with all the features, such as accessibility options, etc.
The โeasiestโ way to do this is to modify an existing display manager. Ubuntu uses GDM (Gnome Display Manager) by default, but itโs rather complicated. SLiM is a simple display manager that you can probably change without much difficulty.
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