The ATI drivers are not open-source. AMD/ATI ships proprietary, binary, closed-source drivers for Windows (Catalyst) and Linux (fglrx) and they're very very much not open-source. There are open-source drivers for linux based on the specifications they released, though. Nothing for windows. So-called third-party drivers (Omega drivers, etc) are the same exact drivers with some registry tweaks enabling sometimes hidden functionality.
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