It seems as though, according to a Half-Life 2 forum user, there is a DirectX 9.0 bug in Half-Life 2. “There are problems with the way that current hardware implements FSAA. If you enable it, you will see a lot of artifacts on polygon boundaries due to the way that they sample texture subrects with FSAA enabled. We are working with the harware companies and the DirectX team to make sure that future hardware doesn’t have this problem.”
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