Actually, Valve is doing much more than just making maps and storylines.
Episode 1 saw a major revision to the source engine (HDR) and Episode 2 has a similar update to the engine so it remains 'current'. Also the programming behind portal and the changes to the engine it requires (orginally in Source you could never see your character). So dont think this is just new levels, it requires quite a lot of programming to get working properly.
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I stand corrected. The 'portal mods' I had seen before this all had the flaw of the player being unable to see themselves through a portal.
Actually, Valve is doing much more than just making maps and storylines. Episode 1 saw a major revision to the source engine (HDR) and Episode 2 has a similar update to the engine so it remains 'current'. Also the programming behind portal and the changes to the engine it requires (orginally in Source you could never see your character). So dont think this is just new levels, it requires quite a lot of programming to get working properly.