Mac users will be able to download Portal, Football Manager 2010 and Torchlight at launch, with more titles added every Wednesday.

Portal 2 co-op, story, gameplay details

Portal 2 features a separate two-player co-operative campaign, playable over the internet, and new gameplay features include physics-changing paint.

That's according to various reports and scans that have popped up since Friday's confirmation that Portal 2 is due out this year on PC and Xbox 360. According to Game Informer's latest preview, it's also heading to Mac.

A GameStop product page filled in some of the blanks. "Portal 2 introduces a cast of dynamic new characters, a host of fresh puzzle elements, and a much larger set of devious test chambers," it explained.

"Players will explore never-before-seen areas of the Aperture Science Labs and be reunited with GLaDOS, the occasionally murderous computer companion who guided them through the original game."

That's not all, either. Although it has subsequently been edited, the page previously confirmed co-op. "The game's two-player co-operative mode features its own entirely separate campaign with a unique story, test chambers, and two new player characters," it said.

"This new mode forces players to reconsider everything they thought they knew about portals. Success will require them to not just act co-operatively, but to think co-operatively."

Those details are confirmed in the full Game Informer preview, seen by Eurogamer, which reveals that the co-op campaign stars a paid of bipedal robots (one derived from a GLaDOS-style "personality core", the other from a turret).

Co-op will work online or in local split-screen, and includes various options to assist with communication, including a picture-in-picture mode.

The main single-player campaign will take place hundreds of years after the original Portal, although you still control main character Chell.

On the story side, the recent changes to the first game's end sequence hint at narrative continuity, confirmed in the preview, and Portal 2 will also absorb other details that may have seemed throwaway at the time, like the rows of GLaDOS-style personality cores illuminating the room with the cake during the last scene of the first game.

These cores were individual AIs, some of whom now play secondary characters you encounter in various contexts.

Although your gun will continue to fire just two portals, physical forces will now bleed between them. For example, a portal positioned below a powerful suction tube will maintain that suction across the portal threshold, affecting objects on the other side. There's also a tractor beam.

Perhaps most interestingly, there's even a range of paints that change surface friction and behaviour, which you can distribute through careful portal use. For example, you can paint the floor orange to increase the speed you move over it.

You will also get to see more behind the scenes of the Aperture Science facility. It sounds as though time hasn't been kind to the labs, and of course the further away from the shop floor you get the greater the chance of finding out more of what's going on in the story.

Finally, the preview confirms that Portal 2 will be significantly longer than its predecessor. Despite all the acclaim it attracted, Valve feels it was taking a chance on the original, whereas this time it is more sure of itself.

As marketing director Doug Lombardi puts it in Game Informer, "Portal was a testbed. Portal 2 is a game."