Taking Killzone into 3D doesn't simply mean more impressive visuals. For Guerrilla Games it means having to worry about performance hits, where to put the game's cross-hair and whether 3D gives players an unfair advantage in multiplayer.
Yes, Killzone 3 in 3D looks great. The sensation of depth is very convincing and, yes, missiles and bullets really do feel like they're whizzing past your ears. Indeed, it's a more persuasive showcase for the technology than we've previously seen from Sony through the likes of Motorstorm and Wipeout.
Perhaps it's because Killzone 3 was built from the ground up to be in 3D, not to have it shoe-horned in later. I asked Guerrilla's Steven Ter Heide, senior producer on Killzone 3, what that meant for the design to be able to build a 3D game from scratch.
"Everything that we have is already 3D suitable, in that everything has a front and a back, there are no cardboard walls with nothing behind. So everything is really there and has volume," says Ter Heide. "In terms of content and the level layouts, that's a 1:1 translation really, from the '2D' game to the 3D, we didn't make any adjustments in that respect."
What Guerrilla did discover, however, was that 3D meant they had to re-think something as seemingly innocuous as the HUD. The team had to think about where all those HUD elements live on the screen; they can no longer just sit there like a 2D overlay, they have to exist somewhere in this 3D space.
"There's a very subtle thing that you don't notice at first with the cross-hair," says Ter Heide. "With 3D the amount of information that you have to take in is a lot more - you're focusing on a certain area, you're looking into the depth - whereas with '2D' you're taking in the entire image. The area around the cross-hair is really important because you're so much more focused on that area. The cross-hair has to behave perfectly and always has to be at the right level of depth if you are able to understand where things are. If it's constantly here [holds his hand in front of his face]then it's weird, it's a very jarring effect.
"What we had to do was devise a system that scans how much depth there is and we adjust the layer of depth that the cross-hair sits at. It constantly looks at the environment and says, okay, now it needs to be in front of this rock. If you're in front of a wall or if there's a beam moving past then it's very subtle, the cross-hair just glides in front of it, so it'll just go into the depth very smoothly. It's something you don't notice at first, but when you start thinking about it you realise this is something we needed to do otherwise it simply wouldn't work."
Of course, the 3D world isn't just static objects whose location is always easy to predict. Being a first-person shooter, Killzone 3 is full of enemies, in both singleplay and multiplayer, and when played in 3D the cross-hair must instantly shift depth to sit in front of a target whenever someone walks in front of you.
Surprisingly, Ter Heide tells me this, once they realised they needed to do it, is a relatively simple thing to code. All they do is draw a line out from the camera - ie. ray casting - and the first thing it hits, that's the level of depth the cross-hair needs to be at.
Although a simple piece of code, it's not without a performance hit. That line has to be drawn many, many times per second to ensure the cross-hair is always in the right spot at the right level of depth. When not running in 3D, the game doesn't need to make those calculations. So one of the areas Guerrilla is really working on right now is the performance of the 3D version.
Right now, the 3D version is running slower.
"Yes, because we have to put two images up and we have to do things like the ray casting," Ter Heide explains. "So the tech team back home is working very hard to make sure we get the performance right. And as soon as we get that performance where we want it to be, we can start adding more visuals effects that Killzone is known for - the filters, the blurs - and see how much of that stuff we can put in the 3D version as well.
When it comes to performance, Guerrilla may be able to regain some ground if the 3D version doesn't even require some of the visual effects in the first place. Ter Heide says that a lot of the visual effects games use these days are about emulating 3D. When your game is already in 3D, why would you emulate it?
"For example, with motion blur, it just looks weird in 3D because it now needs to have a 3D effect to it," he explains. "Unlike when everything was in a '2D' place, you no longer need that motion blur because you can see where things are going. In 2D it gives you a sense of direction, but in 3D the direction is already there."
It's the type of thing that's evident in those aforementioned missiles and bullets whizzing past your ears, something that Guerrilla is keen to take full advantage of whenever they can. I asked Ter Heide whether they took the opportunity to orchestrate set-pieces or design certain aspects of the environment to show off the fact the game is now in 3D.
"In the first section we showed back at E3, where you first come into the ship and just after you fight off the jetpack troopers, there are two guys at the back with rocket launchers. At the very moment they fire those, we render out those rockets and they just zoom past your head," enthuses Ter Heide.
"That's a typical 3D effect, stuff coming at you, that's what everybody knows about 3D. And that works really well, but that scene was already in the normal game as well. The 3D effect enhances that stuff flying at you. It's not a case of us having to design things around it, because the game already does those kinds of tricks."
Having played Killzone 3 in 3D, I can say with authority that it does feel more immersive. I found myself better able to judge distance, while at the same I was reacting more strongly to explosions and being under fire. I wondered whether Guerrilla had noticed any differences in the way people played the game in 3D?
"I've had people come up to me and say they feel they're able to read the environment much better in 3D. They think they may have an advantage when playing in 3D," says Ter Heide. "But other people say, yeah, it looks nicer and they feel like they're part of the world... but they still have to shoot the enemies and the difficulty still feels the same."
It turns out that whether or not 3D gives you an advantage is a major concern for the team. Guerrilla is undergoing a rigorous playtesting period right now to ascertain whether that is the case.
"3D is something that will be game-wide, so we have to know if it'll be an advantage in multiplayer as well," says Ter Heide. "When you have a multiplayer game with somebody 3D, somebody HD and somebody still SD, it needs to be a level playing field. So that's something that we have to carefully test. I think it's probably a similar leap as from SD to HD. With SD, there's just not enough information on the screen for you, while the HD people are able to pick out somebody in the distance. Maybe the same goes for 3D? I think it might be. It's something that we have to test, though."
Killzone 3 is coming to PS3 in February next year in 3D, HD and SD.