Released a new game: O, two players only, for iPad, get it on appstore. It's half price for a week, and so is Glitch Tank.
It's very natural when creating a videogame for more than one player to clearly separate their inputs. In games like Shot Shot Shoot and Glitch Tank, even though players use the same input device, there's a clear delineation of zones - here's my side of the screen, here's yours, and a DMZ between where neither one of us touches. Within the simulation we have a defined formal interaction, and we refrain from coming into contact outside of it.
There are exceptions to this. A Bastard makes the keyboard a shared input space, randomly remapping keys after every move, sometimes coinciding with your opponent's keys (George is working on a touchscreen version too - keep an eye out for it). Centrifeud mostly keeps to clearly separated input zones, but the occasional cry of "touch yourself" directs players to reach into the shared central playing field. Greedy Bankers has two players grabbing gems from each other's side of the touchscreen. Bloop has separate input zones for each player, but they're intermingled across the screen making it difficult not to collide.
When inputs overlap like this, it invites players to interfere with each other physically. But there's no reason why we cannot interact physically in any game. David Hayward has told me that sometimes when he plays Glitch Tank, if his opponent is hesitating over an action, he will bump the ipad up into their finger to hurry them along. You could also just reach over and press one of your opponent's buttons, or physically restrain them so that they can't interact with the game at all. The purest form of this is Chicanery, in which there's no way to win other than physically removing your opponent's fingers from the controls by any means possible. This would be an optimal strategy for any competitive game, if we allowed ourselves to use it.
In many physical sports there is unavoidable overlap in input domains, and guidelines have arisen restricting physical violence; protocols of what's considered good sportsmanship; at a professional level these are enforced by referees. Different expected levels of physicality - non-contact, semi-contact, full-contact. What degree of violence is appropriate depends on the players and the game, but in general crippling your opponents is avoided despite being a dominant strategy. As we develop the space of digital games with overlapping inputs I hope that similar protocols will emerge; taking into account having some level of physical interaction without degenerating into violence. If not, every such game will be played as a Chicanery variant - and while Chicanery is great, that would be less interesting than having a variety of different games.
O came out of a discussion with Jonathan Brodsky, he suggested a variation on Glitch Tank splicing in some ideas from A Bastard: instead of keeping action buttons separated at opposite edges of the screen, have cards spawn randomly across the map which either player can grab and drag to use them. This inspired me with a similar idea; omit the tank element and just have a kind of set-collection game. The idea was beautiful to me so I dropped everything to make it as quickly as possible.
Balls appear on the screen in three colours. You score points by collecting a sequence of balls of the same colour - the first is worth 0, then each after that is worth 1 more. Triangular numbers. This score system ensures that it's not simply a matter of grabbing balls indiscriminately; you get a much higher score by sticking to one colour than by taking anything you can get.
It's very elemental. It feels a bit like a two-player variation of Eliss, it's one of the purest non-trivial touchscreen games.
Both players share the entire touchscreen as their input space, with no restrictions on where each player may touch. As above, this invites physical interference. You will have to work out for yourselves what level of physical interaction is appropriate; there's a lot of depth on the digital side of the game that can be trampled right over and missed out on if you get too violent, but some level of physical push and shove is unavoidable when you're both trying to grab the same balls. I'm not going to tell you how you're 'supposed' to play, but I think it's a lot more interesting when both players actually get to. Think of it as a low-contact sport, like Netball where you're not allowed to grab the ball out of another player's hands - that's partially enforced in O; while someone is touching a ball it won't respond to other touches (you could physically prise an opponent's finger from the screen, but this is not recommended: the natural response is to push harder, and that way risks breaking something). So as a possible guideline: don't prevent your opponent from touching the screen, and don't force them to release a ball that they're touching.