More examples just to drive the concepts home.
Canabalt -> Bit Trip Runner -> VVVVVV -> Super Meat Boy -> Donkey Kong Country Returns -> Super Mario Bros. The degree to which platformers are linear is perhaps the easiest to assess. We typically measure linearity of platformers by the number of paths available to the player and how tight the timing windows for overcoming obstacles. Keep in mind that platforming challenges use the dynamic of space(typically 2D). So slight variations in timing and execution change how large the timing windows of success are. However, in terms of determining lineary gameplay, these slight variations can be inconsequential. If the goal is to make it from point A to point B along a single path filled with moving hazards that players must overcome one at a time, we don't count such a challenge as non-linear gameplay simply because players have control over when they push through each obstacle.
Canabalt is an auto-runner, a sub genre of platformer where the player character automatically runs forward. In this game, the player JUMPs from building to building. You have a short JUMP and a taller JUMP if you hold the button down. Though randomized, the gameplay is very linear. Just JUMP the gaps as they appear and over obstacles as they appear. There's just one button, one mechanic, and one strategy. There's little control and therefore little strategy to this game of survival.
Bit Trip Runner is like Canabalt in that it's an auto-runner. However, Bit Trip offers more layered level design with optional challenges. Specifically, the gold "coins" players can grab highlight the alternate paths players can take. Each level is choreographed around the music. And like Rock Band Blitz the gold "coins" are a lot like the alternate tracks players can opt into. So players can experience different sequences of actions depending on how you play. It may not seem like much, but it's enough to make the levels non-linear.
VVVVVV is a platformer with no JUMP mechanic. Instead players can FLIP the gravity whenever Captain Viridian is standing on a solid surface. The enemies in this game are not very dynamic like in Mario. You can't kill them or do anything to affect them. They don't see the player or alter their routine in any way. You simply avoid them. Also, there are no coin-like objects to collect to give the platforming gameplay a layer of optional challenge. Though I like the variety of challenges in VVVVVV, the micro level gameplay is very linear in that there is mostly just one path to navigate through each room. The macro level of VVVVVV is largely a series of linear areas connected to a non-linear hub area. Because you don't acquire any new abilities throughout the game, players are able to access and complete these areas in different orders.
Super Meat Boy's gameplay generally offers much more freedom in terms of platforming paths. With a very large JUMP distance and the ability to wall-JUMP, players can often find different ways through complex levels. While some levels are very much like VVVVVV where there is one clear path that's the only path players must take to succeed, there are many levels that offer multiple paths. There are also optional challenges that can vary up the sequence of actions and challenges players may take on (bandages and warp zones).
It's important to note that with Super Meat Boy and even VVVVVV to a degree, simply not being propelled by an auto-runner design let's players explore a great variety of different platforming paths and possibilities overcoming even a single obstacle. This in itself is highly emergent because it uses the dynamics of space and time to create variable and complex challenges in a way we can intuitively understand. Instead of the same set up every time (act here at this time or lose), you can move through the space in different ways to alter the challenge. Like I mentioned above, we're looking closely at the sequence of actions and obstacles not the variation with which they can be done.
Donkey Kong Country Returns has a lot of gameplay layers packed into each level. Though you still move from start to finish by moving to the right, there are many secrets, optional collectables, and alternate paths to engage with. So there are many sequences of actions players can take and still reach the end successfully if they explore these optional challenges. On top of these design elements, many levels in DKCR are designed with speed running paths that allow players to bypass playing through the level normally if they have the skills. Playing solo players can collect enough hearts to play as DK and Diddy to take advantage of Diddy's jetpack. New paths and possibilities open up as long as you hold on to this powerup. The game is a great example of non-linear platforming gameplay. But its core design is modeled after the king of platformers.
Super Mario Brothers for the NES is the game that I continually reference on this blog because its solid design holds up to my increasingly exacting analysis. With it, Nitnendo pioneered designing levels with the layers of level challenges, enemies, coins, and secrets. Unlike most platformers then or since, the back and forth interplay with the enemies coupled with the destructibility of the levels gives Mario's gameplay a lot of significant variation. Enemy challenges change when some bricks are destroyed. Kicking Koopa shells can be as offensively powerful as they are dangerous when they rebound. The whole game is designed to create new and interesting challenges based on the subtle and nuanced ways you play and stumble. Add on top alternate paths, solid powerup design, and the ability to warp across worlds, and you can see how the particular type of micro level emergence gameplay in Super Mario Brothers is unmatched while it's macro level campaign is non-linear as well.
Z-rox -> Tetris DS puzzle mode -> Planet Puzzle League puzzle mode -> Boxlife -> Braid -> Portal -> Mario vs Donkey Kong. Just like the rhythm-action genre, puzzle games thrive on linear challenges. By engaging the mind and stressing knowledge skills almost exclusively, puzzle games present scenarios that often challenge players to solve challenges with one solution.
Z-rox is a puzzle game that challenges players to read encoded moving symbols and input the solution, which is a single key on a standard PC keyboard. As simple as the game is, it gets quite difficult. This is as linear as puzzle challenges and puzzle games get. Each level has only symbol and one keystroke for the solution. Players progress through the campaign one level at a time with no way to skip around or take it in any alternate sequence.
Tetris DS puzzle mode and Planet Puzzle League puzzle mode are also linear or nearly entirely so. Most of the challenges have a single solution sequence, but some feature multiple solutions. All the puzzles in Planet Puzzle League in particular are designed to be completely solvable in turn-based fashion. If players so choose, alternate and possibly elegant solutions can be found by manipulating the blocks in real-time to create and catch additional opportunities.
Games like BOXLIFE, Pushmo, and many sliding block puzzle games are more challenging to categorize on the scale of linearity. In many of the challenges in Boxlife there are multiple solutions or combinations of boxes to cut out that will get the job done. Because the puzzle mode for these games don't have fail states (or don't have to be played in a way that forces players to reset the stage to try again), while finding the solution the puzzle challenge can be remixed and reread. It's like getting a different perspective on the same problem. I call this double layered reading. The bottom line is for these puzzle games there isn't a fixed set of steps players need to take to solve the puzzles. So the gameplay can be non-linear depending on the level.
Many puzzles in Braid and Portal have elegant solutions. This means that there is more than one way to solve the puzzles and therefore one of them is more elegant, requiring less real-time skill (timing and reflex) and increased ingenuity (knowledge skills). Simply having multiple solutions makes gameplay non-linear. Mario vs. Donkey Kong is a puzzle-platformer in the purest form. This game features non-linear gameplay design that's like Super Mario Brothers (presents, 1ups, enemies, altering environment) and non-linear gameplay design that's like Braid or Portal (elegant solutions).
There's more to linear and emergent game design to uncover. In part 4 we'll look at the nature of linear gameplay and the issues inherent in emergence.