In the early 90s, a trend developed among shoot-'em-ups that was affectionately (or was it rancorously?) dubbed "bullet hell."
- An example of bullet hell. It gets worse. Much worse.
The impetus behind it was to add visual flair to 2D games in order to compete with the craze surrounding 3D games. Arcade cabinets were more powerful than ever before, so these shooters could handle many more sprites and the extra calculations required for the accompanying collision checks. The approach worked relatively well, providing plenty of "Holy Shit!" moments. However, there was one major issue: playability.
Shoot-'em-ups tended to be one-hit kill games, and simply saturating the screen with harmful projectiles made them incredibly difficult (if not downright impossible). Now arcade games are meant to take your money, but that wouldn't happen if no one played 'em. In order to implement bullet hell without alienating customers, something had to give.
Part of Ikaruga's solution was to make all bullets and your ship one of two colours, and then simply ignore collisions between like-coloured objects. Other titles used shields and various powerups, but the original solution, and, in a way, the purest to the genre, was the (roughly) 1-pixel collision box.
- Ikaruga's dual colour scheme was the focus of its gameplay.
Instead of surrounding the majority of the player's ship with an area susceptible to fire, a single pixel was used to indicate its vulnerable spot. This was a rather elegant solution as it required no other changes and didn't present an extra hit to performance. Players were also less likely to feel cheated if they came out on the positive end of some collision-fiddling. The end result looked something like this.
The visual oddity of having the player's ship fly straight through harmful projectiles was lessened by the nature of the top-down perspective. This view had issues with representing depth/elevation, and that actually made it easier to imagine bullets just skimming over the player's ship. The 1-pixel collision box also had the side-effect of making the player feel more skilled at the game, which -- in the very least -- provided the illusion of empowerment.
Granted the 1-pixel approach is a bit of a simplification. All the games used slightly different collision techniques, but the concept itself was the same: make it difficult for the player to actually get hit.
For more information on shmups, please visit shmups.system11.org where a variety of threads dedicated to the genre's mechanics can be found.
Radek Koncewicz is the CEO and creative lead of Incubator Games, and also runs the game design blog Significant-Bits.