GravSlider       App Store
GravSlider is a game that defies gravity. You tilt the iPhone/Pod/Pad to move a frictionless "slider" around the playing surface in an attempt to put it on target. There are multiple levels of difficulty, and a scoreboard for showing off. Normal rules of motion are obeyed, except for gravity. (Although, if you want to try some gravity, you can do that too.) It started out as a serious experiment with the internal accelerometers, but then it evolved into a challenging game based on the user trials done during the experiment.
While holding the iPhone "normally", say at 45 degrees in front of you, the slider (a thing that seems to slide around on the screen) can be moved by tilting the device front to back and side to side. The main idea of the game is to repeatedly move the slider into the target box and keep it there for about a second. It might not seem hard on low levels, with lots of anti-gravity to help, but it gets much harder on higher levels. You can score significantly more points per target when playing at higher levels, so it's worth it to try if you want to beat a score on the scoreboard.
A Few Details of Game Play
Game play can take place whenever the Play button is enabled (not dim). Pressing the Play button starts a scored game. Pressing it while playing will stop and cancel the game. During a game the current score and remaining game time are shown in the target box. The top scores appear on the central scoreboard, between games. The lowest score on the scoreboard will be displayed, in brackets beside the current score, when the current player is close to getting onto the scoreboard. (Note, the scoreboard can be reset by pressing "Reset SB" on the toolbar within the first few seconds after starting the App.)
When the border line around the playing surface is solid, the slider will stay in play no matter what happens. However, when the border appears as a dashed line, the slider can slide off the game surface. If the slider leaves the game boundary then points are deducted from the score, and the game continues, but with the solid border temporarily back up. Pressing the Lev+/Lev- buttons adjusts the level of difficulty prior to game play. Higher levels of play earn and loose more points per target.
A Bit of Technical Stuff
The slider moves as if it had mass and were on a frictionless surface, like a puck on an air-hockey table. The sliding action is caused by gravity, but not ALL of gravity. Signal processing is used to remove the constant "bias" of gravity, so that the slider does not just fall to the edge of the screen. Pressing the Anti-Grav button (so that it says GRAVITY) will turn off the bias removal processing and allow normal gravity to have an effect. When under normal gravity, the slider can still be balanced and held in the middle of the screen, but it is much more difficult.
Raw x- and y-axis acceleration signal data is continually read from the iPhone/Pod/Pad sensor sub-system, filtered through high-pass and low-pass digital filters, and integrated to produce the slider's velocity and position. The high-pass filter is used to remove low frequency motion, and in fact primarily serves to remove the constant gravity bias; this is the anti-gravity filter. The low-pass filter is used just to remove higher frequency hand-jitter or other motion. Higher levels of game play result from the anti-gravity filter being set so that it just doesn't work so hard on your behalf.