An open source game for PalmOS inspired by Space Invaders
Interlopers is inspired by the classic Space Invaders, but expands upon that game greatly. The objective is to prevent slowly descending waves of aliens from reaching the bottom of the screen (by shooting them of course - how else would you do it?). You have barriers to shield you from the aliens' fire, but then hiding will also give them more time to advance downwards.
The aliens will occasionally drop money when shot, which can be spent on weapon upgrades and special equipment in the between-level shop. Of course, the aliens counter this with improvements of their own in later waves...
Interlopers is free software as described by the GNU General Public License (v3). See LICENSE.md for details.
The game was originally released as shareware on mobilegamelab.com. The open source version has had the registration check disabled so everyone can enjoy the full game.
Interlopers comes in 3 versions:
| Version | Hardware |
|---|---|
| interlopers_color | 8-bit color devices |
| interlopers_gray4 | 2-bit grayscale devices |
| interlopers_gray16 | 4-bit grayscale devices |
All require OS 3.5+
When destroyed, the aliens will either drop money or one of two power-ups. One recharges your shield (displayed at the top-right of the screen) and the other provides energy for your special equipment (top-left).
Interlopers allows you to start on any previously reached wave (see the settings screen). However, on later waves, the starting weapon would be of little use against the stronger aliens.
So, the game records what weapons and equipment were used on every wave (separately for each difficulty level). When starting a new game, you automatically get the best arsenal used in previous games. This shows up as a penalty in the final score.
In addition to your main weapon, you have access to a variety of other useful items via the shop. For example, you start with a teleporter that sends you to a random location - useful for avoiding enemy fire. Other items allow you to repair your shield or repel enemy bullets, among other things.
Special equipment requires energy to run, which is in limited supply. Your current energy is displayed at the top-left of the screen, next to your money and shield. Energy can be restored by collecting star power-ups that the aliens sometimes drop.
The controls can be reconfigured by selecting Keys on the title screen or from the menu, but are set up as follows by default:
| Button | Action |
|---|---|
| Date Book | Move left |
| Address Book | Move right |
| To Do List | Fire |
| Memo Pad | Use special equipment |
| Up | Fire |
| Down | Pause |
Additionally, the stylus can be used to control player movement.
A note on "Allow Key Events" - If checked, this allows the OS to generate events when hard keys used by the game are pressed. This can cause conflicts with 3rd party software, but is needed for some devices to function correctly (e.g. the Treo 600's power button).
Diffculty controls the speed of the aliens as well as how often they fire at you. On Easy mode, trade-in values at the shop are increased to 100%. Nightmare difficulty doubles the damage of enemy fire and generates already-damaged barriers. High scores and starting arsenal are stored separately for each difficulty setting.
Changing the difficulty does not affect games in progress.
Start Wave determines on which wave new games will begin. Only waves that have been previously reached are available. When starting new games beyond the first wave, Interlopers will automatically select the best weapon and equipment item that were previously used on the starting wave.
| Setting | Description |
|---|---|
| Sound Volume | Affects the volume of sounds played by the game. This will override the system preferences unless set to "System Setting". |
| Pen Control | Determines how the game responds to pen taps in the display area (but not the Graffiti area). |
| Confirm Retire | If checked, causes the game to ask before abandoning a game in progress. |
| Quick Restarts | If checked, causes "Try Again" to be automatically selected after you die. |
| Graffiti Control | Allows pen taps in the Graffiti area to control player movement when checked. This is not affected by the Pen Control setting. On some devices, it is possible that the Graffiti area might be detected incorrectly. |
| Compatibility Mode | If checked, alters some of the game's display code, such that it is more likely to function on all devices. Otherwise, faster assembly routines are used. On most newer devices, this option is automatically turned on and will not be displayed. |
Interlopers was originally developed with PRC Tools, which unfortunately has not been maintained for modern operating systems. An easy alternative to build it is to use prc-tools-remix. The project offers pre-built binaries for 64-bit Ubuntu/Debian systems and a convenient setup script to install the Palm SDK.
Run make color, make gray, and make gray4 to build a .prc file for each
of the hardware variants. Transfer the .prc file to a device with appropriate
graphics hardware or the Palm OS Emulator.