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Background/Arena File Format (.BK)
The BK file format is similar in many ways to the AF file format, however, there are some significant differences.
Link to complete data from the different .BK files: https://katajakasa.fi/projects/openomf/cards/
Header
| Unknown | 1 DWORD | 0 for INTRO, 2 for ending cutscenes,4 for main menu/newsroom/VS/Mechlab/melee, 6 for creditsm 8 for Arena0, 16 for Arena1, 32 for Arena2, 64 for Arena3 and 128 for Arena4 |
| Unknown | 1 BYTE | ? 1, 2 or variable |
| Width | 1 WORD | Background width (always 320?) |
| Height | 1 WORD | Background height (always 200?) |
Animations
Each animation is preceded by the offset of the next animation stored as a DWORD, along with an animation number. Reading the animation and advancing the file pointer should result in the file pointer being the same as the next animation offset. An animation number >= 50 marks the end of the animations section. When this occurs, the next animation offset is set to the offset of the current animation.
| Next animation | 1 DWORD | Position of next animation relative to start of file, in bytes |
| Animation number | 1 BYTE | Unique identifier for animation |
Animation Header
| Unknown | 1 BYTE | Nul padding? (not for fire pit wandering orb) |
| Unknown A | 1 BYTE | Animation to chain to if collision/hit (eg. fire orb 15 → fire ball 17) |
| Unknown B | 1 BYTE | Animation to chain to on no collision/hit(?) (eg. fire orb 15 → fire orb fade 16) |
| Repeat | 1 BYTE | 255 for animations to play on BK start (eg. spotlights on main menu, torches in fire pit, newscaster's lips/eyes), Desert animation (spawn the planes) 0 has 254 in this field. |
| Probability | 1 WORD | How likely this is to happen, lower is higher probability, 0 means never. 1 for torches/'cloud of dust' in fire pit, 200 for 'cloud of dust' in desert, 1000 for spikes in spike pit. For example, setting this to 1 for animation 0 in the desert means that planes will always be strafing you. |
| Unknown E | 1 BYTE | Arena hazard damage (eg. spike pit, fire pit fireball, electric walls in powerplant)? |
| Anim string size | 1 WORD | |
| Anim string | N BYTES | Animation string, size is determined by the previous value |
There then follows an AF Animation Structure.
Big-Footer
After the animations there is a footer structure:
| Background image | W*H BYTES | W and H are the width and height in the file header |
| Num palettes | 1 BYTE | Specifies the number of palettes which follow |
Palettes
| Palette data | 256 * 3 BYTES | A palette |
| Palette remapping tables | 19 * 256 BYTES | See below |
The palette remapping tables are maps from image colour indices into palette indices, for various different colour effects. The tables are for:
| Index | Purpose |
|---|---|
| 0-3 | Fade to black, for shadows |
| 4 | Dark blue, for menu backgrounds |
| Most BK files | |
| 5-8 | Saturating to white, then black-and-white (newsroom transition?) |
| 9-13 | Saturating light blue → white (lightning flashes?) |
| 14-18 | Green (?) |
| Fire pit [3 palettes: normal, fire, ice] | |
| ?? | TODO |
| Desert [4 palettes] | |
| 5-18 | Slow fade to white |
Rest of footer
An AF Footer follows.
Sample Implementations
- Deleted on request of Vagabond
