===== Background/Arena File Format (.BK) =====

|[[omf2097_file_formats|<back]]|

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|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#big-footer|AF Footer]] follows.

==== Sample Implementations ====

  * Deleted on request of Vagabond