Background/Arena File Format (.BK)

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The BK file format is similar in many ways to the AF file format, however, there are some signifigant differences.


Unknown 1 DWORD 0 for INTRO, 2 for ending cutscenes,4 for main menu/newsroom/VS/Mechlab/melee, 6 for credits,

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?)


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 repeating animations (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)?
Something size 1 WORD
Something N BYTES Animation string, size is determined by the previous value

There then follows an AF Animation Structure.


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


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]
Desert [4 palettes]
5-18 Slow fade to white

Rest of footer

An AF Footer follows.

Sample Implementations

Sample python parser for .af and .bk files