Background/Arena File Format (.BK)

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

Header

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?)

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	Non zero for wandering orb, ???
Unknown B	1 BYTE	Non zero for wandering orb, ???
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 animation structure (see .AF file format).

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

Unknown

30 BYTES

Sample Implementations

Sample python parser for .af and .bk files