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?
Unknown	1 BYTE
Unknown	1 BYTE
Unknown	1 BYTE
Unknown	1 WORD
Unknown	1 BYTE
Something size	1 WORD
Something	N BYTES	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
Unknown	1024 BYTES
Unknown	3840 BYTES

Rest of footer

Unknown

30 BYTES

Sample Implementations

Sample python parser for .af and .bk files