View source for 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==

{| border='1'
|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.

{| border='1'
|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 ===

{| border='1'
| 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:

{| border='1'
| 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 ===

{| border='1'
| 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:

{| border='1'
! Index !! Purpose
|-
| 0-3 || Fade to black, for shadows
|-
| 4 || Dark blue, for menu backgrounds
|-
!colspan=2| 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 (?)
|-
!colspan=2| Fire pit [3 palettes: normal, fire, ice]
|-
| ?? || TODO
|-
!colspan=2| Desert [4 palettes]
|-
| 5-18 || Slow fade to white
|}

=== Rest of footer ===

{| border='1'
| Unknown || 30 BYTES ||
|}

== Sample Implementations ==

[[PythonAfBkParser|Sample python parser for .af and .bk files]]