From WTFwiki
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
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| Unknown | 1 BYTE | ? 1, 2 or variable
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| Width | 1 WORD | Background width (always 320?)
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| Height | 1 WORD | Background height (always 200?)
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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
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| Animation number | 1 BYTE | Unique identifier for animation
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| Unknown | 1 BYTE | Nul padding?
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| Unknown | 1 BYTE |
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| Unknown | 1 BYTE |
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| Unknown | 1 BYTE |
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| Unknown | 1 WORD |
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| Unknown | 1 BYTE |
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| Something size | 1 WORD |
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| Something | N BYTES | Size is determined by the previous value
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There then follows an animation structure (see .AF file format).
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
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| Num palettes | 1 BYTE | Specifies the number of palettes which follow
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Palettes
| Palette data | 256 * 3 BYTES | A palette
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| Palette remapping tables | 19 * 256 BYTES | See below
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The palette remapping tables are maps from image colour indices into palette indices, for various different colour effects. The tables are for:
| Index | Purpose
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| 0-3 | Fade to black, for shadows
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| 4 | Dark blue, for menu backgrounds
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| Most BK files
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| 5-8 | Saturating to white, then black-and-white (newsroom transition?)
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| 9-13 | Saturating light blue -> white (lightning flashes?)
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| 14-18 | Green (?)
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| Fire pit [3 palettes: normal, fire, ice]
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| ?? | TODO
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| Desert [4 palettes]
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| 5-18 | Slow fade to white
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Sample Implementations
Sample python parser for .af and .bk files
