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XPK Technical Documentation
XPK is a raster graphics format. The format is not only used in Purple Moon games; Presage Software seemed to use it in other games from the same era. In fact, Anthony Kozar and Toastline have previously described the XPK format as it's used in Lode Runner 2. However, there's a key difference between how XPK is used in Lode Runner 2 versus Purple Moon's games ("C02" versus "B01") so I'll try to describe the entire format in this document, without requiring the reader to consult Toastline's research.
A single XPK file may specify one or more individual graphics. For example, it may specify a single still image, or perhaps the individual frames of an animation. It's not currently well understood how the multiple-image format works, so this document currently describes the structure of a single image. With that in mind:
- A single XPK graphic describes a raster of pixels.
- Each pixel is either fully opaque with a color, or fully transparent. ("Boolean" transparency, no alpha blending to worry about.)
- When decoding the image, one starts with a fully transparent canvas, and then fills in some, all, or none of the canvas with "paint" according to the instructions listed in the XPK file. I.e., there's no such thing as "transparent paint."
- The dimensions of the canvas, and other such metadata, is accessible in a header which precedes the drawing instructions.
- Color information is never stored directly in a XPK. Instead, colors are identified only by an index in a color lookup table (CLU). Therefore, one must "attach" a CLU to a XPK in order to properly decode the XPK.
Luna Purpura has not consulted source code nor documentation of any kind from Presage Software. Therefore, the names of instructions are entirely Luna Purpura's invention.
| Prefix | Name | Description |
|---|---|---|
| 0x0 | XPKINST_REPEAT |
|
| 0x2 | XPKINST_XSKIP |
|
| 0x3 | XPKINST_BIGXSKIP |
|
| 0x4 | XPKINST_RLE |
|
| 0x5 | XPKINST_BIGRLE |
|
| 0x6 | XPKINST_DIRECT |
|
| 0x7 | XPKINST_BIGDIRECT |
|
| 0x8 | XPKINST_LINEREPEAT |
|
| 0xA | XPKINST_SETXNL |
|
| 0xB | XPKINST_BIGSETXNL |
|
| 0xC | XPKINST_RLENL |
|
| 0xD | XPKINST_BIGRLENL |
|
| 0xE | XPKINST_DIRECTNL |
|
| 0xF | XPKINST_BIGDIRECTNL |