Westwood CMP Format
| Format type | Image |
|---|---|
| Hardware | EGA |
| Colour depth | 4-bit, 8-bit |
| Minimum size (pixels) | 320×200 |
| Maximum size (pixels) | 320×200 |
| Palette | Shared |
| Plane count | 1 |
| Transparent pixels? | Palette-based |
| Hitmap pixels? | No |
| Games |
The CMP Format is an older version of what would later become the CPS format. It is a 4-bit or 8-bit compressed file which always contains a 320x200 image. It contains no palette.
Like the CPS format, these files tend to be used both for fullscreen 320x200 images, and for collections of sprites.
Note that the .CMP extension is also used by the Lands of Lore SHP format, but that article was named "SHP Format" because the described format is a sprite format, like all other Westwood SHP formats. Both "CMP" and the later "CPS" extensions probably just refer to the file being "compressed".
File format
Header
The file starts with the following header:
| Data type | Name | Description |
|---|---|---|
| UINT16LE | FileSize | File size. This only counts the bytes behind this value, making it two bytes less than the actual file size. On type 4, it is three bytes less than the full size. |
| UINT8 | Type | File type. Determines the bits per pixel and the compression scheme? See the list below. |
For types 3 and 6, this header is followed by another UINT16LE that indicate the length of the uncompressed data. This should always be 64000.
The image data follows immediately after this header.
Image data
The image data can be in different formats, depending on the Type in the header:
| Type | Colour depth | Compression |
|---|---|---|
| 1 | 4-bit | Westwood RLE (horizontal). Reads the extended 16-bit integer repeat values as little-endian. |
| 2 | 4-bit | Westwood RLE (vertical). Reads the extended 16-bit integer repeat values as little-endian. |
| 3 | 8-bit | Westwood LZW-12 |
| 4 | 8-bit | Westwood RLE (horizontal). Reads the extended 16-bit integer repeat values as big-endian. |
| 5 | - | Unknown, probably unused. (In DragonStrike, any value besides 3, 4 and 6 seems to default to loading as type 4) |
| 6 | 8-bit | Westwood LZW-14 |
If the type is "2", the decompressed bytes should be painted onto the image per column. Do note that that type denotes a 4-bit image, so each such column is two pixels wide.
To handle this type, the game decompresses the data straight into the screen buffer vertically using a separate decompression function. For modern applications, it is probably simpler to swap the rows and columns as post-processing step, since that means both compression types can use the same decompression code.
The vertical compression is consistently used on images where compressing the bytes per column results in a lower file size.