3O Format
| Format type | 3D Model |
|---|---|
| Model type | Polygon |
| Maximum number of polygons | 400 |
| Maximum number of vertices | 938 |
| Games |
The 3O Format is used by Chasm: The Rift to store most of the game's 3D objects, such as scenery and collectible items. Monsters and the player use the more complicated CAR Format instead.
! Figure out flags and animation. Animations seem to require a separate .ANI file to be associated with the object.
There aren't any headers, footers or "magic number" in this format. There are, however, some values sandwiched between the vertex and skin data that one might expect to appear in a header instead.
The data in the 3O is divided into three distinct sections, which appear at fixed positions in the file, and only the skin data is allowed to vary in length. Unused entries in the polygon and vertex arrays are generally filled with junk data.
Based on the amount of space alloted to the arrays, the upper limit of the format ought to be 938 vertices and 400 polygons.
Skins are always 64 pixels wide.
| Offset | Type | Description |
|---|---|---|
| 0x0000 | Array | Polygon array. |
| 0x3200 | Array | Vertex array. |
| 0x4800 | UINT16LE | Number of valid vertex entries. |
| 0x4802 | UINT16LE | Number of valid polygon entries. |
| 0x4804 | UINT16LE | Skin height |
| 0x4806 | 64 × skin height array of UINT8 | Skin image, each byte defines a pixel by palette index |
Polygon definition
This format uses quads, rather than triangles. Each polygon uses 32 bytes.
| Relative position | Type | Description |
|---|---|---|
| 0x00 | UINT16LE | Index of first vertex |
| 0x02 | UINT16LE | Index of second vertex |
| 0x04 | UINT16LE | Index of third vertex |
| 0x06 | UINT16LE | Index of fourth vertex |
| 0x08 | UINT16LE | U coordinate of first vertex |
| 0x0A | UINT16LE | V coordinate of first vertex |
| 0x0C | UINT16LE | U coordinate of second vertex |
| 0x0E | UINT16LE | V coordinate of second vertex |
| 0x10 | UINT16LE | U coordinate of third vertex |
| 0x12 | UINT16LE | V coordinate of third vertex |
| 0x14 | UINT16LE | U coordinate of fourth vertex |
| 0x16 | UINT16LE | V coordinate of fourth vertex |
| 0x18 | Four bytes | ! Unknown, always zero? May be filler. |
| 0x1C | Byte | ! Usually 0x01, but sometimes 0x02, 0x03 etc. May be a grouping mechanism of sorts? Changing the value doesn't seem to have an effect on non-animated objects. |
| 0x1D | UINT8 | Bit properties.
! What do the other bits do? What's the deal with the high bits making the poly invisible? |
| 0x1E | INT16LE | Appears to be a UV mapping offset, value added to the V coordinates.
If this directs the mapping outside of the skin data, other data may appear on the model as a skin, causing a garbled appearance. |
Vertex definition
Each vertex in the array uses 6 bytes. The first two coordinates position the vertex on the horizontal axes, while the third is the vertical position.
| Relative position | Type | Description |
|---|---|---|
| 0x00 | INT16LE | X coordinate (signed) |
| 0x02 | INT16LE | Y coordinate (signed) |
| 0x04 | INT16LE | Z coordinate (signed) |
Early version
The Chasm Demo Test v1.02 (Entering Shadow Zone) uses an earlier version of the format. It is almost identical, but there are two important differences:
- The polygon array reserves space for 300 polys (100 fewer than the full version). Vertex array therefore begins at 0x2580.
- While not related to the 3O format per se, this version has a slightly different palette.
Therefore, it is possible to get a bare-bones readable conversion between the two formats by adding/truncating entries in the file's polygon array, but a GOOD conversion will also require fixing the skin graphic to account for the palette differences.