3D Model File Format

Binary, little-endian static (single-frame) model format.

.3D files are static (single-frame) models. For the animated multi-frame variant, see 3DC.md.

Versions

Four versions exist: v2.6, v2.7, v4.0, v5.0. See GOG vs Original CD Differences for which directories contain which versions.

• v2.6 / v2.7 — Different header semantics and face data encoding from v4.0/v5.0.
• v5.0 — Adds Section4 (SubObject BVH).

Header (64 bytes)

v2.6/v2.7 Header Differences

The header is the same 64 bytes, but many fields have different semantics:

v2.6/v2.7 Frame Data Differences

The 16-byte frame data record has different field semantics:

v2.6/v2.7 Face Data Differences

Parsers must remap header fields before use. See External References for the remap logic.

Engine version gate: The shipped engine rejects files where the version byte (offset 0x01) is less than ASCII '4' (0x34). This means v2.6/v2.7 .3DC files on the game disc are not loaded at runtime — they are development-era leftovers. The engine only loads v4.0+ files. External parsers (DaveHumphrey, RGUnity, this project) support both versions for preservation completeness.

Section Layout

Sections appear in this order (offsets from header):

1. Face Data — at 0x40, num_faces variable-size records
2. Vertex Coordinates — num_vertices × 12 bytes
3. Face Normals — num_faces × 12 bytes
4. Frame Data — num_frames × 16-byte records
5. Section4 — SubObject BVH entries (when present)
6. Vertex-Normal Indirection Table — total_face_vertices × u32
7. Vertex Normals — num_vertices × 12 bytes

Frame Data

Located at offset_frame_data. Array of num_frames × 16-byte records.

For .3D files this is always a single entry pointing to the base geometry.

vertex_offset and normal_offset point to the base vertex coordinate and face normal sections (same values as header fields 0x30 and 0x34).

Face Data

Located at offset_face_data (always 0x40). Array of num_faces variable-size records.

v4.0 / v5.0 Face Record

Texture Decoding (v4.0 / v5.0)

Solid color (if texture_data_raw >> 20 == 0x0FFF):

• color_index = (texture_data_raw >> 8) & 0xFF
• tex_hi is always 0xFF for solid-color faces.

Textured (otherwise):

• Texture file ID (BCD-like encoding):

  tmp = (texture_data_raw >> 8) - 4000000
  ones     = (tmp / 250) % 40
  tens     = ((tmp - ones*250) / 1000) % 100
  hundreds = (tmp - ones*250 - tens*1000) / 4000
  texture_id = ones + tens + hundreds
  

  This yields the TEXBSI.### file number.

• Image sub-ID within that TEXBSI file:

  ones = (texture_data_raw & 0xFF) % 10
  tens = ((texture_data_raw & 0xFF) / 40) * 10
  image_id = ones + tens
  

v2.6 / v2.7 Face Record

Texture decode from texture_data (the u16 at offset 0x02):

• Solid color if (texture_data >> 7) < 2: color_index = texture_data & 0xFF
• Textured: texture_id = texture_data >> 7, image_id = texture_data & 0x7F

FaceVertex

UV coordinates are cumulative deltas in 4-bit fixed-point format: each vertex’s absolute U/V = previous vertex’s U/V + this delta. First vertex in the face starts from 0.

The raw i16 values are in 1/16th pixel precision. To convert to pixel-space texture coordinates, multiply by 1/16 (or divide by 16). To normalize to 0–1 UV range, divide by 16 × texture_width (U) and 16 × texture_height (V). This fixed-point scale applies identically to first-vertex values and subsequent deltas.

Face Vertex Count Distribution

Vertex Coordinates

Located at offset_vertex_coords. Array of num_vertices × 12 bytes.

Each vertex is 3 × i32 (signed 32-bit integers), scaled by 1/256.0 to get world-space coordinates.

Face Normals

Located at offset_face_normals. Array of num_faces × 12 bytes.

Each normal is 3 × i32, scaled by 1/256.0. Per-face normals (one per face, not per vertex).

Vertex Normals

Located at offset_vertex_normals (header offset 0x2C). Array of num_vertices × 12 bytes.

Each entry is 3 × f32 (IEEE 754 single-precision). Per-vertex normals (unit vectors). Some entries may be NaN (bit pattern 0xFFC00000 for all three components), indicating no normal was computed for that vertex.

Vertex-Normal Indirection Table

Located at offset_normal_indices (header offset 0x28). Array of total_face_vertices × u32.

Each entry is a file offset pointing into the vertex-normal data section. Maps each face-vertex to its vertex normal, allowing faces that share a vertex to use different normals (for hard edges).

Shading Model Selection

Per-vertex shading is determined by the indirection table and vertex normal validity:

.3D files typically have the indirection table (per-face-vertex normal control). .3DC files always omit it (offset_normal_indices = 0) and fall back to direct vertex-index lookup.

Section4: SubObject BVH

Located at offset_section4 (header offset 0x1C). Contains section4_count variable-size entries. Present when offset and count are non-zero.

Each entry is a bounding-volume node with face references:

External References

• UESP: Mod:Model Files — community-maintained model-format notes and version behavior.
• uesp/redguard-3dfiletest Redguard3dFile.cpp — DaveHumphrey’s C++ parser, including v2.6/v2.7 header remap logic (ConvertHeader27).
• RGUnity/redguard-unity RG3DFile.cs — production parser implementation used by the Unity reimplementation (cross-check for face/texture decoding behavior).