3DC Animated Model File Format

Animated variant of the 3D model format. Same binary layout — identical header and section structure — but with multiple animation frames.

Differences from .3D

Aspect	.3D	.3DC
Frames	Always 1	2+ (animated)
frame_type (frame 0)	0	2, 4, or 8
Animated frame data	None	Frames 1+ with compressed or full-precision geometry
Section4 (SubObject BVH)	Present (v5.0)	Always absent (offset=0, count=0)
Vertex-normal indirection table	Present	Always absent (offset=0)
Version	v4.0 or v5.0	Always v4.0

All shared structures (header, face data, vertex coordinates, face normals, vertex normals, texture encoding) are documented in 3D.md.

Animation Frame Data

The frame data array at offset_frame_data contains num_frames × 16-byte records (see 3D.md — Frame Data for the record layout).

Frame 0 always points to the base geometry (same as .3D files). Frames 1+ contain per-frame vertex positions and face normals in a compact encoding determined by frame_type:

frame_type Values

Value	Meaning	Frame 1+ vertex encoding	Frame 1+ normal encoding
2	Compressed animation	i16 × 3 (6 bytes/vertex)	10-10-10-2 packed (4 bytes/face)
4	Full-precision animation	i32 × 3 (12 bytes/vertex)	10-10-10-2 packed (4 bytes/face)
8	Static (single-frame `.3DC`)	N/A	N/A

frame_type is only meaningful in frame 0’s record. Frame 1+ records always have frame_type = 0.

10-10-10-2 Packed Normal Format

Used for face normals in frames 1+:

Bits  0- 9: nx (10-bit signed, subtract 1024 if >= 512)
Bits 10-19: ny (10-bit signed, subtract 1024 if >= 512)
Bits 20-29: nz (10-bit signed, subtract 1024 if >= 512)
Bits 30-31: unused (values: 0 and 3)

Each component is divided by 256.0 to produce the final normal vector.

The engine’s normal decoder extracts the three 10-bit signed components via sign-extending shifts and discards bits 30–31:

nx = (float)((packed << 22) >> 22) * scale;   // bits  0–9, sign-extended
ny = (float)((packed << 12) >> 22) * scale;   // bits 10–19, sign-extended
nz = (float)((packed <<  2) >> 22) * scale;   // bits 20–29, sign-extended
// bits 30–31 are shifted out by << 2 — never read

The values (0 and 3) are likely packing artifacts — 3 (0b11) can result from sign extension during the build tool’s encoding step. Parsers should mask or discard these bits.

Section Layout

Same as 3D.md — Section Layout, but with animated frame data inserted and .3D-only sections absent:

Face Data — at 0x40
Vertex Coordinates — base frame positions
Face Normals — base frame normals
Frame Data — num_frames × 16-byte records
Animated Frame Vertex/Normal Data — frames 1+ geometry
Vertex Normals — per-vertex normals (f32 × 3)

External References

UESP: Mod:Model Files — baseline notes for shared .3D/.3DC structures and version differences.
RGUnity/redguard-unity RG3DFile.cs — practical reference for animated-frame decoding paths used in a full game implementation.

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