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TEXBSI Texture File Format

Container format for indexed-color texture images. Files are named TEXBSI.### where ### is the texture bank number.

Overall Structure

A TEXBSI file is a flat sequence of image records with no file-level header. The sequence ends when a 9-byte null sentinel is encountered.

[Image Record 0]
[Image Record 1]
...
[Image Record N]
[9 × 0x00]   ← end sentinel

Image Record

Image-record envelope fields are little-endian.

OffsetSizeTypeNameDescription
09[u8; 9]image_nameImage name, null-padded. Format: {type_char}{file_num:02d}{image_idx:03d}. All-zero = end of file.
94u32subrecord_bytesTotal size of all subrecords that follow (excludes this 13-byte envelope).
13…subrecordssubrecordsTagged subrecords until END .

Image Name Encoding

The 9-byte name encodes the file number and sub-image index:

"E01005\0\0\0" → type 'E', file 01, image index 005
"A02003\0\0\0" → type 'A', file 02, image index 003

The image index (last 3 digits) is how 3D model faces reference sub-images via image_id.

Filename/name coupling across shipped TEXBSI.### files:

  • ### % 100 matches the 2-digit file number in image names.
  • ### / 100 maps to type-char family: 0->A, 1->B, 2->C, 3->D, 4->E, 5->F.

Examples:

  • TEXBSI.302 contains D02xxx images.
  • TEXBSI.114 contains B14xxx images.

Type characters range from A through F.

The type character has no semantic meaning — it is a deterministic artifact of the base-40 name encoding used internally. The game converts numeric texture IDs to 6-character strings using the alphabet 0123456789abcdefghijklmnopqrstuvwxyz~_#%, then shifts the first character by subtracting 0x31. The type letter is simply which base-40 digit range the texture ID falls into (A=digit 27, B=28, …, F=32). The game never tests or filters by the type character — it round-trips through the numeric ID.

Subrecord Structure

Every subrecord has an 8-byte header:

Subrecord size words are big-endian.

OffsetSizeTypeNameDescription
04[u8; 4]tagTag: BSIF, IFHD, BHDR, CMAP, DATA, or END
44u32payload_sizePayload size in bytes (not including this 8-byte header)
8…payloadpayloadTag-specific data

Subrecords always appear in this order:

BSIF or IFHD  (mutually exclusive)
BHDR           (required)
CMAP           (optional, only with IFHD)
DATA           (required)
END            (terminates the record — no size field)

Subrecord Payloads

BSIF — Static Image Marker

Payload size: 0 bytes (empty). Marks a static (non-animated) image.

IFHD — Animated Image Marker

Payload size: 44 bytes (always 01 followed by 43 00 bytes). Marks an animated image.

When IFHD is present, the DATA subrecord uses the animated offset-table format.

BHDR — Image Header (26 bytes)

All fields are little-endian.

OffsetSizeTypeNameDescription
02i16x_offsetX position hint (placement on virtual canvas)
22i16y_offsetY position hint
42i16widthImage width in pixels
62i16heightImage height in pixels
81u8has_cmapExport-tool flag, not read at runtime. Set to 1 when the image has an embedded CMAP palette (always co-occurs with IFHD animated images). Per UESP: “images that have 1 are all animated effects such as fire and water.”
91u8export_flagsExport-tool metadata. Values: 0, 1, or 9. Packed into the same u16 as has_cmap during export. Purpose within the build pipeline unknown.
104—reservedAlways 0
142i16frame_count1 = static, 2–16 = animated
162i16anim_delayRead at runtime. Animation frame duration in milliseconds. Converted to DOS PIT timer ticks via round(anim_delay × 18.2 / 1000); clamped to minimum 1. Typical value 71 → 1 tick (~55 ms); value 500 → 9 ticks (~495 ms). Range: 0–500.
184—reservedAlways 0
222u16tex_scaleRead at runtime as a single LE u16. 8.8 fixed-point texture coordinate scale factor: scale = tex_scale / 256.0. Default 0x0100 (= 1.0, neutral) is substituted when zero. Multiplied into polygon UV mapping during rendering. Known values: 0 (defaulted to 1.0), 128 (scale 0.5), 163 (scale ~0.637), 256 (scale 1.0), 512 (scale 2.0). Previously documented as two separate bytes (effect_id / effect_param); they are the low and high bytes of this single fixed-point field.
242i16data_encodingPixel data encoding mode. Selects which compression method the DATA subrecord uses. Known values: 0 = raw uncompressed, 4 = animated offset table. Values 1–3 are engine-supported but unused.

CMAP — Embedded Palette (768 bytes)

256 × RGB triplets (3 bytes each, values 0–255). Same layout as COL files.

Optional — always co-occurs with IFHD (animated images). When absent, the image uses an external .COL palette file.

DATA — Pixel Data

Static images (BSIF present, frame_count == 1):

Payload is width × height bytes of 8-bit indexed color, row-major, top-to-bottom. Each byte is a palette index (0–255). Index 0 = transparent.

Animated images (IFHD present, frame_count > 1):

Payload starts with an offset table of height × frame_count LE u32 entries. Each entry is a byte offset from the start of the DATA payload to the first byte of that row. Rows can be shared across frames (identical rows point to the same data).

offset_table[frame * height + row] → byte offset to row data (width bytes)

END — Record Terminator

4-byte tag "END " (with trailing space). No size field. Followed by 4 zero bytes.

Pixel Decoding

for each pixel byte:
  if byte == 0:    → transparent (alpha = 0)
  else:            → palette[byte] as RGB (alpha = 255)

Palette values are raw 8-bit RGB (0–255). No gamma correction needed.

Palette Selection

When decoding pixel data, palette lookup order is:

  1. External .COL palette — the scene-level palette file passed to the converter (e.g. ISLAND.COL). Which COL file to use is determined per-scene by the game engine.
  2. Embedded CMAP — the 256-color palette stored inside the image record (only present in animated IFHD images).
  3. Grayscale fallback — if neither is available, each index maps to a gray value.

How 3D Models Reference Textures

Each face in a 3D model encodes a texture_id and image_id:

  • texture_id → selects the TEXBSI.### file number
  • image_id → selects the sub-image within that file (0-indexed, matching the 3-digit suffix in the image name)

See 3D.md — Texture Decoding for the BCD encoding.

External References