tech-studies

REZ Archive / Resource Packs

2 min read

REZ Archive / Resource Packs

Lithtech’s LTRES container — a single CLAW.REZ file that bundles every level, sprite, sound, and palette behind a tree of virtual paths, like an uncompressed ZIP for the WAP engine.

Why it matters

CLAW.REZ is the one file the user must legally own, and the root of OpenClaw’s whole asset pipeline — open it and you can reach WWD levels, PID sprites, PCX bitmaps, and XMI music by name. The engine’s resource manager is essentially a REZ reader with a cache on top. Getting the directory walk right is the prerequisite for decoding everything else.

How it works

REZ stores files contiguously and keeps a separate directory tree describing where each lives:

  • Header — magic LTRES, version, and a pointer (offset + size) to the root directory block.
  • Directory entries — each is either a sub-directory (recurse) or a resource (a leaf file).
  • Resource entry — holds the data offset, size, a 4-char type (space-padded, e.g. PID, WAV), the timestamp, and a null-terminated name.
  • Type tags are stored reversed (FOURCC quirk): the on-disk bytes for type "WAV" read back-to-front — read 4 bytes and reverse them.

All multibyte fields are little-endian 32-bit. Lookup is path-based: split "LEVEL1/TILES/ACTION.PID" on /, descend the directory tree matching each component, then seek(offset) and read(size) the leaf. Data is stored, not compressed — the offset/size point straight at usable bytes, so extraction is a plain copy. Contrast a real ZIP, which would need per-entry inflate.

Example

hdr = read(); seek(hdr.dirOffset)
dir = read_dir_block(hdr.dirSize)
entry = find(dir, "CLAW.WWD")     # walk tree by name
seek(entry.offset)
wwd_bytes = read(entry.size)      # raw, no decompress
parse_wwd(wwd_bytes)

A typical CLAW.REZ is ~70 MB holding tens of thousands of entries. Because access is by virtual path and data is uncompressed, you can seek directly to one 4 KB sprite without touching the rest of the archive.

Pitfalls

  • Forgetting to reverse the type FOURCC — comparing raw against "PID " never matches; the on-disk bytes are reversed.
  • Assuming compression — REZ is stored, not deflated; running inflate over it yields garbage.
  • Loading the whole file into RAM — at 70 MB you seek/read per entry; do not slurp it.
  • Case-sensitive path matching — internal paths came from DOS/Win32; match case-insensitively or lookups fail on some builds.

See also

Graph View

Backlinks