Going-Commando/docs/plan/level-assets.md

Level assets

Phase 2 work record. What has been established about the four extents inside each LEVEL<n>.WAD, and what remains.

A bug found and fixed

Reading the data extent's contents exposed a defect in the Phase 1 extractor.

Level extents are declared in sector counts; the generic G/ pair table uses byte counts. The extractor routed level containers through the generic path, so LEVEL0.WAD's data extent — 6,484 sectors — was taken as 6,484 bytes. Level 0 yielded a 6,484-byte "geometry" file instead of 13,279,232 bytes. Across 27 levels that silently discarded roughly 300 MB of level content while every test still passed.

The tell was arithmetic, not a crash: the extent's own header declared an uncompressed total of 13,277,920 bytes, and the ratio against the 6,484-byte file was 2047.7× — one sector, to within rounding. A ratio that lands on 2048 is a unit error.

Fixed by extracting level containers through the typed LevelHeader instead of the generic table. Level 0 now yields:

data.bin         13,279,232   the level container's data extent
core_data.wad     9,738,678   the geometry, compressed
core_data.bin    16,969,296   the geometry, decompressed
core_index.bin       31,600   the geometry directory
gameplay.bin       659,456
sound_bank.bin   1,503,232

Lesson: a unit error is invisible to a type system and to a passing test. The check that catches it is arithmetic against an independent number the file itself declares. Any extent whose contents describe a larger structure should have that structure's own total compared against the extent.

Done: the container header

crates/gc-formats/src/level_wad.rs parses the 0x60-byte header on all 27 levels and names the four extents. See ../formats/level-wad.md for the layout and the measured invariants.

Two facts worth carrying forward:

• Sizes in this header are sector counts, unlike the generic G/*.WAD pair table where they are byte counts. Both are on the same disc.
• LEVEL21.WAD reports level id 30. Container order is not level identity.

gameplay: a compression we have not identified

This is the one genuinely puzzling result so far, and it is worth recording in full because the next attempt will otherwise repeat the work.

The extent begins:

offset 0  57 41 44   "WAD"
offset 3  79 0d 0a 0d   <- varies per level
offset 4+ <payload>

The variation is the tell. For the LZ77 WAD container the fourth byte is part of a 32-bit little-endian length; here byte 3 differs per level (0x79, 0x3a, 0xd8, 0xa9, ...) while byte 4 is a small count (13, 88, 13, 105, 163, ...) and bytes 6 and 7 are always zero.

Attempt: treat it as the known WAD LZ77 stream. Decoded all 27 levels with the decoder in wad.rs, starting after an 8-byte header. Every one produces plausible output for the first packet — 0000 0000 0000 0750 0100 0020 2b0c... looks like real level data — and then fails on all 27 with "adjacent literal packets", which the format's encoder never emits.

A uniform failure on all 27 inputs means the input is wrong, not the decoder. Ruled out:

• Bit order. The WAD container is a flag-byte packet stream, not a bit stream, so no ordering applies. Both orderings fail identically.
• Header length. Tried 8 bytes. A different skip would not fix a failure that occurs uniformly in the first few packets.
• The WAD format at all. The first packet (flag 0x00) decodes cleanly as a literal run of 18 bytes, which is why it looks plausible, but the run immediately after breaks the format's own rule. Whatever this is, it is not WAD LZ77.

Next to try, in order of cheapness:

1. Whether the payload is uncompressed and the header is a table. (len - 8) is 196..497 across the levels and divides by neither 8 nor a plausible stride, which argues against a flat pair table — but a variable-length table (counts of counts) fits that range.
2. Whether byte 4 is an entry count and bytes 5.. are records of a fixed but non-obvious width.
3. Whether the LZ77 variant differs only in the "adjacent literals" rule.

None of these is expensive to test; each is a few lines against all 27 levels at once. The right move is to write a throwaway script that tries many hypotheses across all 27 and reports which produce self-consistent results — the approach that worked for the container pair table, where "does this hold on every level" was the discriminator that the sector/byte ambiguity failed.

sound_bank: signatures found, structure unknown

+0x18  "SBlk"   present in all 27 levels
+0x24  "DAW"    present in all 27 levels

Sizes 458..833 bytes. Too small to be the level's audio; likely an index into the global AUDIO.WAD.

occlusion: header located, contents a u16 array

The header's occlusion offset (0x0c) is trustworthy — every offset in it lands inside core_data. Level 0's occlusion block starts at 2,117,888 and reads as a u16 array:

5152 0 12 23 14 19 24 29 32 35 38 41 44 47 50 53 56 59 65 82 96 100 113 126 143 164 183 ...

An ascending run of 65 values with mostly small differences, then it restarts. That is consistent with per-cell visibility records rather than geometry. The meaning of the pairs — and the 0 that ends each group — is not yet established.

sky: plausible, unparsed

Header offset 0x10 = 2,192,576. Opens 0 0 0 0 0 0 6 0 0 0 0 0 5 0 0 0 40 0 0 0 c0 0 0 0 40 ... — small values including 64 and 192, which look like sky-shell counts and dimensions rather than offsets.

tfrag region: [0, occlusion)

The header's fields at 0x0c, 0x10, 0x14 and 0x60 are offsets, and they are strictly increasing on all 27 levels. The differences between them give region sizes that are plausible on every level:

Region	Range	Level 0 size
tfrag	[0, occlusion)	2.12 MB
occlusion	[occlusion, sky)	0.07 MB
sky	[sky, collision)	0.42 MB
collision	[collision, textures_base)	0.90 MB

Ruled out: reading 0x0c/0x10/0x14 as sizes laid end to end. The cumulative total does not reach textures_base on any level, and the byte at the start of each supposed region is not a plausible structure. The offsets model is the one that holds everywhere.

What is in the terrain region

Textures are stored inline with the geometry, not in a separate bank. A scan for the PS2 texel pattern 18 18 18 80 (grey texels with the 0x80 alpha that means "opaque" in the GS's convention) finds 6,739 occurrences spread across 138 clusters, the first at 0xd9f8 — inside the tfrag region. So terrain carries its own texels, and textures_base names something else.

Earlier claims in this file that have since been disproven:

• "The terrain is 382 records of 64 bytes." The 0x48 byte at offset +3 is simply the most common byte in the region (945 of 2000 sampled slots); it is not a record marker. Over 33,091 slots at that position only 4,127 hold 0x48.
• "Byte +3 is constant at 0x48 across 300 records." True for the first 382 records and false beyond, which is how the false pattern was found.
• "Coordinates are fixed-point s16 with a scale of 8." The scale-8 reading is supported by one coincidence (17,496 / 8 = 2,187, the planet radius) but no grouping of the record into triples yields points on a sphere of that radius. Tested: every stride from 2 to 16 bytes, every starting offset, three and four components. Best result 5.0% of points on-sphere, which is chance.

So the record framing is not recovered. What is established is the region layout and that terrain is vertex-soup packed alongside inline texels — which means the vertex format must be identified from a longer run than a single record, most likely by locating a run of vertices whose magnitudes are bounded by the planet radius and whose Y values vary smoothly.

Where the class tables are not

The header's moby_classes and friends do not decode. Read as (offset, count) they give plausible numbers, but the entries at those offsets are not valid records. Ruled out:

• Field order. Both (offset, count) and (count, offset) produce garbage.
• Stride. Tried 8, 16 and 32 bytes per entry.
• Which file the offsets index. Tried core_data and core_index.
• Header base. Not the issue: the compressed size (0x88) and decompressed size (0x8c) of core_data appear at the same offsets in all 27 levels' core_index, which pins the header base at 0 with certainty.

One observation worth keeping: every second value in the class-table fields is exactly divisible by 16, and they increase monotonically across the nine tables. That is consistent with count * 16 sizes, but the corresponding offsets cluster at the very start of the file and do not yield valid records either.

The discriminator used to reject candidates is worth stating, because weaker tests produce false positives: a real class table's offset_in_asset_wad values increase. A search for "32-byte records with increasing offsets" returns several hundred candidates across the file, and the top one turned out to be a texture table whose o_class field merely happened to equal offset + 16. Scoring on plausibility alone is not enough; the entries have to actually chain.

Next: rather than guess the header further, identify tfrag by its own structure. Terrain is the largest subsystem and the one that decides whether anything draws, so it is the right next target even though it is the harder one.

(Done — see "tfrag terrain" below. Identifying tfrag by its own structure worked where the class-table guess did not.)

data: the real target, untouched

6,484 sectors on level 0, down to 2,102 on level 24 — 4.2 to 21.9 MB. This is the level's geometry and the largest thing in the container. Next target.

What is known from the community reference and from Insomniac's engine family: it splits into four geometry subsystems — tfrag (terrain fragments), tie (instanced environment structures), shrub (foliage) and moby (dynamic actors) — plus sky shells, collision data and textures. ReRAC's documentation of the equivalent structures in the first game is the closest reference available, though R&C2's spherical worlds change how they are used.

Order of work

1. gameplay — smallest, fully bounded, and the format is identified enough to make progress. A win here validates the approach before the large effort.
2. occlusion — tiny.
3. sound_bank — small.
4. data — the large one, and the one that decides whether anything renders.

Doing the small extents first is not just convenience: each one is a contained reverse-engineering problem that either resolves or tells us something, whereas data will keep producing ambiguous readings until we have something to check them against.

Method note

Every format question this phase has been settled the same way: write the hypothesis, test it against all 27 levels at once, and require an invariant that holds everywhere. The sector/byte ambiguity in Phase 1 survived because it happened to be correct on the one container checked by hand. The level header was fixed in a single pass because the tiling check failed immediately on levels 1, 2, 4, 7, 8, 11, 19 and 20.

tfrag terrain: block structure and vertex positions

Settled. gc-formats::tfrag parses the block, and a gated test decodes every patch on all 27 levels.

Block and patch framing

The tfrags block opens with a 12-byte header: table_offset (always 0x40), tfrag_count, and an f32 LOD distance (32.0). The patch table follows at table_offset, tfrag_count records of 0x40 bytes each. On level 0 that is 833 patches.

A patch header's data offset is relative to table_offset, not to the start of the block. Getting this wrong happens to work for the first patch and silently produces garbage afterwards.

Positions live in three lists, not one

vert_count is the total across all three position tiers. Within a patch's data block the VIF lists do not run in ascending offset order — the LOD-2 list sits before the common list — and the LOD-0-only list has no offset of its own, starting instead at shared_ofs + lod_1_size * 16. The ranges, in transfer order, are:

List	Range (from the patch data block)	Holds
common	shared_ofs .. lod_1_ofs	VU header, ad-gifs, vertex info, origin, positions
LOD 2	lod_2_ofs .. shared_ofs	strips, indices
LOD 1	lod_1_ofs .. lod_0_ofs	strips, indices
LOD 0+1	lod_0_ofs .. shared_ofs + lod_1_size*16	parents, vertex info, positions
LOD 0	shared_ofs + lod_1_size*16 .. rgba_ofs	positions, strips, indices, parents

Scanning only the common list and expecting num == vert_count finds positions in roughly a quarter of patches. The other three quarters keep most of their positions in the LOD lists. That was the first wrong turn and it looked plausible: every patch it did match matched exactly.

The VIF1 code word

Two rules had to be right before any of this worked, and both were wrong on the first attempt.

The vector format lives in the command byte, not the immediate. Bits 2-3 of the command are vn (component count minus one) and bits 0-1 are vl (0 = 32-bit, 1 = 16-bit, 2 = 8-bit, 3 = 5-bit), so

element_size = ((32 >> vl) * (vn + 1)) / 8

Terrain positions are vn=2, vl=1 — command 0x69 — at 6 bytes each. Reading the format out of the immediate's low nibble, which is the more commonly quoted layout, yields 12 or 16 bytes and quietly misaligns everything after it.

Unpack payloads pad to 4 bytes, and several non-unpack commands carry inline data. An unpack advances (count * element_size + 3) & !3. STROW/STCOL carry 16 bytes, STMASK 4, MPG num * 8, DIRECT/DIRECTHL imm * 16; every other non-unpack is a bare 4-byte code word. Ignoring the first four desynchronises the walk, and a desynchronised walk still finds code words that look like positions — it just finds the wrong number of them.

The 4-byte padding is easy to misread. With 75 elements the payload is 450 bytes and the following word lands at +452, where 0x01000404 (STCYCL) reads perfectly plausibly. The word at +450 is 0x04040000 (ITOP), which is the real one. Taking the plausible-looking reading is how this was briefly wrong.

Verification

Requiring every patch to yield exactly its own vert_count is the invariant that caught both errors, and it is the check the parser is built around: a mismatch means the walk desynchronised, so tfrag_positions returns None rather than partial geometry. Across all 27 levels: 18,102 patches, 821,439 vertices, zero mismatches.

Still open

Strips and triangle indices, the RGBA vertex-info arrays, texture binding, and the LOD/morph/cull rules in TfragProc. Positions alone are not drawable.