ReaperOfVeriod
16c04db559
A defect in the Phase 1 extractor, found by reading the data extent's contents: level extents are declared in sector counts, the generic G/ pair table uses byte counts, and level containers were being routed through the generic path. So LEVEL0.WAD's data extent -- 6,484 sectors -- was taken as 6,484 bytes. Every test still passed, because a truncated file is still a file. The extent's own header gave it away: it declares an uncompressed total of 13,277,920 bytes, and 13,277,920 / 6,484 is 2047.7. A ratio landing on 2048 is a unit error, not a compression ratio. Across 27 levels this had been discarding roughly 300 MB of level content. Level containers now go through the typed LevelHeader instead. Level 0 yields: data.bin 13,279,232 the 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 core_data is a WAD LZ77 stream -- the format already implemented for the game -- and it decompresses cleanly. The first real level content this project has recovered. New: LevelDataHeader parses the data extent's own header, a table of byte ranges naming the level overlay executable, the geometry directory, the GS RAM image, the HUD banks and the geometry. Note this is a third meaning for a "size" field on the same disc, after sector counts in the level header and byte counts in the generic table; all three are documented together in level-data.md so the next parser does not rediscover this the hard way. The geometry directory's offsets were checked against the geometry rather than taken on trust: all twelve land inside the 16.9 MB of decompressed core_data, which is what confirms the header is being read correctly. The class tables themselves do not yet decode under the documented layout, and that is recorded as open work in plan/level-assets.md rather than papered over. 75 unit tests, 9 integration tests against the real disc. Hash table regenerated at 5,159 files and 5,671,915,799 bytes -- hashes only, no content. Extraction now writes to ~/Documents/gc-extracted rather than /tmp, which is 16 GB and did not have room for a 5.3 GB output.
150 lines
6.4 KiB
Markdown
150 lines
6.4 KiB
Markdown
# Level assets
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Phase 2 work record. What has been established about the four extents inside
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each `LEVEL<n>.WAD`, and what remains.
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## A bug found and fixed
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Reading the `data` extent's contents exposed a defect in the Phase 1 extractor.
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Level extents are declared in **sector counts**; the generic `G/` pair table
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uses **byte counts**. The extractor routed level containers through the generic
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path, so `LEVEL0.WAD`'s `data` extent — 6,484 sectors — was taken as 6,484
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*bytes*. Level 0 yielded a 6,484-byte "geometry" file instead of 13,279,232
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bytes. Across 27 levels that silently discarded roughly 300 MB of level content
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while every test still passed.
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The tell was arithmetic, not a crash: the extent's own header declared an
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uncompressed total of 13,277,920 bytes, and the ratio against the 6,484-byte
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file was 2047.7× — one sector, to within rounding. A ratio that lands on 2048
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is a unit error.
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Fixed by extracting level containers through the typed `LevelHeader` instead of
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the generic table. Level 0 now yields:
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```text
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data.bin 13,279,232 the level container's data extent
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core_data.wad 9,738,678 the geometry, compressed
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core_data.bin 16,969,296 the geometry, decompressed
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core_index.bin 31,600 the geometry directory
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gameplay.bin 659,456
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sound_bank.bin 1,503,232
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```
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**Lesson:** a unit error is invisible to a type system and to a passing test. The
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check that catches it is arithmetic against an independent number the file
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itself declares. Any extent whose contents describe a larger structure should
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have that structure's own total compared against the extent.
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## Done: the container header
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`crates/gc-formats/src/level_wad.rs` parses the 0x60-byte header on all 27
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levels and names the four extents. See
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[`../formats/level-wad.md`](../formats/level-wad.md) for the layout and the
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measured invariants.
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Two facts worth carrying forward:
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- **Sizes in this header are sector counts**, unlike the generic `G/*.WAD` pair
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table where they are byte counts. Both are on the same disc.
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- **`LEVEL21.WAD` reports level id 30.** Container order is not level identity.
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## `gameplay`: a compression we have not identified
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This is the one genuinely puzzling result so far, and it is worth recording in
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full because the next attempt will otherwise repeat the work.
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The extent begins:
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```text
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offset 0 57 41 44 "WAD"
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offset 3 79 0d 0a 0d <- varies per level
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offset 4+ <payload>
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```
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The variation is the tell. For the LZ77 WAD container the fourth byte is part of
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a 32-bit little-endian length; here byte 3 differs per level (0x79, 0x3a, 0xd8,
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0xa9, ...) while byte 4 is a small count (13, 88, 13, 105, 163, ...) and bytes 6
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and 7 are always zero.
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**Attempt: treat it as the known WAD LZ77 stream.** Decoded all 27 levels with
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the decoder in `wad.rs`, starting after an 8-byte header. Every one produces
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plausible output for the first packet — `0000 0000 0000 0750 0100 0020 2b0c...`
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looks like real level data — and then fails on all 27 with "adjacent literal
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packets", which the format's encoder never emits.
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A uniform failure on all 27 inputs means the input is wrong, not the decoder.
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Ruled out:
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- **Bit order.** The WAD container is a flag-byte packet stream, not a bit
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stream, so no ordering applies. Both orderings fail identically.
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- **Header length.** Tried 8 bytes. A different skip would not fix a failure
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that occurs uniformly in the first few packets.
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- **The WAD format at all.** The first packet (flag `0x00`) decodes cleanly as a
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literal run of 18 bytes, which is *why* it looks plausible, but the run
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immediately after breaks the format's own rule. Whatever this is, it is not
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WAD LZ77.
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Next to try, in order of cheapness:
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1. Whether the payload is uncompressed and the header is a table. `(len - 8)`
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is 196..497 across the levels and divides by neither 8 nor a plausible stride,
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which argues against a flat pair table — but a *variable-length* table
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(counts of counts) fits that range.
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2. Whether byte 4 is an entry count and bytes 5.. are records of a fixed but
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non-obvious width.
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3. Whether the LZ77 variant differs only in the "adjacent literals" rule.
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None of these is expensive to test; each is a few lines against all 27 levels at
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once. The right move is to write a throwaway script that tries many hypotheses
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across all 27 and reports which produce self-consistent results — the approach
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that worked for the container pair table, where "does this hold on every level"
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was the discriminator that the sector/byte ambiguity failed.
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## `sound_bank`: signatures found, structure unknown
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```text
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+0x18 "SBlk" present in all 27 levels
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+0x24 "DAW" present in all 27 levels
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```
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Sizes 458..833 bytes. Too small to be the level's audio; likely an index into
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the global `AUDIO.WAD`.
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## `occlusion`: smallest, and unread
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2 to 25 sectors. Small enough to dump whole and stare at.
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## `data`: the real target, untouched
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6,484 sectors on level 0, down to 2,102 on level 24 — 4.2 to 21.9 MB. This is
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the level's geometry and the largest thing in the container. Next target.
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What is known from the community reference and from Insomniac's engine family:
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it splits into four geometry subsystems — `tfrag` (terrain fragments),
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`tie` (instanced environment structures), `shrub` (foliage) and `moby` (dynamic
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actors) — plus sky shells, collision data and textures. ReRAC's documentation of
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the equivalent structures in the first game is the closest reference available,
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though R&C2's spherical worlds change how they are used.
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## Order of work
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1. `gameplay` — smallest, fully bounded, and the format is identified enough to
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make progress. A win here validates the approach before the large effort.
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2. `occlusion` — tiny.
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3. `sound_bank` — small.
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4. `data` — the large one, and the one that decides whether anything renders.
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Doing the small extents first is not just convenience: each one is a contained
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reverse-engineering problem that either resolves or tells us something, whereas
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`data` will keep producing ambiguous readings until we have something to check
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them against.
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## Method note
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Every format question this phase has been settled the same way: write the
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hypothesis, test it against **all 27 levels at once**, and require an invariant
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that holds everywhere. The sector/byte ambiguity in Phase 1 survived because it
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happened to be correct on the one container checked by hand. The level header
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was fixed in a single pass because the tiling check failed immediately on
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levels 1, 2, 4, 7, 8, 11, 19 and 20. |