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Session 2: hybrid VQ codec, two quality profiles, three corrections

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Answers session 1's critical-path question. Flat 4x4 VQ at k=256 was prototyped
and REJECTED by eye: Dirk's face disintegrates and ink outlines break into
4-pixel stair-steps. The 256-colour palettised frame is excellent, so the
palette was never the problem -- block VQ was.

Replaced it with a Cinepak-style hybrid: each 4x4 block is SKIP, one 4x4
codeword, four 2x2 codewords, or RAW literal pixels, chosen per block by
rate-distortion. The RAW escape makes lam=0 pixel-exact (measured 0.00 dB loss),
so the quality knob spans lossless to heavily-compressed in one bitstream.

Per the user's decision, ships TWO quality profiles from that one codec, one
decoder and one bitstream -- only the rate knob differs:
  sasi  45 KB/s  lam=300  34.8 dB   stock 10MHz ACE/EXPERT
  scsi  75 KB/s  lam=100  35.9 dB   Super/XVI or CZ-6BS1

Three corrections to earlier numbers:

1. Session 1's "183 KB/s at 12fps" was a bad extrapolation. Halving the
   framerate does not halve the bitrate -- decimation roughly doubles the
   per-frame delta. Re-measured directly: 340 KB/s for session 1's own RLE,
   247 KB/s for changed-spans+deflate. The lossless floor is 319 MB.

2. A FOURTH false-good result, same family as the three in FINDINGS 4:
   k=1024 codebooks appeared to buy +2.4 dB free, because the rate model
   charged 1 byte for a 10-bit index. Charging the true cost reverses the
   verdict -- k=256 wins at every matched bitrate, and by 5 dB at the low end
   where the SASI profile lives. k=256 ships.

3. Stream inventory: the ~3-5MB clips are 1.2-1.7s, not ~60s, and some 60s
   streams are menus, not content. Any survey must classify before averaging.

Also cleared both candidate sources for the game-logic layer: the SNES project
is MIT and DirkSimple is zlib, so the arcade scene graph can be imported and
the two transcriptions diffed against each other.

Encoder is working end-to-end: extract.py -> vq/vq_hybrid/ratectl -> encode.py,
emitting a big-endian DLX1 container the 68000 can parse with plain moves.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
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@@ -188,3 +188,185 @@ Their 516 chapters are finer-grained than our 224 Blu-ray streams, so mapping
their event table onto our footage means subdividing streams by timecode.
Caveat: all of the above is from README/repo-tree summaries, not their source.
---
---
# Findings — session 2 (2026-08-23)
## 8. CORRECTION to session 1: halving the framerate does NOT halve the bitrate
Session 1 measured 365 KB/s for naive delta+RLE at 24 fps and wrote
"(~183 KB/s at 12fps)". **That extrapolation is wrong.** Decimating to 12 fps
roughly doubles the per-frame delta, so the *rate* stays nearly flat.
Re-measured directly on 12 fps decimated frames (4 scenes, 66 frames):
| codec (all LOSSLESS w.r.t. the 256-colour frame) | B/frame | KB/s @12 | 22 min | ratio |
|---|---|---|---|---|
| raw 8bpp 256x192 | 49152 | 576 | 743 MB | 1.0:1 |
| session 1 row-span + RLE | 29055 | 340 | 439 MB | 1.7:1 |
| XOR vs prev + deflate | 30196 | 354 | 456 MB | 1.6:1 |
| **changed-spans + deflate** | **21110** | **247** | **319 MB** | **2.3:1** |
| changed-spans + LZMA | 18759 | 220 | 283 MB | 2.6:1 |
Session 1's own RLE re-measured at 12 fps gives **340 KB/s, not 183**.
Any plan that assumed 183 KB/s was based on a bad number.
Deflate-class entropy coding on top of the span payload is worth **1.4x** over
hand-rolled RLE, and LZ decode is cheap on a 68000 (byte copies), so the
lossless floor is ~247 KB/s / 319 MB. That is **infeasible on SASI** and
**tight but real on SCSI**.
## 9. Flat 4x4 VQ at k=256 is NOT acceptable — confirmed by eye
The risk flagged in 6 is real. At k=256, 4x4:
| scene | palette-only PSNR | after VQ | VQ loss |
|---|---|---|---|
| 00010 | 38.35 | 29.68 | 8.67 dB |
| 00020 | 39.90 | 32.67 | 7.22 dB |
| 00146 | 35.25 | 29.35 | 5.89 dB |
| 00181 | 41.92 | 32.87 | 9.05 dB |
Visually: Dirk's face disintegrates, teeth and eyes turn to mush, ink outlines
break into 4-pixel stair-steps, colour bleeds across block boundaries.
![flat 4x4 VQ failure](images/flat_vq_failure_00010.png)
*Left: 1080p source. Middle: 256-colour palettised 256x192 — the quality ceiling,
and it is excellent. Right: flat 4x4 VQ at k=256. This is the result that killed
the flat-VQ architecture.*
**Crucially, the 256-colour palettised frame itself looks excellent.** Flat cel
art with a per-scene median-cut palette and no dithering is near-transparent
(35-42 dB). So the palette is not the problem and 256 colours is not the
problem — **block VQ is**. The quality ceiling we should hold ourselves to is
the palettised frame, not the 1080p source.
## 10. Hybrid VQ (Cinepak V1/V4 + SKIP) — this is the codec
Per 4x4 block, choose by rate-distortion: SKIP (reuse previous frame),
V1 (one 4x4 codeword, 1 byte), or V4 (four 2x2 codewords, 4 bytes),
with a 2-bit-per-block mode header. `lam` is the lagrangian rate knob.
Measured, k1=k4=256, 4 scenes (mean of the per-scene table in the session log):
| lam | PSNR | loss vs palette | SKIP% | V1% | V4% | B/frame | KB/s @12 |
|---|---|---|---|---|---|---|---|
| 0 (max quality) | 33.9 | 4.9 | 30.8 | 18.5 | 50.8 | 7574 | 88.8 |
| 200 | 31.9 | 5.9 | 44.0 | 37.6 | 18.4 | 4183 | 49.0 |
| 1000 | 31.6 | 7.3 | 47.4 | 47.7 | 4.9 | 2841 | 33.3 |
| 5000 | 25.5 | 13.3 | 55.6 | 44.4 | 0.0 | 2134 | 25.0 |
At a **matched ~30 KB/s** the hybrid beats flat 4x4 VQ by ~1 dB, and unlike flat
VQ it keeps scaling: at 89 KB/s it reaches within **4.9 dB of the palette
ceiling**, which flat VQ cannot reach at any bitrate.
Note V4% collapses to 0 at lam=5000 — that is the knob doing exactly what it
should: under a hard ceiling, detail blocks are the first thing sacrificed.
## 11. Codebook size sweep (flat 4x4, for reference)
| block | k | PSNR | loss | key B | changed% | KB/s @12 | codebook RAM |
|---|---|---|---|---|---|---|---|
| 4x4 | 256 | 30.46 | 8.39 | 3072 | 52.7 | 28.5 | 8K |
| 4x4 | 1024 | 32.89 | 5.96 | 3840 | 56.6 | 35.6 | 32K |
+2.4 dB for 24K more RAM and 7 KB/s. With 2 MB of RAM, a 1024-entry codebook is
cheap and clearly worth it. (RAM figure is the word-expanded form the blitter
wants: k * 16 px * 2 bytes.)
## 12. Source framing — OPEN
The Blu-ray is **full-frame 1920x1080 16:9 with no pillarboxing**. The arcade
original is 4:3. The extractor currently centre-crops 1440x1080, which is the
arcade-faithful choice but discards image the 2006 remaster added. Options are
`crop` (default), `squash`, `wide` in `tools/encoder/extract.py`.
**Not yet decided; needs an eyeball comparison against arcade reference.**
## 13. Stream inventory correction
Session 1 said "typical scene clip ~60s". Sampled directly: the ~3-5 MB streams
are **1.2-1.7 s** clips — these are the individual arcade death/action moments,
which is exactly the granularity the game logic needs. Some 60 s streams
(e.g. 00203) are **menu screens, not content**. Any survey must classify
menu vs content before averaging, or the bitrate numbers are diluted by static
menus.
## 14. A FOURTH false-good result — and the correction
Add this to the 4 list. The mechanism was new but the shape was identical.
**The false result:** flat and hybrid VQ both showed **+2.4 dB for k=1024 over
k=256** at an apparently similar bitrate, which made a 1024-entry codebook look
like an obvious win. The k=1024 quality ladder rendered from that run looked
great at "45 KB/s".
**The bug:** the rate-distortion model in `vq_hybrid.encode()` charged **1 byte**
per codebook index unconditionally. A 1024-entry codebook needs a **10-bit index,
stored as 2 bytes**. So every k=1024 measurement understated the V1 and V4
payload by exactly 2x, *and* the lagrangian mode decision was choosing V4 on the
belief that four codewords cost 4 bytes when they cost 8.
**After charging the true index cost** (`idx_bytes` is now explicit and defaults
from the codebook size), matched-bitrate comparison on scene 00020:
| KB/s | k=256 (1-byte idx) | k=1024 (2-byte idx) |
|---|---|---|
| ~32-42 | **33.87 dB** @ 32.5 | 28.91 dB @ 42.3 |
| ~44-52 | **34.80 dB** @ 44.1 | 35.13 dB @ 52.5 |
| ~72-86 | **35.87 dB** @ 72.2 | 36.51 dB @ 86.0 |
k=1024 buys +0.3 to +0.6 dB for +19% bitrate — a wash at best — and at the low
end where the SASI profile lives it is **5 dB worse**, because the 2-byte index
floor dominates once V4 is priced out.
**k=256 with 1-byte indices is the shipping choice.** It is also the better
decoder: a plain `move.b` index with no alignment case, and an 8 KB codebook
instead of 32 KB.
**The general lesson, again:** the comparison was not wrong about VQ, it was
wrong about *cost*. When a knob looks like a free win, check that the rate model
is charging for it. Same failure family as 4.1-4.3: a plausible number produced
by a pipeline that was not measuring what it claimed to measure.
## 15. Rate-distortion curve of the shipping codec (k=256, corrected)
Scene 00020 (Dirk screaming, close-up face — the hardest case for linework),
and 00146. Includes the 2-bit-per-block mode header. No entropy coding yet.
| lam | 00020 PSNR | 00020 KB/s | 00146 PSNR | 00146 KB/s | SKIP | V1 | V4 | RAW |
|---|---|---|---|---|---|---|---|---|
| 25 | 38.68 | 182.2 | 31.04 | 193.5 | ~37% | ~24% | ~13% | ~26% |
| 100 | 35.87 | 72.2 | 29.04 | 72.5 | ~41% | ~34% | ~21% | ~4% |
| 300 | 34.80 | 44.1 | 28.28 | 44.4 | ~44% | ~42% | ~14% | 0% |
| 800 | 33.87 | 32.5 | 27.77 | 36.1 | ~46% | ~48% | ~5% | 0% |
| 2000 | 27.57 | 25.5 | 24.88 | 30.2 | ~50% | ~49% | ~1% | 0% |
Palette ceilings: 00020 = 39.90 dB, 00146 = 35.25 dB.
![quality ladder](images/quality_ladder_00020.png)
*The shipping codec across the rate knob. Top: source, palette ceiling, lam=25.
Bottom: lam=100 (`scsi` profile), lam=300 (`sasi` profile), lam=800.
Both shipping profiles hold Bluth's linework; the failure only starts past lam=800.*
Two things to read off this table:
- **The cliff is between lam=800 and lam=2000.** That is where V4 is priced out
entirely and detail blocks have nowhere to go. Do not ship past lam~800.
- **RAW is doing real work at high bitrate** (26% of blocks at lam=25) and
vanishes by lam=300. It is what makes the top of the curve reach the palette
ceiling, and it costs the decoder nothing — RAW is the cheapest mode to blit.
## 16. Licences cleared for the game-logic layer
Both checked this session:
- **astrobleem/SNES-SuperDragonsLairArcade — MIT**, "Copyright (c) 2026 Chad
Doebelin". `data/events/` holds 516 XML chapter definitions with timing and
event data. Reusable with attribution.
- **icculus/DirkSimple — zlib.** Independent from-scratch reimplementation of
the game logic in Lua, scene/timing tables in `game.lua`. Also permissive.
Having **two independent permissively-licensed transcriptions** of the arcade
scene graph is better than one: they can be diffed against each other to catch
transcription errors before any of it is committed to 68000 tables.