# Status & next-session handoff — end of session 2 (2026-08-23)

## Decisions locked

| decision | value | why |
|---|---|---|
| Target CPU | 68000 @ 10MHz (stock) | hardest honest constraint |
| Display mode | 256 colors, 256x192 in 256x256 CRTC mode | every mode is 1 word-access/pixel, so 256c is free vs 16c |
| Double buffer | **none** — page 1 sacrificed | enables `movem.l` 24px bursts; delta coding needs a RAM reference frame anyway |
| **Codec** | **hybrid VQ: SKIP / V1 4x4 / V4 four-2x2 / RAW, per-block rate-distortion** | flat 4x4 VQ was measured and rejected — see FINDINGS 9-10 |
| **Quality modes** | **two: `sasi` and `scsi`** (USER DECISION, session 2) | one codec, one decoder, one bitstream; only `lam` differs |
| Framerate | 12 fps, **explicit decimation** | source has zero duplicate frames; no free "twos" win |
| Emulator | MAME 0.277 x68000 | accurate enough that measured cycles mean something |
| SNES project reuse | **MIT — cleared** | `data/events/` scene graph is reusable with attribution |

### The SASI/SCSI question is RESOLVED
Session 1 left "which machine do we target" open. The user's answer: **ship both**,
as two quality profiles. This is now implemented rather than hypothetical — the
bitrate ceiling is a build parameter in `tools/encoder/ratectl.py`:

| profile | target | lam | quality (00020 / 00146) | machine |
|---|---|---|---|---|---|
| `sasi` | 110 KB/s | 60 | 36.9 / 29.6 dB | stock 10MHz ACE/EXPERT |
| `scsi` | 280 KB/s | 10 | 39.4 / 32.3 dB | Super/XVI, or CZ-6BS1 board |

**WARNING — `scsi` does not currently fit 4 Mbps.** The user's working bandwidth
figure is **4 Mbps = 488 KB/s**. `scsi` means 52% of that but **peaks at 96.4%**
(FINDINGS 18), and a late frame is a dropped frame. Until `ratectl.py` is wired
into `encode.py`, `scsi` must either drop to `lam=25` (194 KB/s mean) or not
ship. Derive profiles with `tools/encoder/profile_gen.py --bw-mbps 4`, not by eye.

`scsi` is now within **0.5 dB of the palette ceiling** on 00020. These were
initially set at 45 / 75 KB/s, which was 12% / 7% bus utilisation — read off the
RD curve rather than derived from the hardware. See FINDINGS 17.

Codebooks are **k=256 with 1-byte indices** in both profiles. k=1024 was measured
and rejected — see FINDINGS 14, it was a false-good result from a rate model
that undercharged the index. Do not ship past `lam~800`; FINDINGS 15 has the cliff.

Because of the RAW escape mode, `lam=0` is **pixel-exact** against the palettised
frame (measured 0.00 dB loss). The profiles are two points on one continuous
rate-distortion curve, not two codecs.

---

## What session 2 settled

1. **The critical-path question is answered.** "Does VQ soften Bluth's linework
   unacceptably?" — **flat 4x4 k=256 VQ: yes, badly. Hybrid VQ with k=1024: no.**
   Verified by eye, not just PSNR. See `docs/FINDINGS.md` 9-11.
2. **Session 1's 12fps bitrate was wrong** (183 KB/s claimed, 340 KB/s measured).
   Halving the framerate does not halve the bitrate. FINDINGS 8.
2b. **A fourth false-good result was produced and caught this session** — k=1024
   codebooks looked like a +2.4 dB free win because the rate model charged 1 byte
   for a 10-bit index. FINDINGS 14. The k=256 configuration ships.
3. **The 256-colour palettised frame is the real quality ceiling** and it looks
   excellent. Judge the codec against that, not against 1080p.
4. Encoder exists and produces a real bitstream: `tools/encoder/`.

---

## Encoder — working

```
python3 tools/encoder/extract.py 00020 /tmp/fr_00020 12 crop
python3 tools/encoder/encode.py  /tmp/fr_00020 out.dlx --profile sasi --preview p.png
```

| file | role |
|---|---|
| `extract.py` | .m2ts -> 256x192 PNGs, 12fps, spatial-only denoise |
| `vq.py` | palette, blockify, hand-rolled k-means (no sklearn on this box), PSNR |
| `vq_hybrid.py` | the codec: 4 block modes + lagrangian mode decision |
| `ratectl.py` | SASI/SCSI profiles, leaky-bucket rate control |
| `encode.py` | CLI + `DLX1` container writer |

`DLX1` container layout is documented in the `encode.py` docstring. All
multi-byte fields are **big-endian** so the 68000 reads them with a plain `move`.

### Known encoder gaps
- **Rate control is written but not yet wired into `encode.py`** — the CLI uses a
  fixed `lam` from the profile. `ratectl.encode_rate_controlled()` exists and
  builds a lam-ladder per frame; it needs hooking up and validating.
- **Payload is deliberately NOT entropy-coded** — deflate decode does not fit in
  the 68000's frame budget (FINDINGS 17.2). Do not "optimise" this later.
- Codebooks are per-scene and rebuilt from scratch; no inter-scene reuse.
- `_paint` is a Python per-block loop — fine for prototyping, slow for a full
  disc encode. Vectorise before the 224-stream run.

---

## Working setup (unchanged from session 1, re-verified)

**MAME ROMs** — `~/mame/roms/x68000.zip`. Must pass **`-bios ipl10`**.
```
mame x68000 -bios ipl10 -video none -sound none -nothrottle -seconds_to_run 3
```
**Assembler** — `tools/vasm/vasmm68k_mot -Fbin -o out.bin in.s`

**Blu-ray** — `udisksctl loop-setup -r -f DRAGONS_LAIR.iso` -> `/media/reala-misaki/BDROM`
(still mounted as of end of session 2).

**MAME Lua harness** — `tools/bench/*.lua`, working. Three gotchas (retain the
notifier subscription in a global; the stack register is `SP` not `A7`;
`autoboot_script` fires at PC=0 before boot) are documented in FINDINGS.

**Two shell traps, both hit again this session:**
- piping MAME (or any long job) through `grep` block-buffers — write to a file.
- `pkill -f <pattern>` matches your own shell and kills it (exit 144).
  Use `pkill -x` or kill by PID.

---

## STILL BLOCKED: disk throughput benchmark

Unchanged from session 1 — `IOCS _B_READ` returns -1 uniformly. Full diagnosis
and the four untested hypotheses are in session 1's notes (git history of this
file, commit 65112b9).

**This is now CRITICAL-PATH, not optional.** Session 1 dismissed it because
"VQ at 30 KB/s is correct whether SASI does 300 or 600 KB/s". That reasoning is
dead: the profiles now sit at 110 and 280 KB/s, close enough to the folklore
ceilings that the error bars change the product. Specifically —

**If SCSI sustains >=800 KB/s, the correct `scsi` profile is `lam=0`: pixel-exact
video, ~450 KB/s, and only 38% of the CPU budget.** Whether this port ships
transparent or lossy on SCSI is waiting on one measurement.

Next move is the untried SCSI path: `-exp1 cz6bs1 -hard disk.chd`.

---

## Next steps, in priority order

1. **Wire rate control into `encode.py`** — now the highest-value work in the
   repo, not a loose end. It is worth a full step on the quality ladder
   (lam=25 -> lam=10, +0.7/+1.2 dB) because it lets us size for the mean
   instead of the peak. Validate the bucket holds on an action scene.
2. ~~Entropy-code the payload~~ — **ABANDONED, see FINDINGS 17.2.** Deflate
   decode is ~216% of the frame budget on a 68000 and LZ4 is ~54%; there is no
   room beside a 38% blit. All bitrates are raw payload. This also demotes the
   "247 KB/s lossless" figure in FINDINGS 8 to a compression bound, not a design.
3. **68000 decoder skeleton**: parse `DLX1`, expand codebooks to word-per-pixel,
   blit V1/V4/RAW/SKIP. Measure real cycles with the existing MAME Lua harness —
   this is the first time the harness gets used for its actual purpose.
4. **Full-disc survey** — classify menu vs content first (FINDINGS 13), then
   measure bitrate across all 224 streams per profile.
5. **Resolve the framing question** (FINDINGS 12: crop vs squash vs wide).
6. Unblock the disk benchmark via the SCSI path, then re-set profile bitrates.
7. Import the SNES project's `data/events/` (MIT, cleared) as the scene graph.
   Cross-check against DirkSimple (zlib) which has the same data independently.
8. ADPCM audio: MSM6258, 15.6kHz mono, 7.8 KB/s — already budgeted in `ratectl`,
   not yet extracted or encoded.

## Not yet started
- Any 68000 player code
- ADPCM audio extraction/encoding
- Disk image packaging
- Game logic (scene branching, input windows, death clips)