Find the sustained action sequence: it breaks both profiles
The open risk since session 2 was "a sustained action sequence could still break the bitrate", with every clip measured so far being 1.2-1.7 s. Closed by measurement rather than by sampling clips by hand. 07_motion_survey.py scans a whole stream at 96x72 for the hottest sliding window of inter-frame difference. On 00223 the spread between the quietest and hottest sustained 10 s windows is 10.6x, which is the argument for not eyeballing it. Hottest is t=539.4s, the Singe endgame. There, with the fixed lam the CLI uses, sasi overshoots 110 -> 129.6 KB/s (+18%) and scsi 280 -> 373.8 KB/s (+34%). Rate control moves from "insurance, not a fix" to required, and is promoted above the full-disc survey. The bus is not broken -- 381.6 KB/s still fits the 488 KB/s figure -- so FINDINGS 21 survives, at 78% of the pipe instead of a comfortable margin. Three further corrections fall out: - The two largest streams on the disc are bonus material. 00216 is the feature with a burned-in commentary PiP; 00215 is the commentary. 00223 is the clean 9.4 min. A size-ranked survey would have encoded live action. - On hard content the 256-colour scene palette (31.33 dB) binds well before the X68000 display (40.81 dB); scsi is already within 0.51 dB of it. - FINDINGS 24.5's architecture question resolves to "both paths, chosen per frame": 30-53% of frames sit above the 70% crossover. Picking per frame costs a median 37.0% of the frame budget and caps at 53.6%. Reporting for this is wired into encode.py, which previously only printed a mean over all frames -- the one statistic that cannot answer a per-frame question. extract.py takes optional start/dur; 08_mode_map.py renders source | decoded | block-mode map to .webm. Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
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prosolis
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@@ -96,7 +96,29 @@ rate-distortion curve, not two codecs.
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3. **Reading the source frame is exactly half the blit cost** (V1 53.6% vs a
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write-only floor V3 of 27.1%). That is what makes the architecture question
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below live.
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4. **The decoder architecture now hinges on one unmeasured number.** Writing
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4. **That number is now measured, and the answer is "implement both paths".**
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On the worst sustained window found on the disc, 30% of frames (`sasi`) to
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53% (`scsi`) sit above the 70% crossover and want the flat blit; the rest
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want direct-to-GVRAM. A player that picks per frame — the mode headers are
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parsed before any pixel is written, so the count is free — pays a **median
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37.0%** and is **capped at 53.6%**. FINDINGS 25.6.
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5. **The sustained action sequence exists, was found by measurement, and breaks
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both profiles.** `tools/analysis/07_motion_survey.py` scans a whole stream
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for the hottest sliding window; on 00223 it is t=539.4s, the Singe endgame,
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at 2.01x the stream mean. There, fixed-lam `sasi` overshoots 110 -> 129.6
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KB/s (+18%) and `scsi` 280 -> 373.8 KB/s (+34%). **Rate control is no longer
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insurance — it is required.** FINDINGS 25.3.
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6. **The two largest streams on the disc are bonus material, not game footage.**
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00216 is the feature with a burned-in commentary PiP; 00215 is the commentary
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itself. **00223 (9.4 min) is the clean one.** A size-ranked survey would have
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encoded live action. FINDINGS 25.1.
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7. **On hard content the scene palette, not the display, is the binding
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ceiling** — 31.33 dB on the Singe window against 39.90 dB on 00020 and 40.81
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dB for the X68000 display. `scsi` is already within 0.51 dB of it.
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FINDINGS 25.4.
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### Superseded within session 5
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4a. **The decoder architecture hinged on one unmeasured number.** Writing
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codewords straight into GVRAM costs 76.6% of the frame budget for a *full*
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frame (V4 — the 1024-byte stride kills the `movem.l` burst), but scales with
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the non-SKIP block fraction and needs **no RAM reference frame at all**,
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@@ -284,7 +306,11 @@ SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 -video soft -window \
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## Next steps, in priority order
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1. **Measure the non-SKIP block fraction.** *(new top priority, session 5)*
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1. ~~**Measure the non-SKIP block fraction.**~~ **DONE, session 5** — FINDINGS
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25.6. Answer: implement **both** display paths and pick per frame; median
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37.0% of the frame budget, capped at 53.6%. Reporting is wired into
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`encode.py`. Original framing kept below because the reasoning still governs
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the decoder's inner loop:
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FINDINGS 24.5: compose-in-RAM-then-blit costs a flat 53.6% of the frame
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budget; decode-direct-to-GVRAM costs 76.6% x (fraction of blocks that are not
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SKIP) and needs no RAM reference frame. **They cross at 70%.** Which side of
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@@ -297,6 +323,15 @@ SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 -video soft -window \
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scene-cut frame is ~100% non-SKIP and a held frame near 0%, and the mean of
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those two is a number describing no actual frame.
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1b. **Wire rate control into `encode.py`. NOW REQUIRED (was priority 4).**
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FINDINGS 25.3: on the worst sustained window both profiles overshoot their
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targets with the fixed `lam` the CLI uses — `sasi` by 18%, `scsi` by 34%.
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The note that used to sit here, "no longer a blocker (FINDINGS 21)", was true
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of the 1.2-1.7 s clips measured at the time and is not true of this one.
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`ratectl.encode_rate_controlled()` already builds a per-frame lam ladder; it
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has never been hooked up. Do this before the full-disc survey or the survey
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measures an encoder nobody will ship.
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2. **68000 decoder skeleton**, with the inner loop chosen by (1). Parse `DLX1`,
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expand codebooks, blit per block mode. The display path is verified *by 68000
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code* now (FINDINGS 24) and the harness pattern is `tools/bench/blit.s` +
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@@ -312,16 +347,18 @@ SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 -video soft -window \
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rejection, which was argued as "54% LZ4 with no room beside a 38% blit". The
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conclusion gets *stronger*, not weaker, but the arithmetic should be restated.
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3. **Full-disc survey.** Only 4 clips of 1.2-1.7 s out of 224 streams have been
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measured, and 00146 already runs 23% hotter than 00020. A *sustained* action
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sequence is the one thing that could still break the bitrate. Classify menu
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vs content first (FINDINGS 13) or the averages are diluted by static menus.
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**Vectorise `_paint` before this run** — it is a Python per-block loop.
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Pairs naturally with (1): the same run produces both numbers.
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3. **Full-disc survey.** Now scoped by session 5 rather than open-ended: the
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worst *sustained* window is measured (FINDINGS 25), so what remains is the
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distribution over content, not the worst case.
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- Classify **content / menu / bonus** — not just menu vs content. FINDINGS
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25.1: the two largest streams are bonus material and look like content by
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size, duration and bitrate alike.
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- Run `tools/analysis/07_motion_survey.py` per stream first; it is cheap
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(96x72 greyscale) and gives a hot-window shortlist so the expensive encode
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only runs where it matters.
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- **Vectorise `_paint` before this run** — it is a Python per-block loop.
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- Do it **after** rate control (1b), or it measures an encoder nobody ships.
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4. **Wire rate control into `encode.py`.** No longer a blocker (FINDINGS 21), but
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it is what gives a deterministic ceiling over content not yet measured, which
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was the original reason for choosing VQ. Insurance, not a fix. Pairs with (1).
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5. **Confirm DMA vs PIO in MAME** (see the benchmark section above) — cheap, and
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the only thing that could still move CPU into the binding position.
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6. **Resolve the framing question** (FINDINGS 12: crop vs squash vs wide).
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@@ -425,3 +462,22 @@ V1's output. To check that snapshot is still pixel-exact:
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Not added to `check.sh`: `check.sh` asserts pixel-exactness, and asserting wall
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timings there would make the green-light check sensitive to host load.
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## Reproducing the sustained-action result (session 5)
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```
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python3 tools/analysis/07_motion_survey.py 00223 10 # -> hottest window t=539.4s
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python3 tools/encoder/extract.py 00223 tmp/fr_singe 12 crop 539.4 10.0
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python3 tools/encoder/encode.py tmp/fr_singe tmp/singe_sasi.dlx --profile sasi
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python3 tools/encoder/encode.py tmp/fr_singe tmp/singe_scsi.dlx --profile scsi
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python3 tools/analysis/08_mode_map.py tmp/fr_singe tmp/singe_modes.webm \
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--profile sasi --scale 2
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```
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`extract.py` now takes optional `[start_s] [dur_s]` — needed because 00223 is
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9.4 min and the windows that stress the codec are seconds long.
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`08_mode_map.py` renders palettised source | decoded | block-mode map at 12fps.
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Output format follows the extension; **prefer `.webm`** — GIF re-quantises to
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256 colours, which is a poor fit for output whose subject is colour fidelity,
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and runs larger. It uses `yuv444p` because the mode map is flat saturated colour
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on a 4-pixel grid and chroma subsampling smears exactly those edges.
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