vt1200/bad-apple/REPORT.md

Bad Apple 30 s vector prototype — report

Date: 2026-11-01 Design: 2026-10-31-bad-apple-vector-design.md

What was built

• Host pipeline: video → temporal median → blur/threshold → potrace straight-line polygons → RDP → even-odd rasteriser → 32×32 tile deltas → movie.bin.
• Payload: TMS34010 C decoder + FILL L primitive, 16-bit marker page, display-interrupt pacing, UDP receive path.
• Tools: MAME debugger loader, VRAM checker, UDP sender, rdebug load script.

Measured numbers (first 30 s, 1,800 source frames)

quantity	value
source	960×720, 60 fps, first 30 s
logical grid	640×512, 2×2 screen blocks
records	1,757 (43 source frames dropped by the cost gate)
data	2,401,942 bytes (cap 3.03 MB, gate 3.0 MB)
dirty tiles/frame	mean 44.9, p50 47, p95 72, max 320
rectangles/frame	mean 393, p50 409, p95 670, max 1,175
estimated cost/frame	mean 7.47 ms, p50 7.95 ms, p95 12.01 ms, max 37.99 ms
gate	13.9 ms (one 72 Hz refresh)

The stream fits with ~0.6 MB spare. The p95 frame is inside one refresh; the max is a scene cut that necessarily takes more than one.

Exactness evidence (MAME)

tools/check_vram.py unpacks the 256-byte-stride VRAM dump, downsamples 2×2, and compares bit-for-bit against the Python application of the records.

run	movie frame	result
test pattern	0	EXACT
5 s slice	0	EXACT
5 s slice	180	EXACT
5 s slice	297	EXACT
30 s stream	0	EXACT
30 s stream	900	EXACT

The one-frame MAME run also proved the FILL geometry: a logical black rectangle 10..109 × 20..69 became exactly screen x 20..219. The first version was exactly twice as wide; the cause was the OS leaving PSIZE at 2 bpp. The primitive now re-asserts PSIZE=1 on every call.

Potrace emits y-up GeoJSON; the framebuffer is y-down. The first build was vertically mirrored, and the VRAM checker still said EXACT because it consumed the same flipped data. trace() now flips y before simplification, and test_trace_preserves_vertical_position pins the orientation. The preview PNGs were the visible symptom.

Pacing

wait_vsync() programs DPYINT=1, clears INTPEND, and polls the DI bit at 0xC0000120. The loop reads each record's ticks from the index and advances a 1/360 s accumulator by 5 per vsync, so a 1/60 s record needs 6 units and a held record needs 12. Measured with BA_COUNT=60: status at MAME frame 2580, 74 frames after takeover at 2506 — 60 source frames in 72 display frames plus startup, i.e. the 60-on-72 cadence is real, not free-running.

The performance gap MAME cannot close

MAME needed ~2.1 emulated frames per movie record while replaying frames 0..900 of the full stream. The encoder's model says a mean record is 7.47 ms of a 13.9 ms refresh, so MAME's TMS34010 interpreter is several times slower than the model. MAME proves the picture and the delta chain; it cannot prove the unit's 60 fps. The unit's measured FILL rate (0.40 µs per 16-pixel word) is the number the cost model uses. A real-unit run is the remaining performance measurement.

Network delivery

player-net.bin uses the OS UDP socket API (socket_, bind_, recvfrom_, malloc_) to receive movie.bin on port 7001 into a heap buffer, then DINTs and plays. The sender is tools/send_movie.py. Packets are 10-byte headers (offset, length, seq, crc16) plus body; a final offset==length-of-file / 4-byte-checksum packet is required, and the receiver refuses to jump unless the whole-file checksum matches. Out-of-range packets are dropped without reading or writing outside the buffer.

Status: the packet format, header offset, checksum path and sender are unit-tested; the MAME tap was down in this worktree, so the UDP receive was not exercised end-to-end. The same OS socket API is already proven on the unit by harness/udpstream/udpstream.c. This is the one delivery-path gap.

Regression

The no-env MAME baseline at 200 frames is unchanged:

--- distinct PCs sampled: 2
  PC=3FDCE400  samples=199
  PC=3FDCE350  samples=1

Remaining gaps

1. Real-unit 30 s performance: needs a power-cycle run with tools/load.sh.
2. UDP receive on the unit: socket API proven, this payload not yet.
3. Incremental painting can show a brief wipe on a cut; accepted for this prototype.
4. Potrace/RDP temporal jitter was reduced with a 3-frame median but not measured as a motion score.