# Ch416 — native XYZ 12.4 coverage

## Objective

Carry the captured GS fractional screen coordinates through the fixture,
primitive FIFO, and raster coverage test.  This is a fidelity chapter: draw
population, texture state, fog fold, bilinear filtering, Z behavior, and
chronological ordering remain the Ch415 baseline.

## RTL cut

- `gs_stub` has an opt-in `SUBPIXEL_XY` path that retains each vertex's four
  X/Y fractional bits alongside the existing small primitive FIFO.
- Edge equations use 12.4 endpoints and sample the pixel center at `+8` in
  that domain.  The operands are bounded to signed 17-bit coordinates and
  signed 35-bit edge results.
- The production `GS_SH3_LPDDR_FB` profile enables the path.  Legacy profiles
  retain integer corner-sampled coverage.
- Attribute gradients remain on the proven integer coordinate basis for this
  first structural cut.  Moving gradient setup to 12.4 is explicitly deferred
  and is not hidden inside this coverage change.

## Fixture and generator controls

`--subpixel-xy` emits native XYZ2 low-half 12.4 coordinates.  Capacity
scheduling measures the same 12.4 pixel-center coverage so narrow visible
triangles cannot be discarded after integer collapse.  Runtime-CLUT fixtures
retain the proven single staging CBP and adjacent identical assets without the
obsolete preloaded-palette uniqueness gate.

The full `zsrt139f17` fixture uses `--legacy-strip-kicks` to preserve the exact
Ch415 strip population.  This makes the experiment one-variable: changing the
newer strip-population interpretation at the same time would remove broad
scene layers and invalidate the A/B comparison.

## Evidence so far

- Legacy raster regression: 16/16 pixels PASS.
- Directed subpixel raster regression: 3/3 pixel-center samples PASS.
- Authentic three-draw integration: 5,909 fragments fed, 5,907 Z-passing,
  zero drops, 0/307,200 Z mismatches, and 0/5,858 color mismatches.
- Full software reconstruction versus PCSX2 improves from Ch415 MAE 21.0852 /
  RMSE 32.3385 to Ch416 MAE 19.8099 / RMSE 28.8334.  Lit RGB pixels rise from
  174,167 to 212,915.
- Full 263-epoch RTL integration PASS: 1,139,213 fragments fed, 969,301
  Z-passing, zero drops, 263 markers and ordered drains, no color FIFO or AXI
  response errors, 0/307,200 final Z mismatches, and 0/235,942 final color
  mismatches.
- The Ch416 RTL framebuffer improves against PCSX2 to MAE 20.4941 / RMSE
  30.0163 with 199,104 lit RGB pixels.  This is a large improvement over the
  Ch415 board result at MAE 23.6438 / RMSE 35.0255 and 110,329 lit RGB pixels.
- RTL versus the Ch416 software reconstruction measures MAE 10.1578 / RMSE
  19.1650.  Native coverage closes a substantial part of the hardware/software
  gap, but fractional attribute-gradient setup remains visible fidelity debt.

Artifact SHA-256 identities:

- full epoch descriptor: `ba1484d23cc9a0caba9d4f82af7ed157fbe692c54a9138d5c0aa02f5ca9f3d87`;
- RTL framebuffer MEM: `6aa98a68ee59b5df8457b651baf12decbbb7b71c70c4029ff2cb144a3defa1de`;
- RTL framebuffer PNG: `709d6a1438355531b38723654f27b0b07e49524af10d33388fdbb744739cf7b3`;
- software reference PNG: `63944f31b8c5f55b70bbddbb89db4ac0ed782a4daf8bd6dad8bb8ad8ec9ad006`.

## First owner fit and timing repair

The first owner-controlled Quartus 26.1 GUI fit completed, but is not eligible
for deployment.  EMIF user-clock setup was -0.433 ns WNS / -6.069 ns TNS;
hold was 0.000 ns with zero TNS, and recovery, removal, and minimum-pulse checks
were clean.  The top-ten setup failures were one physical family from the EMIF
`lock_sync_inst|dreg[1]` reset source into duplicated
`u_zc_emit|u_req|...|address_reg_b[2]` registers.  This was reset routing into
the request FIFO's synchronous-read RAM implementation, not the new subpixel
coverage datapath.

`gs_async_fifo` no longer resets `rdata_q` in `REGISTERED_READ` mode.  That
value is invalid until an accepted read and every consumer already qualifies it
with the read transaction, so the reset value had no functional meaning.  The
cut removes the 5,331-fanout EMIF lock/reset route from the duplicated RAM
registers without adding a timing exception or changing FIFO latency.

Post-repair evidence:

- generic asynchronous FIFO scoreboard PASS: 3,284 writes and reads, zero
  duplicate/drop/order errors;
- Z/color emitter PASS: 1,500 fragments across three epochs plus directed
  `ZTST=ALWAYS`, exact final Z/color, `col_ovf=0`, `bresp_err=0`;
- three-epoch native-subpixel integration PASS: 5,909 fed, 5,907 passed, zero
  drops, exact 0/307,200 Z and 0/5,858 color mismatches.

## Owner fit gate

The repaired owner-controlled Quartus 26.1 GUI fit closed cleanly: EMIF setup
is +0.047 ns WNS / 0.000 ns TNS, hold is 0.000 ns WNS / 0.000 ns TNS, and
recovery, removal, and minimum-pulse checks are clean.  The fit uses 37,959 /
46,800 ALMs (81%), 5,261,744 / 7,331,840 memory bits (72%), 336 / 358 RAM
blocks (94%), and 157 / 376 DSP blocks (42%).  The deployed RBF SHA-256 is
`965e15eabdba5260a8d3c795b39a39bebc707d31fee70e3af857ec7e03f94f82`.

## Board acceptance

The full 263-epoch board run passed every texture CRC, runtime-palette sum,
staging-record count, ordered drain, and fragment-drop gate.  The scheduler
finished `rc=0`; the FPGA manager remained `operating`.  The captured board
framebuffer has 235,346 nonzero words, bounds `(0,0)..(511,479)`, sum32
`0x19291e79`, and xor32 `0x05d6395b`.

Most importantly, the board framebuffer is bit-for-bit identical to the full
RTL simulation.  Both MEM files have SHA-256
`6aa98a68ee59b5df8457b651baf12decbbb7b71c70c4029ff2cb144a3defa1de`,
and both PNG files have SHA-256
`709d6a1438355531b38723654f27b0b07e49524af10d33388fdbb744739cf7b3`.
Board versus the bilinearly resized PCSX2 frame measures MAE 20.4941 / RMSE
30.0163 with 199,104 lit RGB pixels.  Ch416 therefore realizes its simulated
fidelity gain on hardware and is accepted.