d4r/engine/rr/README.md

# Ray Reconstruction stages (DLSS-D, NGX feature 13)

d4r's own Vulkan compute shaders for the denoiser in `nvngx_dlssd.dll` 310.7 (presets D and E). One shader per
NVIDIA kernel of the frame; the engine records them in NVIDIA's order:

| stage | NVIDIA kernel | shader | grid (640x360 render, 1280x720 output) |
|---|---|---|---|
| exposure | `cuda_dldn_engine_luma_convert_kernel`, `reduce_sum` x2-3, `auto_exposure_copy_kernel` | `engine/exposure_k.comp` (MODE 1) | |
| enc0 | `cuda_dldn_engine_swin_enc0_kernel` (D: `..._enc0_diamond_wallaby_kernel`) | `enc0.comp` | 81x49, 32 threads |
| enc1-enc5 | `cuda_dldn_engine_swin_enc1_kernel` ... `enc5` | `enc1.comp` ... `enc5.comp` | |
| dec4-dec1 | `cuda_dldn_engine_swin_dec4_kernel` ... `dec1` | `dec4.comp` ... `dec1.comp` | |
| dec0 | `cuda_dldn_engine_swin_dec0_kernel` (D: `..._dec0_diamond_wallaby_kernel`) | `dec0.comp` | 81x49, 32 threads |
| output | `cuda_dldn_engine_hkpn_output_kernel_transformer` (D: `..._diamond_wallaby`) | `hkpn.comp` | 80x45, 16x16 threads |

The D variants of enc0, dec0 and the output kernel are different code; the other nine kernels are shared and only
their weights differ between presets.

## Stage contract

Every stage shader reads the same data NVIDIA's kernel reads, in the same layouts, so each one can be checked on
its own against a captured launch (`test_layer.py`) and the stages chain exactly like NGX's kernels:

- binding 0: the parameter block, `std430` readonly. It is NVIDIA's argument block byte for byte, except that every
  pointer into the plane arena holds its **byte offset in the arena** (a 64-bit value), the weight pointer is zero,
  and a texture or surface handle is 1 where NGX bound one and 0 where it left the argument null (the kernels test
  optional inputs' handles for null). Fields are read at NVIDIA's argument offsets.
- binding 1: the layer's weight image exactly as NGX uploads it (argument 40's allocation), `std430` readonly.
- binding 2: the plane arena, `std430` read/write.
- bindings 3, 4, ...: the textures in ascending argument offset (combined image samplers, clamp-to-edge,
  normalized coordinates, linear or nearest as NVIDIA's texture object; 8-bit unsigned arrays are sampled as UNORM,
  `SNORM_INT8X4` as SNORM), then the surfaces in ascending argument offset (storage images, `rgba16f` or
  `rgba8_snorm`).
- The dispatch grid is NVIDIA's launch grid. The workgroup size is the shader's choice.

## Checking a stage

Captures (bridge launch and replay dumps of a harness run, see `rr_capture.py`) live outside the repository.

```sh
glslangValidator -V --target-env vulkan1.3 engine/rr/enc1.comp -o build/rr/enc1.spv
python3 engine/rr/test_layer.py CAPTURE swin_enc1 2 build/rr/enc1.spv build/rr/work-enc1 [--time 50]
```

It starts from the arena as it was before NVIDIA's launch, with zeroed surfaces, and compares every surface and
the whole arena with what NVIDIA's launch left. `build/rr/kvk` is `vulkan/kvk.cpp`
(`g++ -O2 -std=c++17 -o build/rr/kvk vulkan/kvk.cpp -lvulkan`).

## Native stages (preset E)

`build_native.py CAPTURE MODEL_DIR` turns a `compile_rr.py` model directory into one where every Swin stage is
d4r's own cooperative-matrix shader instead of translated PTX. Each was derived phase by phase from ZLUDA replays
(`phase_dump.py`: every mma's operands of one window) into an exact numpy reference, then written against it:

| stages | reference | shader | weights |
|---|---|---|---|
| enc1-enc5, dec4-dec1 | `swin_ref.py` (one generic model: C, heads, merge / expand) | `swin_m.comp` | `swin_pack.py` |
| dec0 | `dec0_ref.py` | `dec0_m.comp` | `dec0_pack.py` |
| enc0 | `enc0_ref.py` (after the per-pixel features) | `enc0_m.comp` from `gen_enc0_m.py` | `enc0_pack.py` |

`enc1_m.comp` / `dec1_m.comp` are the hand-written C = 64 instances `swin_m.comp` generalises; `enc1_ref.py` holds
the shared numerics (inter, mm, the cubic softmax, the norm trees). enc0 keeps ptx2rr's translation of NVIDIA's feature
gather (waves 0-2 run its pixel loop) and is native from the embedding on.

Binding 1 differs from the stage contract above: it holds the packed weights (enc0: NGX's image, then the packed
blob). Everything else is the contract, so `test_layer.py --weights FILE` checks a native stage the same way. Against
capE3 every native stage leaves the arena byte-identical to NVIDIA's; surfaces differ in a few values by one binary16
ulp (`exp2` approximation, and decoder texture 88 where NVIDIA's bilinear unit rounds differently), and the whole
network's output is ~96 dB from NVIDIA's.

The shaders run every product transposed (weights x activations) so results feed the next product in registers;
`enc1_m.comp`'s header explains the layouts and `rr_exact.glsl` the binary16 rounding RADV needs (it folds
`float(float16_t(x))` and does not fuse f16 `fma`).

## Native models and preset D

`engine/rr/build_native_model.sh CAPTURE OUT_DIR [NVNGX_DLSSD_DLL]` builds a model whose eleven network stages are
the native shaders (`swin_m.comp`, `dec0_m.comp`, the generated `enc0_m.comp`) for the preset of the capture, with the
translated output stage.

The weights are read from the user's DLL itself (`dll_weights.py`), not from the capture. The DLL holds each model's
tensors as plain binary16 arrays in natural `[out][in]` order, stage after stage; nothing names them, so they are
found by structure: every attention head has a 100 x 100 relative position bias table whose entries depend only on
the offset between two window positions, and a model is eleven consecutive stages with 2 2 4 4 8 8 8 4 4 2 2 heads.
What NGX uploads is the same data rearranged: FP8 codes in the kernels' fragment order, the norm gains times
sqrt(C), the bias times 2 sqrt(2) for the kernels' 64 x 96 query / key order, and (preset E) the 4096-byte noise
table encoder 0 dithers with. `dll_weights.py DLL OUT --model N --check DIR` compares a reading with a phase-dump
extraction: for 310.7 every tensor of both presets is identical, and so are the packed weight files. The capture
only selects which of the DLL's models it ran (and supplies the two argument-block templates); a DLL whose network
kernels are not the ones the shaders were written against (`native_kernels.txt`, SHA-256 of the PTX modules) is
refused unless `D4R_RR_ANY_KERNELS=1`.

`compile_rr.py` uses the translated output kernel for both presets: `engine/rr/hkpn.comp` has the reset path only
(`--reset-only` selects it for captures that reset every frame).

Preset D (`..._diamond_wallaby` kernels) differs from E in three places:

- **enc0**: another feature gather (kept as the translation, like E's) with 30 input features for E's 32, and no
  noise table (its weight image is 4096 bytes smaller).
- **dec0**: the eight bytes per pixel for the output stage are channels 12, 13, 0, 18, 19, 20, 21, 0
  (`-DRR_PRESET_D`; E: 12, 13, 18-23). Everything else is E's kernel.
- **output stage**: its own kernel (closest-depth motion from texture 296, a 4x4 bicubic history fetch). It is the
  translated stage; it has no subgroup operations and runs with 64-wide waves (`rr.txt`).

The reset flag (enc0 byte 365, output stage byte 440) follows `Reset`, not the jitter. With `Reset = 0` the output
stage reprojects and blends its history: check stages against a temporal capture (the harness with
`D4R_HARNESS_JITTER_SCENE=1`, and `D4R_CUDA_REPLAY_DUMP_LIMIT=400` so every launch is saved), not only against
captures that reset every frame. `engine/rr/hkpn.comp` implements the reset path only.

Preset D, RX 9070 XT, RADV 26.2.4, 2026-10-12, 1280x720 -> 3840x2160 (`d4r-rr`, `D4R_RR_TIME_REPEAT=20`): 5.0-5.2 ms
per frame (network 4.5, output 0.55-0.65), from 17.5 ms with every stage translated. Every network stage reproduces
the capture's arena byte for byte on reset and temporal frames. The presented image against the CUDA path over
temporal frames: 96-106 dB on the reset frame, then 41-52 dB (one-ulp differences in the filtered surfaces are
amplified by the recurrent feature; twelve frames stay at 41-45 dB).

RE Requiem (2026-10-12, 1280x720 -> 3840x2160, R9G9B9E5 colour, roughness packed in the normals' w): with
`hkpn.comp` the picture was 3-pixel blocks that shook with the jitter (no history); with the translated output stage
presets D and E both give a stable, reconstructed image on the engine. Not compared with the CUDA path in that game.

## Matching the reference bit for bit (2026-10-12)

The reference is the CUDA path through ZLUDA on the same GPU, so its `rcp.approx`, `ex2.approx` and bilinear texture
fetches are the GPU's own instructions and texture unit, not correctly rounded arithmetic. Two places in the native
filters had been "corrected" to exact arithmetic and were one ulp off in ~0.01-0.2% of the filtered surfaces:

- `rcp16` (the filters' softmax normalisation and gate) is now the plain hardware reciprocal;
- the decoders' texture 88 goes through the bilinear sampler at NVIDIA's coordinates (`rr_decfilter.glsl`,
  `dec0_m.comp`) instead of a hand-computed 1/4, 3/4 blend.

With both, every network stage (arena and surfaces 112 / 120 / 128 / 144 / 304) is identical to the capture on every
frame of capDT12, capD4K, capD1080 (preset D) and capE3 (preset E). What remains is the translated output stage:
~0.004% of surface 224 / 240 one ulp off (single channels, values just above 0.5; cause not found). The output is
107-112 dB on frames where the network's inputs are still exact. At 640x360 -> 1080p that holds for all twelve
frames of capDT12 (was 96 dB, then 41-45); at 1280x720 and 1920x1080 -> 4K the history's differing values reach
encoder 0 through texture 496 and the output falls to 52 / 45 / 43 dB (capD4K), 53 / 49 dB (capD1080) as before.

Speed: the compiled stages spend about half their instructions on float conversions. `spv_exact_convert.py` marks
the conversions NoContraction so RADV keeps a binary16 rounding without the opaque OR (`RR_EXACT_CONVERT`), which is
bit-exact and ~8% faster: preset D 4.8 ms at 1280x720 -> 4K, 9.5 ms at 1920x1080 -> 4K (was 5.1 / 10.3).
`D4R_RR_RELAXED=1|2` builds drop NVIDIA's double rounding instead (8.2 / 7.6 ms at 1080p) but are not exact: about 1%
of a layer's FP8 codes change and the output is 39-44 dB from the reference on every frame.

Profile of a full-resolution stage at 1080p (dec0, 3.0 ms): expand 0.31, key norm 0.16, attention 1.37 (per head:
projections 0.15, scores + softmax 0.26, P V 0.17, out-projection 0.10), MLP 0.69, output filter 0.50. The 136
matrix products themselves are 0.45-0.5 ms; weight and table memory traffic is under 8%. enc0's translated feature
gather is 0.28 of its 3.0 ms.

## Optional inputs: specular hit distance and disocclusion mask (2026-10-12)

RE Requiem passes `DLSSD.SpecularHitDistance` (R32_FLOAT) and `DLSS.DisocclusionMask` (R16_FLOAT). Harness captures
with synthetic ones (`D4R_HARNESS_RR_EXTRA=hit,mask`; capDX, capDXhit, capDXmask) show the wiring:

| input | encoder 0 texture | encoder 0 flag byte | output stage |
|---|---|---|---|
| specular hit distance | argument 248 | 536 | flag byte 160 |
| disocclusion mask | argument 296 | 539 | - |

Encoder 0 also tests the handle at argument 248 for null before it takes the hit distance path (it writes a
reprojected specular motion into channels 2 and 3 of surface 304), which is why the parameter block now carries the
handles' presence. With both bound the network is identical to the capture on every frame (capDX). The output
stage then differs from the CUDA path on the image's border pixels only (63-67 dB on temporal frames): NVIDIA's
kernel fetches neighbours outside the image there, CUDA clamps such integer fetches to the edge (every denoiser
texture's address mode is clamp; the translation and rrsim do the same), and ZLUDA's image load returns zero. With
zero outside the image the stage matches the capture like before (99.997%); the engine keeps CUDA's clamp.

Exact speed-ups since the first native build, all bit-identical per stage: conversions kept by NoContraction instead of
an opaque OR, the MLP activation and its FP8 conversion from a 64 KiB table (`enc1_pack.act_table`), FP8 conversions in
pairs, no saturation clamp on attention probabilities. Preset D: 4.4 ms at 1280x720 -> 4K, 8.6 ms at 1920x1080 -> 4K.

Output stage, hypotheses tested for its remaining ~0.004% one-ulp values and rejected (2026-10-12): correctly rounded
exp2 / reciprocal (rrsim's exact arithmetic is further from the capture than the GPU), fusing the kernel's 34
single-use multiply-subtract pairs into fma, merging the two constant multiplications after the log transform,
`precise` on the inlined exp2, NoContraction on its conversions. The differing values come in spatial clusters, one
colour channel at a time, always one ulp, mostly just above 0.5 in surface 224.

## HIP against Vulkan, and the fused rounding (2026-10-12)

Encoder 1 ported line for line to a standalone HIP kernel (`kernels/rr/swin_native.h`, runner
`kernels/tools/rr_swin_native_test.cpp`; capD1080 frame 2, 5 launches per round): 835 us against the Vulkan stage's
770-788 us, the arena byte-identical. A HIP port of the network is not worth making. What the port did show:

- The FP8 product alone costs 0.61 us per product per wave on this grid, 2.07 us when each is followed by the
  f32 -> binary16 -> f32 rounding of its eight accumulator registers. Products and that rounding are about half of
  a stage on either backend.
- Keeping the accumulator as binary16 and taking f16(P + acc) with `v_fma_mix` (as `kernels/m` does) halves the
  rounding's instructions: HIP 780 us. RADV emits the same instruction for `float16_t(p + float(acc))`, so the
  Vulkan stages have it as a build option: `D4R_RR_MIX=1 build_native_model.sh ...` (`RR_MIX`, the shaders' K32).

`RR_MIX` is one rounding of the exact sum where the reference has two, so it is not bit-exact by construction.
Preset D: every network stage still identical to the capture on all 21 frames of capDT12 / capD4K / capD1080 / capDX
/ capDXcp / capDXhw (the same figures as the default build), 4.22 ms at 1280x720 -> 4K and 8.3 ms at 1920x1080 -> 4K
(default 4.45 / 8.7). Preset E (capE3): frames 1 and 3 have a value of encoder 0 one step off (127 dB), which the
later stages amplify to 41 dB at the output; frame 2 is identical. The default build stays the exact one.

## Cyberpunk 2077: more inputs and settings (2026-10-12)

Found on harness captures (`D4R_HARNESS_RR_EXTRA=particles,sss`, `D4R_HARNESS_RR_HW_DEPTH=1`, `D4R_RR_PRESET=N`):

| what the game passes | where NGX puts it |
|---|---|
| `DLSSD.ScreenSpaceSubsurfaceScatteringGuide` (R16F) | encoder 0 texture 264 (its handle is the only switch) |
| `DLSSD.ColorBeforeParticles` (RGBA16F) | output stage texture 416, flag byte 432, and a second output-size history: surface 384, read back next frame as texture 392 (linear) |
| `DLSS.Use.HW.Depth = 1` | encoder 0 byte 980, output stage byte 443 |
| preset hint | 5 runs preset E; every other value (0-4, 6, 0x80000002, 0x80000005) runs preset D |

Integer texture fetches outside the image return zero (`fetch0`), which is what the reference does; CUDA would clamp.
With Cyberpunk's set (hit distance, particles, subsurface guide, hardware depth) the engine's presented image is
byte-identical to the CUDA path through the shim on the frames the harness can compare (640x360 -> 1920x1080), and
with RE Requiem's set the network is identical on every frame (output 107-109 dB). At 1280x720 and 1920x1080 -> 4K
without these inputs the output still falls to 43-53 dB after the first frame (the output stage's one-ulp values);
a correctly rounded log2 there matches the capture worse than the GPU instruction does.

## The output stage's one-step differences: fused multiply-convert (2026-10-13)

The rare one-binary16-step values of the translated output stage (above: ~0.004% of surfaces 224 / 240, "cause not
found") were the driver fusing an f32 product (or sum) with the conversion that follows it into `v_fma_mix`, which
rounds the exact result once. The kernel rounds twice, to f32 and then to binary16, and the two differ when the f32
result is exactly a binary16 midpoint, as `log2(x) * c1 * c2` of the output stage's tone curve sometimes is.
`ptx2rr.py` now ORs an opaque zero into the f32 bits before every `cvt.rn.f16.f32` (and into the binary16 bits
after, against the fold of `float(float16_t(x))`). With it presets D and E are byte-identical to the CUDA path on
every frame of every capture (capDT12, capD4K, capD1080, capDX, capDXcp, capDXhw, capE3; history and output), at
+0.05 ms: 4.27 / 4.45 ms at 1280x720 -> 4K. Found with `build/rr`-style bisection: probe every register of a block
through ZLUDA (debug stores added to the PTX, `zluda_replay.py`) and through the translated shader
(`ptx2rr.py --probe`) and list the first that differ.

Other reference behaviours the translation now follows, all found the same way on preset F's kernels:

- formatted stores to normalized surfaces round half away from zero in f32 (ZLUDA's `surface_formatted_store_bits`);
  UNORM 10:10:10:2 arrays are RGBA32F images holding the quantised values `q / 1023` (the correctly rounded quotient,
  from a table: the driver divides by multiplying with a reciprocal);
- `tanh.approx.f32` is OCML's `tanhF` with LLVM's AMDGPU lowering of `exp` (`lowerFEXP`, fast-FMA path): the same
  operations, the fused ones through f64 (the driver's f32 `fma` is a product and a sum whenever the product also
  exists on its own); `tanh.approx.f16` is the correctly rounded value (a table);
- the driver's GLSL `fma` on binary16 is a true fused operation, as the reference's is: no emulation there;
- texture fetches with a binary16 result keep that rounding.

## Preset F: DLSS 4.5 Ray Reconstruction (2026-10-13)

`nvngx_dlssd.dll` 310.9 has a third model with its own kernels, `rr2_*`, and runs it for every preset hint but 4 (D)
and 5 (E): none, 6, Cyberpunk 2077's 0x80000002. The engine runs it from a `D4RR0002` model:

```sh
python3 kernels/tools/extract_dlss_ptx.py NVNGX_DLSSD_DLL ~/.cache/d4r-rr-corpus-f       # its PTX (zstd in 310.9)
python3 engine/compile_rr2.py CAPTURE OUT_DIR --native NVNGX_DLSSD_DLL [--particles CAPTURE_WITH_PARTICLES]
```

The runtime is `engine/runtime_rr2.inc` (behind `RModel` / `REngine`); the shim uses `engine\rr-f` (or
`D4R_ENGINE_MODEL_RR_F_DIR`) and, when that model exists, NGX 310.9's hint mapping.

A frame, as NGX launches it:

| pass | NVIDIA kernel | engine |
|---|---|---|
| exposure | `rr2_histogram_auto_exposure_basic_kernel` | `exposure_f.comp` (two passes) |
| enc0 | `rr2_enc0_kernel` | translated feature gather and guide tail around the native block (`gen_rr2_enc0.py`) |
| enc1 - enc5 | `rr2_enc1_kernel` ... | `rr2_m.comp` |
| dec4 - dec1 | `rr2_decN_kernel_0`, `rr2_decN_kernel_1` | `rr2_m.comp` |
| dec0 | `rr2_dec0_kernel_0`, `rr2_dec0_kernel_1` | `rr2_m.comp`; the second: native block, translated output tail |
| output | `rr2_post_kernel`, `rr2_downsample_kernel_static_hdr[_no_transparency]` per tile, `rr2_pooled_history2_kernel` | translated (`ptx2rr.py`) |

The network (`rr2_ref.py` is its exact numpy reference and documents the block): D/E's levels, window grids and
arithmetic (FP8 operands, binary16 sums rounded per 32-deep step, the cubic softmax, the MLP activation), with a
different block. Each kernel is one block on 8x8-token windows that alternate their shift per frame: heads that
share one projection for queries and keys (so the scores are symmetric; bias `(T + T^T) sqrt(2)` of a 64 x 64
relative position table), all reading the block's input and summing their out-projections; MLP branches likewise;
both sums enter the residual times a binary16 scalar from the argument block. Encoders merge 2x2 for the next level,
a decoder's first kernel expands the level below and adds the skip times a third scalar. Next to the tokens every
level carries a three-channel guide image: encoders gate it (`sigmoid(head) * s0`) and average it 2x2 for the next
level, decoders upsample it bilinearly, gate it and blend it with the encoder's. All stages exchange data through
the arena (3152 units of W0 * H0 / 32 bytes; NGX's region offsets, which scale with the render size), tokens as FP8
planes of 32 channels, guides as four binary16 per token.

Weights: the DLL holds the model as plain binary16 tensors in natural `[out][in]` order, kernel after kernel
(`rr2_ref.dll_weights`; found by the sixteen kernels' relative position tables, 2 2 4 4 8 8 8 8 4 4 4 4 2 2 2 2
heads). `rr2_pack.py` packs them for `rr2_m.comp`; nothing of the network comes from the capture. The capture still
supplies every kernel's argument block (the runtime keeps their constants and fills the geometry, the frame's
values and the pointers) and encoder 0's second table.

The output stage filters an intermediate image of 1.5 times the output size in tiles (columns of at most 720, rows
of at most 1620 intermediate pixels, 4 pixels of overlap inside), reduces each tile to the output, and keeps two
histories for the next frame: the intermediate image, and a half-size copy whose quarter-size reduction
(`rr2_pooled_history2_kernel`) encoder 0 also reads (UNORM 10:10:10:2 arrays, see above). NGX runs filter and
reduction concurrently with progress counters; the kernels skip that when the counter pointers are null, which is how
the engine runs them, in order. They have no matrix products and run with 64-wide waves (their warp-level shuffles
address the lane's own half).

Options (harness captures capFX*): encoder 0 byte 466 packed roughness, 568 inverted depth, 569 / texture 272
`DLSSD.SpecularHitDistance`, 571 / texture 336 `DLSS.DisocclusionMask`, texture 288 the subsurface guide, 1028
`DLSS.Use.HW.Depth`; `DLSSD.ColorBeforeParticles` selects the other reduction kernel (model stage `downp`, texture
208, an output-size history 256 <-> 264, flag bytes 196-198). Encoder 0's surfaces 376 / 384 are the output filter's
textures 64 / 72. Pre-exposure does not reach these kernels.

Checked (`d4r-rr` on `rr_inputs.py` inputs; every stage surface and the presented image against the CUDA path):
byte-identical on every frame of capF (640x360 -> 1080p, 3 frames), capF900 (800x450 -> 1600x900), capF4K
(1280x720 -> 4K, 16 tiles) and of the option captures (motion, a real camera, inverted and hardware depth, packed
roughness, hit distance, disocclusion mask, subsurface guide, particles); through the shim (D3D12 harness under
Proton) the engine's frames are byte-identical to the CUDA backend's too (`D4R_RR_PRESET=6 D4R_RR_DLL=<310.9 DLL>
scripts/run_rr_engine_compare.sh`; that script still compares frame N with frame N, and the harness presents the
CUDA result one frame late in the jitter scene).

Speed, RX 9070 XT, 1280x720 -> 3840x2160: 6.2 / 6.6 ms per frame (exposure 0.03, network 4.1 - 4.2, output 2.1 -
2.4), from 14.5 ms with every kernel translated; preset D is 4.3 / 4.5. Per stage (us): enc0 1076 (its block 521),
dec0_1 775, dec0_0 630, enc1 340, dec1_0 316, dec1_1 269, the rest under 180; filter and reduction about 130 per
tile, 16 tiles. Not done: native output passes and feature gather.

### Where the translated parts' time went (2026-10-13, later)

Now about 5.8 / 6.2 ms (5.7 / 6.1 with the 10-bit histories that were later withdrawn), still byte-identical on every capture and through the
shim. Profiling the translated code (cutting it at points behind an opaque condition, `D4R_ENGINE_RR_SKIP=post,down,
pooled`) showed its arithmetic is spread evenly - no hot spot a rewrite would remove - and that the losses were in
how the passes were arranged:

- **Encoder 0 in two passes** (-0.2 ms). One shader ran NVIDIA's 32-lane gather on one of the block's four waves
  while the other three waited: 0.40 ms for the gather, 0.24 ms as a pass of its own in 32-thread groups.
  `gen_rr2_enc0.py --split gather | block` writes both; the gather pass (`stage enc0g`, the same bindings and
  parameter block as enc0) stores each window's record in the arena after the tile slots: the first product's A
  fragments (1024 words) and, per lane, the values the tail reads from the gather (six binary16 on the reference
  build; the generator finds them and recomputes the tail's index arithmetic). The record's address is a word
  compile_rr2.py appends to enc0's argument block. Cost: 76 bytes per render pixel of arena.
  `D4R_RR2_SPLIT=0` builds the single shader.
- **Tile passes in overlapping groups** (-0.15 ms). The 16 tiles' post and down passes ran one after another, each
  behind a barrier, and the gaps between 32 short dispatches added up. Each tile now has a slot (its two regions;
  eight slots, 58 MB at 4K) and a group's post passes run without barriers between them, as do its down passes
  together with the next group's post passes on the other half of the slots - what NGX does with two slots and its
  progress counters. `D4R_ENGINE_RR_SKIP=serial` restores the old order.
- **Pooled histories as real 10:10:10:2 images: withdrawn.** 4 bytes a pixel instead of RGBA32F's 16 saved
  0.1 ms and was identical at 3x and 2x scaling, but the hardware's bilinear filter rounds a UNORM image differently
  from a float one: at 1.5x (capFQ) temporal frames came out at 44 - 51 dB. The default is RGBA32F again;
  `D4R_RR2_POOLED=rgb10_a2` builds the other form.

Tried without gain: 32-bit arena accesses for the translated code's paired 16-bit ones (0.01 ms), dropping the
subgroup shuffles or the depth search (they cost nothing measurable). What is left is NVIDIA's arithmetic: gather
0.24 - 0.33 ms, decoder 0's output tail 0.18, post 1.1 - 1.3, down 0.7.

### Other scaling ratios, and what a game-like configuration showed (2026-10-13, later)

Asked whether instability seen in RE Requiem came from this implementation, the engine was compared with the CUDA
path through the shim in configurations the earlier captures did not cover (`D4R_RR_PRESET=6 D4R_RR_DLL=<310.9>
scripts/run_rr_engine_compare.sh`, frames aligned by one):

- **Render widths that are not a multiple of 32 were wrong** (1114x626 -> 1080p: 15 dB): the post pass's argument 8
  is the row stride of the network's output, the padded width W0, not the render width. Fixed; identical now.
- **Below 4/3 scaling NGX runs another output filter**, `rr2_post_kernel_3_2` (1456x820 -> 1080p and larger render
  sizes, DLAA included; 1440x810 and smaller use `rr2_post_kernel`). The model has no such stage, so the shim hands
  those features to the CUDA backend. `rr2_histogram_auto_exposure_histogram_kernel` was never launched in any run.
- **DLSS.Roughness.Mode 1 with a roughness texture bound**: NGX follows the mode, the shim followed the texture.
  Fixed for preset F (RE Requiem binds none, so it was not affected).
- 1.5x, 1.72x, 2x and 4/3 scaling, and RE Requiem's inputs together (inverted and hardware depth, hit distance,
  mask, packed roughness, scaled motion vectors, a moving scene): identical.
- **Open: the camera matrices are not NGX's bit for bit.** NGX computes them in single precision: argument 644 is
  the f32 product view x clip, 836 its inverse by cofactors times 1 / det (glm::inverse's operation order; both
  reproduced bit for bit in build/rr/f/matfit.py), 900 is an inverse of the pixel matrix (it carries that inverse's
  residues, and 0.9999999 for 1 at 1114x626) and 964 was not reproduced. The engine computes them in double and
  rounds, a last-bit difference in a few elements. With a non-identity camera, hit distance and mask bound and a
  moving scene that showed as 40 - 50 dB on later frames in a synthetic scene whose affected feature is rounding
  noise around zero in both paths; no other configuration was affected.