DarkRoom/docs/frame-budget.md

# What a frame costs

**Status:** Measured · 2026-08-27
**Companion to:** [display-and-extension.md](display-and-extension.md) §2–3 ·
[requirements.md](requirements.md) §3.4 FR-DSP-2, FR-DSP-3, FR-DSP-4
**Instrument:** [`core/dr-gpu/examples/frame_budget.rs`](../core/dr-gpu/examples/frame_budget.rs)
**Guard:** [`core/dr-gpu/tests/frame_budget.rs`](../core/dr-gpu/tests/frame_budget.rs)

[display-and-extension.md](display-and-extension.md) §2 fixed a decision rule in
advance and made three measurements the thing that settles it. This file is
those measurements, and the recommendation they support.

Rerun with:

```sh
cargo run --release -p dr-gpu --example frame_budget
```

and diff this file. That is the whole point of committing numbers: a regression
should be a diff rather than somebody's recollection of how fast it used to be.

---

## The answer, first

**FR-DSP-2 should be rewritten, not implemented.** M1 and M2 sit inside the
16 ms budget at the 99th percentile for every chain of point operations at every
viewport size measured, fit and at 1:1 — the widest case, every operation that
contributes a fragment to the fused shader at 4K, costs **4.5 ms** on the GPU and
**8.2 ms** including the composition that precedes it. Tiling the interactive
path would be optimising something that is already using a quarter of its budget.

**But the measurement did find a budget-breaker, and it is not the one tiling
fixes.** The neighbourhood stage — clarity in particular — costs **34 ms at 4K
on its own**, twice the whole budget, and tiles do not help it: a tile of a
convolution has to read its halo, so tiling raises the total tap count rather
than lowering it. §2 predicted this exactly ("a separable blur at a large radius
is the plausible budget-breaker, not the fused pass"), and the fix it needs is
the one `local_contrast`'s own module documentation already names — a base
computed at reduced resolution — which is a change to `crate::detail`, not a
tile scheduler.

There is a third finding nobody was looking for: **shader composition costs
3–5 ms of CPU per frame on a full chain**, on the UI thread, before any GPU work
is submitted. That is a fifth to a third of the budget spent formatting strings,
and it is invisible to any amount of tiling.

---

## Conditions

| | |
|---|---|
| Adapter | NVIDIA GeForce RTX 3050 6GB Laptop GPU (Vulkan) |
| Source | 9504 × 6336 synthetic (60.2 MP, 482 MB as `rgba16f`) |
| Frames | 100 measured per row, 12 warm-up frames discarded |
| Percentile | Nearest-rank, so p99 of 100 frames is the second-worst frame |
| Build | `--release` |
| Date | 2026-08-27 |

`shader` is `EditGraph::compose` alone. `cpu` adds the detail chain and the
invalidation hash — everything `DevelopSession::render` does per frame before it
dispatches. `gpu` is submit plus wait-for-idle, which serialises the GPU work
into the frame that caused it and is therefore pessimistic. `TOTAL` ranks
`cpu + gpu` summed **within each frame**, which is the column the budget is
judged on; adding two percentiles instead would invent a stutter that no frame
actually had.

Chains: `one` is exposure. `five` is exposure, contrast, highlights/shadows,
blacks/whites, vibrance. `point` is every operation in the default chain that
contributes a fragment to the fused shader, film stock included. `all` is `point`
plus the four neighbourhood operations — noise reduction, capture sharpening,
clarity and texture.

---

## M1 — the fused pass at proxy resolution

The develop view: the whole frame fit to the viewport.

|        size | chain | shader | cpu p99 | gpu p50 | gpu p99 |   TOTAL |      |
|------------:|------:|-------:|--------:|--------:|--------:|--------:|:-----|
| 1920 × 1200 |   one | 0.08ms |  0.10ms |  1.02ms |  1.23ms |  1.31ms |      |
| 1920 × 1200 |  five | 0.18ms |  0.20ms |  1.01ms |  1.20ms |  1.36ms |      |
| 1920 × 1200 | point | 2.79ms |  2.82ms |  1.98ms |  2.18ms |  4.83ms |      |
| 1920 × 1200 |   all | 3.65ms |  4.73ms |  6.86ms |  7.37ms | 12.02ms |      |
| 2560 × 1600 |   one | 0.08ms |  0.10ms |  1.73ms |  2.00ms |  2.12ms |      |
| 2560 × 1600 |  five | 0.24ms |  0.27ms |  1.73ms |  2.26ms |  2.46ms |      |
| 2560 × 1600 | point | 2.82ms |  2.85ms |  2.37ms |  2.65ms |  5.38ms |      |
| 2560 × 1600 |   all | 3.37ms |  4.35ms | 14.31ms | 15.65ms | 18.42ms | OVER |
| 3840 × 2160 |   one | 0.10ms |  0.14ms |  3.09ms |  3.31ms |  3.42ms |      |
| 3840 × 2160 |  five | 0.30ms |  0.32ms |  3.03ms |  3.40ms |  3.61ms |      |
| 3840 × 2160 | point | 3.62ms |  3.65ms |  4.12ms |  4.52ms |  8.23ms |      |
| 3840 × 2160 |   all | 4.12ms |  5.07ms | 37.73ms | 40.17ms | 43.24ms | OVER |

Read the `point` rows: **the fused dispatch scales with pixels and almost not at
all with chain length.** Going from one operation to the entire point chain at
4K costs 1.2 ms of GPU. Going from 2.3 M pixels to 8.3 M costs 2.3 ms. Both are
small, and the second is the one tiling would address.

The `all` rows go over, and the `point` rows in the same block are what say why:
the difference between them is the neighbourhood stage, measured on its own in
M3 and arriving at almost exactly the same figure.

## M2 — the same, zoomed to 1:1 on the 60 MP source

FR-DSP-5's case. `Framing::view` shrinks the sampled region while the render
target keeps its size, so one render pixel lands on one source pixel.

|        size | chain | shader | cpu p99 | gpu p50 | gpu p99 |   TOTAL |      |
|------------:|------:|-------:|--------:|--------:|--------:|--------:|:-----|
| 1920 × 1200 |   one | 0.12ms |  0.14ms |  0.42ms |  0.66ms |  0.75ms |      |
| 1920 × 1200 |  five | 0.27ms |  0.30ms |  0.49ms |  1.14ms |  1.22ms |      |
| 1920 × 1200 | point | 3.49ms |  3.52ms |  1.18ms |  1.39ms |  4.85ms |      |
| 1920 × 1200 |   all | 3.64ms |  5.18ms |  8.96ms |  9.55ms | 14.30ms |      |
| 2560 × 1600 |   one | 0.11ms |  0.12ms |  0.56ms |  0.99ms |  1.06ms |      |
| 2560 × 1600 |  five | 0.22ms |  0.25ms |  0.77ms |  1.02ms |  1.17ms |      |
| 2560 × 1600 | point | 2.96ms |  2.99ms |  2.03ms |  2.52ms |  5.61ms |      |
| 2560 × 1600 |   all | 5.24ms |  7.35ms | 18.80ms | 21.62ms | 25.81ms | OVER |
| 3840 × 2160 |   one | 0.10ms |  0.12ms |  1.31ms |  1.52ms |  1.63ms |      |
| 3840 × 2160 |  five | 0.14ms |  0.27ms |  1.39ms |  1.64ms |  1.75ms |      |
| 3840 × 2160 | point | 3.14ms |  3.16ms |  4.04ms |  4.50ms |  7.21ms |      |
| 3840 × 2160 |   all | 4.84ms |  6.78ms | 47.22ms | 48.79ms | 54.47ms | OVER |

**A 1:1 view of a 60 MP file is cheaper than the fit view of the same file**, for
every point chain and at every size — 1.52 ms against 3.31 ms for one operation
at 4K. That is not a rounding artefact and it is worth stating plainly, because
it is the opposite of what "full resolution" sounds like it should cost. The
dispatch is the same number of pixels either way; what changes is where those
pixels read from. A fit view walks the whole 482 MB texture on a stride, and a
1:1 view reads a contiguous window of it that fits comfortably in cache.

So the resolution FR-DSP-5 promises costs nothing extra on the fused path.
Zooming is not an expensive mode to be dreaded and progressively refined into;
it is the cheap one.

The `all` rows are worse at 1:1 than fit, and that is the detail stage again for
a specific reason: noise reduction's radius is stated in *source* pixels, so
`RenderScale::ratio` climbing to 1.0 widens its kernel. Clarity's is stated as a
fraction of the frame and does not move. M3 separates the two.

## M3 — the neighbourhood stage alone

Timed with the fused dispatch deliberately reused: only a detail parameter moves,
so `render_detailed` skips the colour pass (FR-DEV-3d) and what remains is the
convolutions. `colour` counts fused dispatches over the measured frames and is
zero on every row, which is what makes these numbers mean "detail alone" rather
than asserting it.

|        size |    stage | view | pass | radius | colour | cpu p99 |     p50 |     p99 |
|------------:|---------:|:-----|-----:|-------:|-------:|--------:|--------:|--------:|
| 1920 × 1200 |  clarity | fit  |    2 |     29 |      0 |  1.60ms |  5.42ms |  5.99ms |
| 1920 × 1200 | all four | fit  |    7 |     29 |      0 |  1.71ms |  5.78ms |  6.16ms |
| 1920 × 1200 |  clarity | 1:1  |    2 |     29 |      0 |  1.12ms |  7.51ms |  8.01ms |
| 1920 × 1200 | all four | 1:1  |    9 |     29 |      0 |  2.71ms |  8.25ms |  9.11ms |
| 2560 × 1600 |  clarity | fit  |    2 |     38 |      0 |  1.03ms | 12.02ms | 12.44ms |
| 2560 × 1600 | all four | fit  |    7 |     38 |      0 |  1.87ms | 12.49ms | 13.16ms |
| 2560 × 1600 |  clarity | 1:1  |    2 |     38 |      0 |  1.87ms | 15.82ms | 16.60ms |
| 2560 × 1600 | all four | 1:1  |    9 |     38 |      0 |  2.71ms | 17.24ms | 18.06ms |
| 3840 × 2160 |  clarity | fit  |    2 |     52 |      0 |  1.75ms | 33.11ms | 33.89ms |
| 3840 × 2160 | all four | fit  |    7 |     52 |      0 |  1.76ms | 34.21ms | 35.03ms |
| 3840 × 2160 |  clarity | 1:1  |    2 |     52 |      0 |  1.08ms | 40.39ms | 41.86ms |
| 3840 × 2160 | all four | 1:1  |    9 |     52 |      0 |  2.37ms | 43.29ms | 44.72ms |

`radius` is the widest halo any pass reads, in render pixels.

Clarity alone is 97% of the cost of all four neighbourhood operations together,
at every size. Its σ is 1.2% of the shorter edge and it truncates at 2σ, so its
radius is 29 px on a 1200 px viewport and **52 px at 4K** — two separable passes
of 105 taps each, over 8.3 M pixels, which is 1.7 billion texture reads. That is
the whole of the problem, and the numbers scale as `radius × pixels` exactly as
that description predicts: 5.99 → 12.44 → 33.89 ms for radii of 29 → 38 → 52 over
2.3 → 4.1 → 8.3 M pixels.

The extra cost at 1:1 is noise reduction and capture sharpening, whose radii are
properties of the sensor rather than of the frame. That is the correct behaviour
— it is why `RenderScale` has two units — and it is bounded by the kernel caps
those operations already declare.

---

## Reading this against §2's decision rule

§2: *"If M1 and M2 sit inside 16 ms at the 99th percentile, FR-DSP-2 is
rewritten rather than implemented … If they do not, the measurement tells us
which stage to tile."*

Both halves of the rule fire, on different stages, and the honest reading takes
both.

### FR-DSP-2 — rewrite it

For the fused pass the rule passes with a wide margin. Every point chain at
every size, fit and at 1:1, is inside 16 ms — the worst `TOTAL` is 8.23 ms and
the worst GPU figure is 4.52 ms. There is no viewport size on a desktop display
where recomputing the entire point chain over every visible pixel is a problem.

Two further reasons not to build the tile scheduler as written:

1. **Panning, which is the case ARCH §5.3's tile cache is designed for, gets no
   benefit here.** Reusing already-valid tiles saves recomputation. Recomputing
   the whole 4K viewport costs 4.5 ms, so a perfect tile cache could save at most
   4.5 ms of a 16 ms budget, at the price of a cache keyed by
   `(VersionId, tile, zoom, graph_hash_prefix)` that has to stay correct across
   every parameter change in the graph. That is a large correctness surface
   bought with a small number.

2. **It would make the actual problem worse.** The stage that misses the budget
   is a convolution, and a tiled convolution reads a halo per tile. At a 52-pixel
   radius, 256-pixel tiles would read (256+104)² instead of 256² — very nearly
   *twice* the taps. Tiling is the wrong tool for the one stage that needs a
   tool.

So FR-DSP-2 becomes what §2 said it actually is for this architecture: a
scheduling concern for export and thumbnailing, both of which already run off
the frame path. The interactive path does not tile.

### The stage that does need work — and it is not tiling

The measurement's real product is naming the stage. It is `local_contrast`, and
the fix is stated in that module's own documentation:

> The right optimisation is a base computed at reduced resolution, which needs a
> detail stage that can write a smaller target than it reads; that is a change to
> `crate::detail`, not to this file.

A Gaussian base at a quarter resolution is 1/16 the pixels at 1/4 the radius —
about 1/64 of the work — and the result is visually identical because a base at
σ = 26 px has no content above the quarter-resolution Nyquist to lose. That is a
change to two files with a bounded blast radius, and it is what the 34 ms buys
back. It should be tracked as its own item rather than smuggled in under a
requirement about tiles.

### FR-DSP-3 — the clause that should be narrowed

§3.3 proposes narrowing "when a full-resolution result is needed it is computed
asynchronously, and the proxy result remains on screen until it is ready" to
export and 1:1 zoom, or striking it.

**M2 says strike it.** The clause exists to hide the latency of a
full-resolution render behind a proxy. There is no such latency: the 1:1 view is
*faster* than the fit view on the fused path, and there is no second
full-resolution code path to be asynchronous about — `Framing::view` is the
whole mechanism. Export renders its own frames on a worker already. Keeping the
clause would mean building a progressive-swap machine to conceal a render that
completes in 1.4 ms.

### FR-DSP-4 — satisfied vacuously, on the fused path

§4 makes progressive refinement conditional on M1 failing. On the fused path M1
passes, so reduced-quality rendering during a drag would buy nothing and cost the
visible softness the requirement itself warns against.

The neighbourhood stage is the exception, and it is worth being precise: what
that stage needs is not *progressive* refinement — it is a permanently cheaper
base, computed at reduced resolution and correct at any moment the user stops.
"Render coarse while dragging, sharpen when it settles" would paper over the same
34 ms with a visible swap. Fix the stage.

---

## Which GPU, on a machine with more than one

**Measured 2026-08-29** on a laptop holding an Intel Iris Xe (RPL-P) and an AMD
RX 5700 XT, same binary, adapter forced with `VK_ICD_FILENAMES`.

The question was whether an integrated GPU is the better choice for this
application. The argument for it is good: a 24 MP frame is ~96 MB of RGBA, and
on a discrete card every upload and every export readback crosses PCIe, where
an iGPU shares memory with the CPU and crosses nothing. It also does not empty
a battery.

The compute says otherwise, and not marginally.

| 2560×1600, p99 | AMD RX 5700 XT | Intel Iris Xe |
|---|---|---|
| fused pass, `point` | 5.19 ms | 7.75 ms |
| fused pass, `all` | 11.70 ms | **66.42 ms** |
| M3 clarity, fit | 4.67 ms | **38.28 ms** |
| M3 all four, 1:1 | 6.44 ms | **57.67 ms** |

| 1920×1200, M3 clarity, fit | 2.35 ms | **19.99 ms** |
|---|---|---|

The fused colour pass is within a factor of 1.5 — it is one read and one write
per pixel, which an iGPU does perfectly well. The **neighbourhood stage is
5–8× slower**, and that is what decides it: clarity at 1920×1200 costs 20 ms on
the Iris Xe, so it leaves the budget on its own at the smallest size tested,
before anything else in the chain runs.

**So the default adapter preference stays `Performance`** (`dr_gpu::AdapterPreference`).

Two things this does *not* show, and neither is a reason to revisit the default
without measuring them:

- **It does not refute the transfer argument.** This harness renders from a
  resident texture and never uploads or reads back, so the PCIe cost an iGPU
  avoids does not appear in any column above. Import, export and the thumbnail
  sweeps are transfer-heavy and compute-trivial, and may well go the other way
  — but they are not what FR-DSP-3 bounds, and one device is opened at startup
  and shared with the compositor, so there is currently no way to use a
  different adapter for a different task.
- **It says nothing about power.** `Efficiency` remains offered
  (`DARKROOM_GPU=integrated`) because a user on battery may rationally accept a
  slower detail chain, and because someone whose discrete card has failed needs
  a way to keep working.

## What is not measured here

Stated because §7 of [display-and-extension.md](display-and-extension.md) asks
for it, and because each of these could move the numbers.

- **Local adjustments.** The mask stack is a separate chain per layer and is not
  in any row above. `render_masked` takes them and the fused shader addresses
  them per layer, so a heavily masked edit costs more than `all`.
- **Spot repairs.** These add detail passes, and their cost is per spot.
- **Lens corrections.** Not part of `EditGraph::default_chain` — they are built
  from a matched profile — so the `point` row does not include the warp chain.
- **Demosaic.** Once per photograph on a worker, not on the frame path.
- **Presentation.** The bench waits for the device to go idle inside the frame it
  measures. A real compositor overlaps frames, so these figures are an upper
  bound rather than an estimate.
- **One adapter.** A discrete laptop GPU. The Intel iGPU on the same machine, and
  Android, will be slower — which is an argument for the conclusion rather than
  against it: the stage with no headroom has none to lose.

## The CPU finding, which deserves its own item

`EditGraph::compose` costs 2.8–5.2 ms per frame on a full chain, at every
resolution, because it is resolution-independent: it assembles a WGSL string and
hashes it. On the `all` rows it is a third of what is left of the budget after
the GPU has taken its share, and at 1920 × 1200 it is larger than the entire
fused dispatch.

Nothing in this document's recommendations changes it, and it is the cheapest
remaining win. The generated *source* depends only on the structure of the graph
— that is what `structure_hash` already identifies, and it is precisely what does
not change while a slider is being dragged, which is why the pipeline cache in
`AdjustPass` does not recompile. The uniforms do change, but assembling them is a
handful of floats per operation. So caching the source string against the
structure hash and rebuilding only the uniforms would take these milliseconds to
approximately nothing, on the path that needs them most. Worth its own entry in
[technical-debt.md](technical-debt.md).