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Assert the frame budget, and commit the numbers behind the FR-DSP-2 verdict

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FR-DSP-3 states a latency requirement and nothing checked it, which makes it a
wish. This adds the check and the measurements it guards.

`docs/frame-budget.md` is the bench's output with the reading of §2's decision
rule attached. The short version: every point-operation chain at every viewport
size, fit and at 1:1, is inside 16 ms at the 99th percentile — the widest is
4.5 ms of GPU at 4K — so FR-DSP-2 should be rewritten rather than implemented.
The measurement did find a stage that misses the budget, and it is the one §2
predicted: clarity's 52-pixel separable kernel costs 34 ms at 4K. Tiles make
that worse rather than better, since a tiled convolution reads a halo per tile;
the fix `local_contrast` already names for itself is a base computed at reduced
resolution.

The test guards the fused path and says so, at length, rather than quietly
excluding the expensive stage and letting the tag imply otherwise (§7). What it
asserts is exactly the claim the recommendation rests on: one dispatch over a
viewport-sized target, at a full chain, is comfortably inside a frame.

Two things the numbers forced:

- The two cases are one `#[test]`. As two they ran on a thread each, contended
  for the same device, and took the 1:1 case from 2.5 ms to 14.9 ms — a
  measurement of the harness that would have flickered either side of the
  budget forever.

- The CPU half of the frame is judged only in an optimised build. Composition
  is real per-frame work on the UI thread and belongs in the budget, but the
  workspace builds its own crates at `opt-level = 0` in dev and `cargo test` is
  a dev build, so measuring it there measures rustc. The GPU half is asserted
  either way.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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# 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.
---
## 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).