Make the video SynthID operating point measurable and hard to move silently

The shipped profile was certified by one oracle row, but only noise_std was
pinned: long_side and fps -- two thirds of what the verifier was actually shown
-- could move with a green suite. The test now derives the pin from
data/evaluations/video-synthid-oracle.csv, so a default without a certifying row
fails.

The certified profile is a perturbation-to-signal ratio, not a bare noise_std.
sd-vae-ft-mse publishes no scaling_factor key, so 0.18215 comes from the
AutoencoderKL class default under an upper-unbounded diffusers pin. The loader
now gates that value, carries it on VideoVaeRuntime, and passes it into encode
and decode so the validated value is the applied value. video_synthid_sweep.py
loads through the same function: the harness producing the certified rows was
the one path exempt from the gate it exists to feed.

psnr_db is measured against the already-resized frame and before the encoder, so
it cannot see the downscale, the decimation, or the codec, and no in-loop metric
can. scripts/video_fidelity_probe.py scores the delivered file end to end,
streaming the way the engine does and sharing its frame-selection rule rather
than copying it -- a frame-count check cannot catch a rule that reorders frames
without changing how many.

The manifest gains source geometry, vae, track, verbatim verdict and session
fields. The two 2026-07-31 rows keep them empty: they were never recorded and
are not recoverable. Verdicts now have four states, because the verifier's
unclear reading logged as not_detected is the silent regression the manifest
exists to prevent.

docs/video-synthid-quality-research.md records the research behind this: the
noise axis is worth about 2 dB and is nearly exhausted, resolution is the real
prize but is an uncertified destruction axis rather than a free win, and every
proposed autoencoder swap was refuted. First local measurements included.

Verified: engine output is byte-identical before and after the refactor on a
locally built clip, at noise_std 0.00 and 0.15.

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

docs/index.md

@@ -37,4 +37,5 @@ The current behavior is defined by the code, tests, README, and user guides.
 - [Doubao reverse-alpha research](research-doubao-distillation.md)
 - [SynthID identity research](synthid-robust-identity-research.md)
 - [SynthID identity follow-up](synthid-robust-identity-research-2026-06-08.md)
+- [Video SynthID quality research](video-synthid-quality-research.md)
 - [Text protection research](text-protection-research.md)

docs/known-limitations.md

@@ -118,6 +118,14 @@ rather than clip duration. Runtime still grows linearly with duration, and the
 separate multi-candidate research sweep deliberately retains its short sampled
 prefix so it can reuse identical latents across candidate strengths.
 
+The reported `psnr_db` is measured against the already-resized frame and before
+the H.264 encode, so it excludes the downscale, the frame decimation, and the
+encoder. It measures the VAE round trip plus latent noise at the working geometry.
+[`video-synthid-quality-research.md`](video-synthid-quality-research.md) records
+what the manifest rows constrain, what a higher-resolution or higher-frame-rate
+profile would need in order to be certified, and the audio-track question this
+path has not yet answered.
+
 ### Strength is content and seed dependent
 
 The two profiles resolve an unset strength differently, because different things

docs/module-internals.md

@@ -252,7 +252,35 @@ quality measurements. Neither is a watermark detector. The high-level result
 reports completed removal without a separate verification-status flag. The companion
 `scripts/video_synthid_sweep.py` imports the same engine helpers to build a
 matched control and candidate grid, preventing research and shipped
-regeneration paths from drifting. The full-clip oracle floor is
+regeneration paths from drifting.
+
+The engine's `psnr_db` is measured against the already-resized frame and before
+the encoder, so it scores the VAE round trip plus latent noise and cannot see
+the downscale, the decimation, or the codec. No in-loop metric can: the
+candidate frame is captured before it reaches the encoder pipe.
+`scripts/video_fidelity_probe.py` covers the rest by decoding the delivered file
+after muxing, upscaling it back to the source geometry, and scoring it against
+the untouched source frames. It also reports the delivered file's bitrate, so a
+fixed-crf bitrate rise cannot read as unchanged quality; because the mux copies
+source audio verbatim, that figure is a container bitrate, not a video one. The
+probe streams and accumulates the same way the engine does, so its peak memory
+does not grow with clip length. It drives the source through the engine's own
+`_iter_sampled_frames` at the source geometry rather than repeating the
+selection rule: a frame-count check cannot catch a rule that reorders frames
+without changing how many, so the rule itself has to be shared.
+
+`load_video_vae_runtime` asserts the default model's latent scaling factor
+against `VIDEO_SYNTHID_VAE_SCALING_FACTOR` and warns that no certified profile
+exists for any other model. The published `sd-vae-ft-mse` config carries no
+`scaling_factor` key, so the value is a `diffusers` class default under an
+upper-unbounded pin, and the certified profile is a perturbation-to-signal ratio
+rather than a bare `noise_std`. A library bump that moved that default would
+otherwise rescale every perturbation with a green suite. The validated factor is
+carried on `VideoVaeRuntime` and passed into encode and decode, so the gated
+value and the applied value are one measurement rather than three independent
+reads. `scripts/video_synthid_sweep.py` loads through the same function: the
+harness that produces the certified rows is the last place that should be exempt
+from the gate. The full-clip oracle floor is
 `noise_std=0.15`: on the public eight-second Veo carrier, `0.10` remained
 detected while `0.15` did not.
 

docs/synthid.md

@@ -372,6 +372,11 @@ itself. `0.15` is therefore the shipped default. The tracked manifest
 `data/evaluations/video-synthid-oracle.csv` records the public source URL,
 hashes, fidelity metrics, and verdicts without committing generated videos.
 
+What that calibration does and does not constrain, the ranked experiment program
+for trading less quality for the same removal, and the change candidates that were
+refuted along the way are recorded in
+[`video-synthid-quality-research.md`](video-synthid-quality-research.md).
+
 The VAE perturbation follows the general regeneration-attack construction from
 Zhao et al. The video-specific control and temporal metric are local additions.
 VideoMarkBench motivates testing frame aggregation and matched perturbations,

docs/video-synthid-quality-research.md

@@ -0,0 +1,493 @@
+# Video SynthID quality research (2026-08-05)
+
+> Research archive. This page records experiments and decisions from the date
+> above. It may mention prototypes or defaults that were later changed. Use the
+> user guides and current source code for the supported interface.
+
+Cited research behind the question **"can the video SynthID path keep removing the
+mark while giving up far less quality than the shipped 512 px / 12 fps profile?"**
+Produced by a 13-agent workflow: 6 parallel scouts (pipeline ablation, SynthID
+internals, attack literature, autoencoder landscape, perceptual masking, experiment
+design), 11 change proposals from 3 independent design angles, and 3 adversarial
+critics (signal theory, repository engineering, verifiability). No experiment was
+run against the provider oracle for this page; every claim is labeled MEASURED,
+REPORTED, or INFERRED.
+
+Repository claims below were re-verified against source after the workflow
+returned: the PSNR reference frame, the audio stream copy, the missing `_fit_size`
+clamp, the absent `enable_tiling` call, the crf asymmetry, the test pins, and the
+absent `scaling_factor` key in the cached checkpoint config.
+
+## Context
+
+`remove_video_invisible` regenerates video pixels through `sd-vae-ft-mse`: frames
+are decimated to 12 fps, resized to a 512 px long side, encoded to latents with
+`latent_dist.mode()`, perturbed by one seeded spatial noise field shared across
+every frame, decoded, and streamed to an H.264 encoder at crf 18. A separate
+stream-copy mux re-adds source audio and strips metadata. The shipped
+`noise_std=0.15` is certified by one oracle row on one carrier.
+
+The quality complaint is real: a 1080p source is delivered at roughly a quarter of
+its linear resolution and half its frame rate. This page asks what part of that
+cost is buying removal and what part is buying nothing.
+
+## What the two oracle rows do and do not prove
+
+`data/evaluations/video-synthid-oracle.csv`, one carrier (Veo 3 off-road,
+sha256 `79a552b9...`), one seed, one geometry:
+
+| noise_std | long_side | fps | verdict | psnr_db | temporal_residual_ratio |
+| --- | --- | --- | --- | --- | --- |
+| 0.10 | 512 | 12 | detected | 26.2932 | 1.0072 |
+| 0.15 | 512 | 12 | not_detected | 25.3911 | 1.0578 |
+
+They prove exactly one thing: the envelope of 512 px, 12 fps, crf 18, and a full
+VAE round trip does **not** silence the oracle on its own. The verdict flip is
+bought by the last 0.05 of latent noise.
+
+They cannot decompose that conjunction. No row varies `long_side` or `fps`, so the
+contribution of the downscale is unmeasured. The frequent reading "the 512 px
+downscale was probably doing the removal work" is not supported, and neither is its
+opposite.
+
+## Measured facts from the repository
+
+- **The fidelity metric is blind to the expensive steps.** `_iter_sampled_frames`
+  applies `cv2.resize` at [`video_invisible.py:236`](../src/remove_ai_watermarks/video_invisible.py);
+  the same generator feeds the loop at `:401-407`; the accumulator at `:429-435`
+  zips those already-resized frames against `regenerated` captured at `:423`, which
+  is before `frame_pipe.write` at `:430`. `psnr_db` therefore excludes the
+  downscale, the decimation, and the encoder. It measures the VAE round trip plus
+  latent noise at the working geometry, nothing else.
+- **Frame decimation cannot destroy the carrier.** `_iter_sampled_frames`
+  (`:224-238`) is pure subset selection: the only statement inside the threshold
+  test is `yield cv2.resize(...)`, with no arithmetic across frames. The
+  perturbation is equally time-blind: `shared_noise.expand(latents.shape[0], -1, -1, -1)`
+  at `:295` broadcasts one CHW field across the batch and never mixes across time.
+  Any fps effect on the verdict is a detection-probability effect, not carrier
+  destruction.
+- **`_fit_size` has no clamp at 1.0.** `scale = long_side / max(width, height)` at
+  `:88` upscales any source whose long side is below 512, and both axes are floored
+  independently to a multiple of 8 at `:89-96`, which introduces an anamorphic
+  shift at scale 1.0 (1366x768 becomes 1360x768).
+- **`enable_tiling` is never called** anywhere in `src/`, `scripts/`, or `tests/`.
+  `enable_slicing()` at `:135` already splits both encode and decode to single
+  frames, so activation VRAM is set by one frame and does not scale with
+  `--batch-size`.
+- **crf asymmetry:** 18 on the invisible path (`:321`, `:386`) against 14 on the
+  visible path ([`video_visible.py:1343`](../src/remove_ai_watermarks/video_visible.py))
+  through the same encoder.
+- **No HDR guard on the invisible path.** `_HDR_TRANSFERS` (`video_visible.py:97`)
+  and the `component_depth > 8` rejection (`:1322`) exist only for visible removal.
+- **Audio is byte-copied.** `-map 0:v:0`, `-map 1:a?`, `-c copy` at
+  [`video_encoding.py:413`](../src/remove_ai_watermarks/video_encoding.py).
+- **Only `noise_std` is pinned.** `tests/test_video_invisible.py:285` asserts
+  `DEFAULT_VIDEO_SYNTHID_NOISE_STD == 0.15`. Neither `long_side` nor `fps` is
+  pinned anywhere, so changing either breaks no test.
+- **`scaling_factor` 0.18215 is a class default, not a checkpoint fact.** The
+  cached `config.json` for `stabilityai/sd-vae-ft-mse` has no `scaling_factor` key
+  at all (verified locally: `_class_name`, `latent_channels`, `block_out_channels`,
+  `sample_size`, and block types only). The value comes from the `AutoencoderKL`
+  class default under `diffusers>=0.38.0` with no upper bound (`pyproject.toml:94`,
+  `uv.lock` resolves 0.39.0), while `maintain.sh` runs `uv-outdated`. A dependency
+  bump can move the certified operating point with a fully green test suite.
+- **The manifest schema cannot record what a real program needs:** no source
+  geometry, no control row, no track column, no verbatim verdict, no indeterminate
+  state.
+
+## Measured facts from external sources
+
+- Gemini video verification quota: 10 checks per rolling 24 hours, up to 5 minutes
+  of video total, under 90 seconds and 100 MB per file. The verifier reports which
+  parts of the video carry the mark, and it has a **third** state beyond detected
+  and not detected: unclear, with documented causes including "not enough details
+  to watermark".
+- The verifier scores **audio and visual tracks separately**. Google's published
+  example verdict reads as SynthID detected in the audio over a time range with no
+  SynthID detected in the visuals.
+- SynthID-Image (arXiv:2510.09263) is a post-hoc, model-independent pixel-space
+  watermark: a separate encoder network stamps an already-decoded image rather than
+  being injected into the generator's latents. Its published payload figure is 136
+  bits within a 512x512 image, and its product setup runs at 1536x1536.
+
+## Inferred, with the reasoning that makes them weak
+
+- **The noise axis is nearly exhausted.** Fitting `MSE = A + B * noise_std^2` to the
+  two measured rows gives A = 124.5 and B = 2815, so a pure round trip
+  (`noise_std = 0`) lands near 27.2 dB. The entire noise budget is worth at most
+  **+1.79 dB**; everything else is the autoencoder. This is a two-parameter fit to
+  two points with zero degrees of freedom, and the assumptions that MSE is
+  quadratic in `noise_std` and that reconstruction and noise errors are orthogonal
+  are untested. One local run at `noise_std=0` replaces it with a measurement.
+- **Statistical power of the current certification.** With zero failures at n = 1,
+  the exact one-sided Clopper-Pearson bound `1 - 0.05^(1/n)` is 95%: the data are
+  compatible with removal failing almost always. n = 15 gives 18.1%, n = 30 gives
+  9.5%.
+- **Size of the geometry prize.** At a 1280x720 source, `_fit_size(1280, 720, 512)`
+  is `(512, 288)` (pinned in `tests/test_video_synthid_sweep.py:30`), so 6.25x of
+  the pixels are discarded by the downscale and another 2x by decimation from a
+  24 fps source. Ladder rungs: 768 gives 2.25x the current area, 1024 gives 4.0x.
+  The carrier's own geometry is recorded nowhere, so this is conditional.
+- **The direction of the resolution effect is disputed, and both sides are
+  inference.** Against raising it: the absolute frequency ceiling below which an f8
+  VAE reconstructs faithfully is tied to the latent pitch, so 512x288 (a 64x36
+  latent) preserves roughly up to 32 cycles per frame width while 1920 (a 240x135
+  latent) preserves up to about 120. Raising resolution moves the carrier band out
+  of the regime the decoder synthesizes and into the regime it reproduces
+  faithfully, handing the detector more evidence. For raising it: Google's product
+  operating point is 1536x1536, so native is closer to the distribution the
+  watermark encoder targets. Note that the second argument cuts against the
+  proposal rather than for it.
+
+## First local measurements (2026-08-05)
+
+Run without the oracle on a locally built carrier: a 6-second 1280x720 24 fps
+clip panning across `data/fixtures/provenance/doubao-1.png`, processed by the
+shipped path at 512 px / 12 fps on MPS. Generated media stayed outside the
+repository.
+
+| noise_std | engine `psnr_db` | end-to-end PSNR | end-to-end SSIM | bitrate |
+| --- | --- | --- | --- | --- |
+| 0.00 | 27.8049 | 27.6935 | 0.6955 | 2564 kbps |
+| 0.15 | 25.8853 | 25.8541 | 0.6703 | 2716 kbps |
+
+Three readings, all MEASURED, none of them about SynthID:
+
+1. **The two-point fit's shape survives contact with independent content.** The
+   pure round trip lands at 27.80 dB against the 27.2 dB the carrier fit
+   predicted, and the whole noise budget costs 1.92 dB against the predicted
+   1.79 dB. These are not the carrier's numbers, but the decomposition holds:
+   the autoencoder is the floor and the entire `noise_std` axis is worth about
+   2 dB.
+2. **On this content the downscale is nearly free in PSNR terms.** End-to-end
+   PSNR sits within 0.11 dB of the in-loop number, meaning the 512 px geometry
+   cost almost nothing next to the VAE damage. This clip is a pan over a smooth
+   generated image with little high-frequency detail, so it is the friendly
+   case: real camera texture should widen that gap. Run the probe across content
+   types before trusting any estimate of the geometry prize.
+3. **`temporal_residual_ratio` is not meaningful on a near-static shot.** It read
+   1.82 at `noise_std=0` and 2.47 at 0.15, far outside the [1.0072, 1.0578] band
+   ever observed before. A slow pan gives the source almost no motion residual,
+   so the `max(temporal_baseline, 1e-6)` denominator collapses and the ratio
+   inflates. This is the predicted defect, now observed rather than argued.
+
+## The single most important unknown
+
+**How much the 512 px downscale contributes to removal.** Every quality gain routes
+through this question, and the critics moved it from "probably free" to "direction
+unknown", which is exactly what makes it the highest-information experiment
+available.
+
+## Ranked experiment program
+
+Ranked by information per oracle query. The budget is roughly 10 checks per 24
+hours (MEASURED), so the program is paced by calendar, not by money.
+
+### E0. The zero-oracle tier (0 submissions) - do this first
+
+Not an experiment on the oracle; the precondition that makes everything else
+interpretable.
+
+- `ffprobe` the carrier and record `source_width`, `source_height`, `source_fps`.
+  The actual downscale factor of the existing rows is currently unrecoverable.
+- Extend the manifest schema: variant/control, source geometry, `vae`, `track`
+  (audio|visual), verdict state in {DETECTED, NOT_DETECTED, INDETERMINATE,
+  REFUSED}, verbatim verdict text, detected time range for the output,
+  `session_id`, content stratum.
+- **Measure the `noise_std = 0.0` round trip locally** on the same carrier. This
+  converts the inferred 27.2 dB ceiling into a measurement and bounds the whole
+  noise axis for one GPU pass and zero oracle cost.
+- Freeze the current defaults' metrics on a fixed clip set, matching the
+  record-then-diff rule in `.claude/rules/development.md`.
+- Determine whether the carrier has an audio track and whether it carries SynthID.
+
+### E1. Instrument validation: multiplexing and a session anchor (1-2 submissions)
+
+The verifier reports time ranges, so one file can carry several doses. A 24-second
+file of `[control 8 s | certified 0.15 8 s | control 8 s]` should read detected on
+the outer segments and not detected on the middle one, both halves already known
+from the manifest.
+
+`encode_video_frames` requires matching dimensions and one frame rate, so
+multiplexing works **only along the `noise_std` axis**, not across geometries: one
+multiplexed file per geometric envelope.
+
+Separately, submitting the existing detected file first in each session costs 10%
+of the quota, turns "the oracle may have changed" into a per-session gate, and
+measures the flip rate on a byte-identical file, a quantity every plan silently
+assumes is zero.
+
+### E2. Resolution ladder at fixed dose - the decisive axis (2 submissions per rung)
+
+A fixed value, not a ceiling. Rung 1 is `long_side = 768` with fps held at 12 so
+exactly one destruction axis moves. Jumping straight to 1920 is a bad first rung:
+a 3.75x jump makes a detected verdict uninformative about where the boundary lies.
+
+- **Matched control:** 768 / 12 / `noise_std = 0.10`, expected DETECTED. It is
+  strictly stronger than a plain re-encode control because it discharges the
+  envelope, the VAE round trip, and a nonzero dose at once, and it diffs directly
+  against the existing 512 row.
+- **Candidate:** 768 / 12 / 0.15.
+- **Positive:** the resolution axis is open; next rungs 1024 and then native with a
+  clamp, 2 submissions each.
+- **Negative:** the downscale contributes to removal. This is the most valuable
+  available negative: it kills the native-resolution proposals outright and turns
+  the question into how much dose must be spent to buy resolution back, at the
+  known exchange rate B = 2815.
+- Watch for a monotonicity violation: 768 / 0.10 reading NOT_DETECTED would
+  overturn the critics' spectral argument.
+
+### E3. Frame rate at fixed geometry and dose (2 submissions)
+
+Control 512 / 24 / 0.10 expected DETECTED; candidate 512 / 24 / 0.15 expected
+not detected. The mechanism is provable from source, so the prior is high, but the
+only counter-mechanism - per-frame count aggregation - is probabilistic and n = 1
+on an 8-second clip does not measure it. Certify on the longest clip that fits the
+90 s / 100 MB limits, because the risk compounds with frame count.
+
+Ranked below E2 because the outcome is nearly predetermined and the prize (judder)
+is smaller than the geometry prize. It is the safest bet if a guaranteed win is
+wanted.
+
+### E4. `noise_std = 0.0` as an oracle probe (1 submission) - deliberately low rank
+
+The local half of this probe is the cheapest high-information measurement in the
+program and already sits in E0 at zero oracle cost. The **oracle** half ranks low:
+it buys no quality by construction, and its likely DETECTED outcome is nearly
+deducible from the 0.10 row under monotonicity. Its one real value is that
+NOT_DETECTED here would be non-monotone against 0.10 and would refute the
+assumption underneath every ladder and bisection in this program. Worth one query,
+after E2 and E3.
+
+### E5. Stratified certification of the surviving operating point (15-30 submissions)
+
+Only after E2 and E3 produce a winner. Zero detections across at least 15
+stratified carriers (at least 3 per stratum: face closeup, fast motion, flat sky,
+dark night, text overlay) bounds the failure rate at 18.1%; 30 carriers bound it at
+9.5%. Add the project's 1.5x margin convention, at least 2 seeds, at least 2
+sessions, and per-track verdict recording.
+
+For the flat-sky stratum, check the source reads detected first: Google documents a
+"not enough details to watermark" state, so a clean reading there may say nothing
+about removal.
+
+## Surviving change candidates
+
+**S1. Fix the metric's reference frame (0 oracle submissions).** Add
+`source_psnr_db` (candidate upscaled back to native against the untouched source
+frame) **and** a separate post-mux pass that decodes the delivered file. The
+critics killed the original claim that this would expose crf 18: the candidate is
+captured at `:423`, before `frame_pipe.write` at `:430`, so no in-loop metric can
+see the codec. Only the post-mux pass covers resize, decimation, crf, and mux
+together. Use `INTER_AREA` or a fixed analytic size for the upscale so the CPU
+resize does not dominate the loop, and update both other consumers of the
+generator in the same change (`read_sampled_frames` at `:188-209`, and
+`scripts/video_synthid_sweep.py:147` where `np.stack(frames)` must keep receiving
+resized frames). Do not redefine the existing `psnr_db`: the two manifest rows stay
+comparable to each other only while that field keeps its exact current meaning.
+
+**S2. Frame rate as a fixed certified value (after E3).**
+`DEFAULT_VIDEO_SYNTHID_FPS` 12.0 to 24.0, keeping `min(fps, source_fps)`. A
+ceiling of 60.0 was **rejected**: it makes the delivered operating point a function
+of the user's source, so one sample from a family gets certified while the rest
+ship uncertified, and the risk direction is unfavorable. Port the VFR/PTS bridge
+(`probe_video_timestamps`, `timestamped_input`, currently only in
+`video_visible.py`): at native frame rate the output stops reading as a proxy, and
+silent CFR-ification becomes a master-quality defect. Add `fps` and `long_side`
+pins next to `tests/test_video_invisible.py:285`, tied to the certifying manifest
+row.
+
+**S3. Clamp and align `_fit_size` (0 oracle submissions, with a caveat).**
+`scale = min(1.0, long_side / max(width, height))` at `:88`; align the long side
+and derive the short side from the true aspect, rounding to the nearest multiple of
+8. The pinned `_fit_size(1280, 720, 512) == (512, 288)` survives. The caveat: the
+clamp is an **uncertified operator change for a whole class of users**, since a
+320x240 source is upscaled to 512x384 today and would run at 320x240 afterwards,
+and neither has been tested. Ship it as its own documented change, not as a
+drive-by fix.
+
+**S4. Extend the manifest schema and oracle protocol (0 submissions).** A
+precondition for the whole program. Record the verbatim verdict text, because an
+unclear state logged as not detected is exactly the silent regression the protocol
+exists to prevent.
+
+**S5. A sigma normalization contract (0 submissions, applies to the current path).**
+No VAE swap survived, but two elements are real risks today: `vae.config.scaling_factor`
+cannot be trusted, so log `latents.std()` at `:276` and assert the effective
+scaling factor at load; and any cross-model sigma transfer must match on the **RMS
+of the decoded pixel perturbation**, not on latent units.
+
+**S6. Additive texture masking - contingency on an E2 negative only.** Not energy
+preserving: the certified sigma stays as a floor everywhere and the mask only adds
+on top in textured regions. The original energy-preserving form was refuted - the
+mechanism normalizes and then clips, which breaks the claimed invariant by a
+content-dependent amount, and the proposed unit test asserted the invariant before
+the clip and would have stayed green. Energy preservation is also unsafe in
+principle: the detector does not need a full frame, and the verifier reports
+per-segment, so a flat region perturbed only at the floor is a crop carrying nearly
+the full carrier. The salvaged version has **no standalone PSNR win**; its only
+value is freeing distortion budget to spend on resolution if E2 shows the dose must
+rise. It additionally needs motion compensation for the mask (flow is computed at
+`:442`, after the decode at `:423`, so the loop must be restructured) and one
+deliberately bad candidate so `temporal_residual_ratio` acquires a known failing
+value: it has only ever been observed in [1.0072, 1.0578], so "the ratio looks
+fine" is currently an unfalsified claim.
+
+## Refuted proposals - do not resurrect
+
+- **Carrier-scale, resolution-invariant noise field.** "Carrier scale" is set by
+  our VAE's latent pitch, not the carrier's. The motivating arithmetic treats a
+  field on the latent grid as white at pixel resolution and is wrong by exactly the
+  square of the scale factor: white noise on a 240-wide latent already spans
+  0-120 cycles per frame, entirely inside the band the decoder reproduces. The
+  construction actually band-limits the perturbation from 0-120 to 0-32 cycles,
+  making it smoother - the first thing any spread-spectrum extractor's content
+  suppression removes - while moving its energy toward the peak of the contrast
+  sensitivity function, making it more visible. Bilinear upsampling is also
+  heteroscedastic, and global renormalization by `noise.std()` stamps a visible
+  amplitude lattice rather than fixing it.
+- **Native resolution plus tiling plus a carrier-scale field.** Inherits the
+  arithmetic above, concedes that native processing destroys less carrier without
+  proposing a replacement mechanism, and bundles four destruction axes into one
+  candidate on a 4-query budget so a detected verdict is undecomposable. Its cost
+  model was also wrong by roughly 10x. The salvageable parts moved into S1 and S3.
+- **`AutoencoderKLTemporalDecoder`.** Refuted independently by three critics.
+  Removal is a property of the encode-decode composition, so a strictly more
+  faithful decoder preserves more carrier at the same sigma. The class implements
+  neither slicing nor tiling, so `enable_slicing()` at `:135` raises
+  `NotImplementedError` and the documented bounded-memory property dies. Its
+  temporal receptive field needs windows of tens of frames, its license carries a
+  revenue restriction incompatible with an Apache-2.0 default, and its own
+  published table shows a **worse** FID (9.17 against 7.61).
+- **Wan or another temporal video VAE.** Hard refusal in code: `AutoencoderKLWan._encode`
+  uses `iter_ = 1 + (num_frame - 1) // 4`, so at the shipped `batch_size = 4` only
+  frame 0 is encoded and a 4-frame batch decodes back to 1, which makes
+  `zip(..., strict=True)` at `:429` raise. The proposed remedy of carrying the
+  causal feature cache across chunks is impossible through the public API, since
+  `clear_cache()` runs on both entry and exit of `_encode` and `_decode`.
+  `float(vae.config.scaling_factor)` at `:270` and `:292` also raises, because Wan
+  exposes `latents_mean`/`latents_std` instead. Logically, feeding 4:1 temporal
+  compression frames 83 ms apart either reconstructs well (carrier preserved, no
+  removal gain) or hallucinates (no quality prize); the two claims are mutually
+  exclusive.
+- **A 16-channel f8 VAE.** Self-refuted and confirmed by the critics. Quadrupling
+  latent channels roughly doubles L in Zhao's theorem, so sigma must roughly double
+  to hold removal, and the VAE frontier in the UnMarker results is monotone with no
+  published point where a more faithful autoencoder removes **more**. Predicted net
+  loss of 2-7.5 dB by its own arithmetic. Its config also carries
+  `scaling_factor = 0.2614`, so an unchanged `noise_std = 0.15` is about 30% weaker
+  in raw latent units. Only the normalization contract survived, as S5.
+- **A per-channel luma/chroma probe.** The arms match in absolute latent units but
+  not in relative dose per channel, since `scaling_factor` normalizes the aggregate
+  latent, so the very reading the probe exists for is confounded with dose. The
+  chroma arm is additionally subsampled by the yuv420p encode, which no current
+  metric sees. Maximum prize under 1 dB by its own fit.
+- **Post-processing with unsharp plus grain, as a shipped stage.** The
+  carrier-reimport argument is sound and was verified line by line: `unsharp_mask`
+  reads only its own argument ([`humanizer.py:79`](../src/remove_ai_watermarks/humanizer.py)),
+  and `adaptive_polish` touches the source only through one float. But reimport was
+  never the binding risk:
+  `cv2.addWeighted(img_f, 1.0 + amount, blurred, -amount, 0.0)` at
+  `_ADAPTIVE_MAX_UNSHARP = 1.0` multiplies the **surviving carrier residue** by up
+  to 2x as the terminal operation before encoding, and whether that re-arms a given
+  file depends on that file's unobservable margin. With no local decoder and only a
+  sampled oracle the stage is structurally uncertifiable, not merely expensive to
+  certify. The only rescue is moving sharpening **above** the VAE stage so the
+  removal operator stays terminal. Grain, which monotonically lowers detector SNR,
+  is the safe half and can be separated.
+
+## Measuring quality properly
+
+The current `psnr_db` cannot show the improvement this work exists to produce.
+Minimum upgrade:
+
+1. **`source_psnr_db`** - candidate upscaled back to native against the untouched
+   source frame. Good for ranking configurations against a common reference,
+   dominated by unrecoverable high-frequency content, so not a measure of what the
+   VAE costs.
+2. **A post-mux end-to-end pass** - decode the delivered file after
+   `mux_encoded_video` (`:456`) and compare against the source. The only measure
+   covering resize, decimation, crf, and mux together. Keep it out of the streaming
+   loop so `test_stream_batches_consumes_only_one_batch_ahead` stays valid.
+   Implemented as `scripts/video_fidelity_probe.py`, which streams, reports the
+   delivered file's bitrate, and shares the engine's frame-selection rule rather
+   than copying it.
+3. **DISTS** (arXiv:2004.07728), built to tolerate texture resampling - exactly
+   what the VAE does to foliage, skin, and grass, and exactly what PSNR and LPIPS
+   punish even when the result is perceptually equivalent. It separates "the VAE
+   resampled the grass" from "the VAE destroyed an edge".
+4. **VMAF** through `ffmpeg` `libvmaf`, cheapest to add, whose ADM/DLM feature
+   names this exact complaint. Its temporal term is only a mean absolute luma
+   difference between neighboring frames, so it is not a flicker detector: keep the
+   motion-compensated ratio.
+5. **Encoded file size as a third axis.** Every current metric is taken before the
+   pipe, so a bitrate explosion currently reads as "quality did not suffer". This
+   matters most for any perturbation that varies frame to frame: at fixed crf it
+   raises bitrate rather than lowering quality.
+6. **`temporal_residual_ratio` repairs**: add a p95 across frames next to the mean
+   so localized flicker stops being averaged away, add a long-horizon term against
+   the first frame, and replace `max(temporal_baseline, 1e-6)` with an explicit
+   undefined result on static shots.
+7. **A no-reference metric** (DOVER or FAST-VQA) for the case where no
+   full-reference measure can compare a 512/12 output against a 1024/24 output on
+   one scale. Relative ranking within a sweep only; absolute values are
+   uncalibrated for this artifact class.
+
+Nothing local measures removal. `remove_video_invisible` checks `get_ai_metadata`
+on the output, which is metadata, not pixels. Only a manifest row counts.
+
+## Risks
+
+**Error asymmetry is the governing constraint.** Shipping a leak means a user
+receives a watermarked file believing it is clean, with no local pixel decoder to
+catch it and no feedback path that would surface it. Staying conservative means a
+user receives 512 px / 12 fps, a cost that is visible, bounded, and reversible with
+an explicit flag. A symmetric test is therefore inadmissible, and the burden of
+proof sits entirely on the new default.
+
+**n = 1.** The current default is certified by one success on one carrier with one
+seed. Any quality change inherits that weakness and must not deepen it.
+
+**Content and seed dependence** is MEASURED on this project's image branch:
+survivors switch by content type, and near the threshold the same input flipped
+between runs on seed alone.
+
+**Oracle instability.** Three verdict states, not two. Per-segment reporting.
+Separate audio and visual tracks - and the pipeline byte-copies audio, so a Veo 3
+clip with generated sound leaves with its audio SynthID intact, and the manifest
+cannot even record which track the 2026-07-31 negative referred to. That is
+potentially a shipped product hole, not only an experimental confound. Session
+drift is currently unfalsifiable.
+
+**Observability narrows exactly where quality rises.** At fixed crf the bitrate
+grows with pixels per second, so the maximum duration fitting under 100 MB falls as
+fps and resolution rise. The configuration users actually receive becomes **less**
+observable than the one it replaces. That is a permanent property, not an
+inconvenience.
+
+**Ceilings instead of fixed values.** Any ceiling makes the delivered operating
+point a function of the user's source, and with it the perturbation's cycles per
+frame - the quantity carrying the certified margin. Ship fixed values.
+
+**Documentation and test surface.** Only `DEFAULT_VIDEO_SYNTHID_NOISE_STD` is
+pinned. Moving `long_side` or `fps` touches hardcoded numbers in `README.md`,
+`docs/known-limitations.md`, `docs/cli.md`, `docs/python-api.md`,
+`docs/module-internals.md`, `docs/synthid.md`, and `docs/verification-plan.md`.
+Drift-prone operational numbers should not live in seven places of prose; their
+source of truth is the manifest.
+
+**Silent configuration drift.** `sd-vae-ft-mse/config.json` carries no
+`scaling_factor`, so 0.18215 is a class default under an upper-unbounded
+`diffusers>=0.38.0` while `maintain.sh` runs `uv-outdated`. A library bump can move
+the certified operating point with a green suite.
+
+**Missing guards that get worse at native resolution.** There is no HDR or >8-bit
+rejection and no VFR/PTS bridge on the invisible path. A 512 px output plainly
+reads as a proxy; a native-resolution output reads as a master, and the cost of
+silently flattening a 10-bit PQ source to 8-bit SDR rises accordingly.
+
+**Do not bundle axes.** Raising `long_side`, raising `fps`, and lowering crf all
+reduce total destruction and all require recertification. The metric reference-frame
+fix is the only exception, because it changes the measurement rather than the
+pixels. Do not put a crf 18 to 14 change in the same candidate as a resolution
+change.