The rolling window held frames as decoded — `images_.push_back(f.image)` — and
left the downscale to the backend. TransNetV2's input is 48x27, so the buffer
held roughly 590 MB at 1080p to feed a model that needs about 380 KB. The
config note for `dense_scale` says as much outright: "TransNetV2 downsamples to
48x27 regardless".
This is not a channel capacity, so no amount of tuning channel depths would
ever have found it. It is a `std::deque<cv::Mat>` member, and it is the single
largest allocation in the scene branch.
It is also redundant work. Windows overlap by `kWindow - stride`, so a frame
appears in several of them and was re-downscaled once per window it appeared
in; now it is downscaled once, on arrival.
**The risk here is the invariant, not the memory.** Every model gets the input
it was trained for — a model run off-distribution returns confident, plausible,
wrong output, and for a boundary detector that means fabricated cuts, which are
indistinguishable from real ones in the output. So this reproduces the
backends' preprocessing exactly rather than doing its own: both
ort_backend.cpp and trt_backend.cpp guard mis-sized input with
`convertTo(CV_8UC3)` and then
`cv::resize(..., {kFrameW, kFrameH}, 0, 0, cv::INTER_AREA)`, in that order, and
`to_model_input` performs the same two operations. The backend guard then sees
a correctly-sized frame and does nothing, so the tensor the model receives is
unchanged. The interface has always specified this as the caller's job — "Each
frame must already be kFrameW x kFrameH, BGR, CV_8UC3" — so the node now meets
a contract it was already given.
The tests assert equivalence, not size. They perform the backend's own two
operations independently and compare byte for byte, on a gradient rather than a
flat fill, since INTER_AREA averages and a constant image would compare equal
under almost any resize. Order is pinned too: converting a 4-channel frame
after downscaling averages alpha into the colour channels and gives different
pixels.
Verified in both directions. With INTER_LINEAR substituted for INTER_AREA —
the most plausible way to get this subtly wrong — three assertions fail. With
the backend's own operations, byte-identical at 1920x1080, 640x360 and 720x480.
149/149.
Still unmeasured on real content, as with the previous commit: the equivalence
argument says the model sees the same tensor, but a run comparing scenes.json
before and after on a real clip is what would settle it, and I could not launch
one here.
TRACES: AR-004, AR-010 | SR-002
Closes both violations SPEC.md named under "Every model gets the input it
was trained for". They are one bug, not two.
The dense stream defaulted to 12 fps, so a 100-frame TransNetV2 window
spanned ~8.3 s against the ~4 s it was trained on: half-speed motion over
twice its temporal context. Boundary timestamps stayed correct throughout,
which is exactly why the degradation was invisible and why the compressed
separation it produced (~0.50 baseline against ~0.7+ peaks) was read as a
property of the ONNX export rather than of the input.
Dedup then merged boundaries closer than a literal 0.04 s — one frame at
25 fps, and wider than a frame at 30, so two cuts on consecutive frames
became one. Nothing in scenes.json showed it; the file simply had fewer
boundaries. Native rate is where that constant did the most damage, which
is why fixing the decode rate without fixing the dedup would have made
things worse.
dedup_window_sec() now takes the median interval the detector was actually
fed and halves it. Half a frame rather than a whole one: the only thing
being merged is one frame scored by two overlapping windows, and two
distinct frames are a full interval apart.
Cost is real — dense decode is the pipeline's cost driver. It is accepted;
dense_scale and scene_stride remain the reductions that do not run the
model off-distribution. scene_threshold 0.60 was fitted against the 12 fps
input and is now stale, so VR-006 goes from Low to Medium: it is no longer
a refinement, it is a constant that no longer describes the input.
AR-002 rides along because it was already implemented, just untagged and
unverified — the register said Planned while the code was correct. The size
filter becomes FaceDetectorFunc::drop_undersized(), tested at the threshold
and at dense_scale 0.5, and checked end to end against the superhero dump,
whose smallest face is exactly its recorded 32 px minimum, so the fixture
check cannot pass vacuously.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
TRACES: AR-002, AR-011 | SR-002 | UT-002, UT-003, IT-001
dtourolle
