Latency Budget

Because latency is a functional requirement, the per-frame cost of feed_image (front-end tracking + one backend update step) has a budget enforced as a test (#47).

The budget (a tunable knob)

branes::sdk::eval::FrameLatencyBudget<T> records the per-frame budget, keyed by the estimator scalar. The numbers below are pinned to that source constant — this page is generated from the code, so it can’t drift:

Scalar	per_frame_ms (current gate)	Real-time target
double	75 ms	~30 ms (≈33 Hz)
float	50 ms	tighter than double

Both the median and the 99th percentile of measured frame latencies must stay at or under the gate. The numbers are a ratchet: the real-time target is ~30 ms, but the MVP front end re-runs full-frame FAST detection every frame, which on a reference x86 host at -O2 measures ≈38 ms median / ≈40 ms p99 at EuRoC’s 752×480 (a reference measurement, not a gate). So the recorded gate starts loose (with headroom for slower hosts) and is meant to be tightened toward 30 ms as the front end gains incremental detection / SIMD and the backend is optimized.

How it’s measured

A microbenchmark times VioEstimator::feed_image over a stable stream and computes order statistics with a nearest-rank percentile() helper (rank = ⌈p·n⌉, so the tail isn’t underestimated; it rejects empty input and NaN probabilities). Warm-up frames are excluded.

• CI guard — runs on a stable synthetic 752×480 stream (so it needs no dataset).
• Dataset bench — the real V1_01_easy feed_image latency, gated on CORTEX_EUROC_V101.

Why it’s enforced only in optimized builds

A real-time budget is meaningless under -O0 (the same synthetic frame measures ~270 ms unoptimized). The assertion is therefore gated on NDEBUG: in a Debug build the test reports the measured numbers and skips the assertion. Since CI builds the sitl-debug preset, the budget is exercised when you run an optimized (sitl-release) build — ctest stays green either way.

Reproducing locally

cmake --preset sitl-release
cmake --build --preset sitl-release
ctest --preset sitl-release -R sdk_vio_latency --output-on-failure