The detectors

Rekkefly has two wing detectors. The robust detector (method = "wing_blur", Ignore legs in the toolbar) is the default. The classic detector (method = "edge") is Kinefly’s approach, kept for comparison and for existing setups.

The problem: narrow things in the wing sector

A beating wing photographed over many wingbeats is a translucent stroke envelope: a broad shoulder of blur tens of degrees wide. Its leading edge is what wingbeat amplitude means.

A leg, an antenna, a bristle or the arista is narrow. It is localized in angle, and usually in radius, and it can sit inside the blur or outside it. It need not be darker than the wing: because it is in focus, its edges are sharper than the wing’s soft edge. A detector that reports the sharpest edge therefore jumps onto it, and L − R spikes by tens of degrees, exactly when a closed-loop experiment can least afford it.

Front legs reaching forward. White dashes: the true leading edge. Orange: the detected edge. Classic detector (left) and Rekkefly (right).

Classic: the strongest edge

1. Average the sector along each ray into one angular profile: intensity against angle.
2. Smooth it (three-bin box) and take its derivative (central difference over ±2 bins).
3. Report the steepest edges, strongest first.

This is fast and precise on a clean wing, and fails as soon as something sharp enters the sector.

Robust: the edge of the blur shoulder

The robust detector tracks the shoulder rather than the sharpest edge:

1. Bands. The sector is split into up to 8 radial bands, each giving its own angular profile.
2. Contrast is capped at contrast_cap above each band’s background, so an opaque leg cannot outweigh a translucent wing.
3. Narrow features are removed. A morphological opening deletes anything narrower than spike_width_deg (default 5°) in each band, however dark it is. A broad shoulder passes through unchanged, edges included.
4. Median across bands. Something present at only some radii, as a leg or an antenna usually is, is out-voted.
5. The shoulder decides. Each remaining edge is scored by its steepness times the blur shoulder behind it: the mean level over a wide window on either side (four times spike_width_deg, between 12° and 45°: 20° by default). A narrow line has almost no shoulder, even where it touches or crosses the wing; the wing’s true edge has the whole envelope behind it. An edge whose window is cut short by the end of the sector counts for less.

The angle itself still comes from the steepest point of the winning edge, so the robust detector is as precise as the classic one: the median error on synthetic flies with known angles is about 0.25°.

The debug traces show both stages: in the angular profile panel, thin is the raw profile and bold is what the robust detector ranks edges on, with legs and antennae removed.

Which edge is reported

A blurred wing has two edges in its sector: the leading edge on the head side, and the trailing edge toward the tail. primary_edge chooses:

The leading and trailing edges are told apart by polarity (which side is wing and which is background), not by position. A stray feature beyond the wing, such as an antenna or the arena wall, therefore cannot take the leading edge’s place. If no edge of that polarity is found, the outermost edge in that direction is reported.

Up to max_edges edges (default 2) are reported in angles_rad, the primary one first. Edges closer than 4 bins, or on the same derivative lobe, are reported once.

Threshold

threshold is the minimum edge strength (the absolute derivative of the normalized profile, 0–1). At 0, the default, the strongest edge is always reported, however faint. Raise it so that frames with no wing visible (the insect stopped flying) report a missing angle instead of noise. The live peak strength in Settings shows what a good edge scores on your rig.

Centroid

method = "centroid" reports the intensity-weighted mean angle of the sector. It is meant for the head and the abdomen, which are solid and move as a whole.

Limits

• Features wider than spike_width_deg, such as a badly out-of-focus leg 10 px wide, can still win now and then. Raise Ignore features narrower than for very thick legs, keeping it narrower than the wing’s blur.
• A feature that touches the leading edge along its whole length is indistinguishable from the wing itself.
• Marker placement matters most. No detector fixes a hinge in the wrong place or a sector that clips the body; see Placing the markers.

The evidence for these claims, on synthetic flies with known angles and on the real Bogong moth, is in Evidence.