elliptic_axis_xld — 2D `features` op

• Data kinds: contour → match

• Call: fullseye.apply(img, "elliptic_axis_xld", a=0.5, b=0.5) (the 2-D model is one image plus two scalar knobs a,b∈[0,1])

• HALCON equivalent: elliptic_axis_xld (the HALCON reference is a useful guide to its meaning and parameters)

• GPU: none (CPU reference implementation only)

elliptic_axis_xld: input → output

*The figure is the actual output on a synthetic 128×128 input. Left: input, right: output. Point clouds are drawn as a top-down scatter (brightness = z), 1-D series as a line plot, volumes as the maximum-intensity projection along z, videos as the middle frame, complex images as magnitude; return values that are not pictures are shown as the values themselves.*

*Knob a does not change the output (measured: identical at 0.1 / 0.5 / 0.9).*

*Knob b does not change the output (measured: identical at 0.1 / 0.5 / 0.9).*

Stages (the ops that come before → this op, left to right):

▸ elliptic_axis_xld: stages (docs site)

Usage

Fits an ellipse to the contour with the most points (cv2.fitEllipse) and returns (Ra, Rb, Phi) as a 3-component match sort vector — Ra/Rb are the semi-major and semi-minor axes [px], Phi the main-axis angle (radians, from the column axis counter-clockwise on screen with rows pointing down, greater than -π/2 and at most π/2). The same three values as HALCON's elliptic_axis_xld (Parameters of the equivalent ellipse of contours or polygons.).

★Until 2026-11-03 the sign of Phi was reversed (cv2.fitEllipse's angle turns clockwise on screen; a bar turned 20 degrees counter-clockwise gave -0.3505 rad).

The same form as the region version elliptic_axis. Because it is a least-squares fit to the contour points, it does not match the values from the region's moments (30x70 rectangle: region (40.41, 17.31, 0.0) vs xld (44.53, 16.99, 0.0018)).

★Until 2026-09-26 it returned a single scalar, the minor/major axis ratio (docs/hardening/dimensional-features-were-normalised.md).

a, b are unused. Returns 0 (a scalar) when no contour has 5 or more points. Requires cv2.

Detailed usage guide

• gallery2d_features family guide

References (sample data, literature)

• Sample-data catalog (download URLs / licences) — 2-D uses skimage.data (BSD/public domain) plus synthetic images; 3-D lists download URLs for real data sources (Stanford, PDS, …).

• Operator provenance and references — the sources of the research/methods this op family came from.

• The canonical algorithm (author, year) and its uses are named in the family usage guide above.

Try it in Studio

The program below has been verified to run (same input as the figure). In Studio's help this block becomes buttons that load and run it on the spot.

threshold 0.50 0.50
sk_find_contours 0.50 0.50
elliptic_axis_xld 0.50 0.50

▸ Load this pipeline · Load & run

Runnable examples (verified samples that actually call this op)

• gallery2d_features — py -3.11 examples/gallery2d_features.py

Ops the type connects to (they accept `match` as input)

identity

Same category (`features`)

effective_bit_depth · blob_count · area_frac · count_contours · total_length · vol_count · sk_euler · sk_entropy_feat

*Provenance: ops.py — 2D operator registry. This per-op note is generated by tools/opdocs.py md (do not hand-edit).*

elliptic_axis_xld — 2D features op