frei_dir — 2D edges op

• Data kinds: image → image

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

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

Usage

An edge-direction (angle) map using Frei-Chen-type kernels. Computes `arctan2(gy, gx) from the convolution responses of the horizontal kernel _FREI[0] (row/vertical-direction gradient) and vertical kernel _FREI[1] (column/horizontal-direction gradient), and maps it to [0,1] using the same convention as sobel_dir/prewitt_dir` (0/1 = -180 degrees, 0.5 = 0 degrees). a and b are unused.

> The detailed description below is the original text — the summary and the headings are translated.

HALCON の `frei_dir（Frei-Chen 演算子でエッジの振幅と方向を検出する演算）のうち方向成分に相当する近似。振幅は frei_amp` 側が担当する。

Detailed usage guide

• gallery2d_edges 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.

Runnable examples (verified samples that actually call this op)

• gallery2d_edges — py -3.11 examples/gallery2d_edges.py

Ops the type connects to (they accept image as input)

identity · gaussian · mean_box · bilateral · unsharp · median · min_filter · max_filter

Same category (edges)

sobel_mag · prewitt_mag · roberts_mag · dog · grad_dir · log · corner_response · sk_scharr

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

© 2026 Kazufumi Furuse — Fullseye operator documentation. Licensed under Apache-2.0.