color op• Data kinds: qimage → qimage
• Call: import fullseye as fs; fs.ledger.quat_color_filter(qimage, direction_rgb, mode) -> 'np.ndarray' (to call the implementation directly, import quatimage; quatimage.quat_color_filter(qimage, direction_rgb, mode) -> 'np.ndarray'; from the registry, opsquat.get("quat_color_filter"))
Keep or remove one colour direction, exactly. → (H, W, 4).
mode="remove" returns v - (v.g) g for the unit RGB direction g:
the component along g is exactly zero everywhere afterwards, to machine precision (measured max residual 5.8e-16 and 6.5e-16 on two random colour images — seed-dependent only at the 1e-16 level), and remove + keep reproduces the input to 0.0 exactly. mode="keep" returns the complementary (v.g) g. The scalar part is passed through untouched in both.
There is no default mode. The two are opposites, both return a valid picture, and neither raises — so choosing for the caller would be a coin flip that never announces itself.
The remove branch is the specular-invariant projection of Mallick et al. (2005), which this repository already implements as specularity.specular_free_transform for the rgbimage sort. Rather than write the same three lines twice, this operator *delegates* to it — so agreement between the two sorts is by construction rather than by luck, and a future fix in one is a fix in both. What is added here is the keep branch (which has no counterpart there) and the qimage sort, so the projection composes with quat_color_rotate and qft2.
A per-channel filter applies a diagonal matrix, and I - g g^T is diagonal only when g is a coordinate axis. For g = (1,1,1)/sqrt(3) — remove the grey axis, i.e. keep only chromatic content — the *best possible* diagonal approximation is off by ||P - diag(P)||_2 = 0.666667 in operator norm. Concretely, a pure red pixel (1, 0, 0) must become (0.666667, -0.333333, -0.333333); the best diagonal filter can only reach (0.666667, 0, 0), an error of 0.471405 — it cannot put anything into the green and blue channels, because it multiplies each channel by a number and both start at zero. The impossibility is structural, not a tuning gap. (A full 3x3 colour matrix, of course, does it exactly; see quat_color_rotate for that half of the accounting.)
Raises ValueError: *qimage* is not a valid (H, W, 4) field; *direction_rgb* is not a finite non-zero 3-vector; *mode* is not 'remove' / 'keep'.
• quaternion_monogenic family guide
• 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.
• quaternion_monogenic — py -3.11 examples/quaternion_monogenic.py
qimage as input)quaternion_to_rgb · quat_norm · quat_conjugate_image · quat_normalize_image · quat_image_multiply · monogenic_amplitude · monogenic_phase · monogenic_orientation
color)*Provenance: quatimage.py — QUAT 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.