xyz_to_lab — IMGMETRICS colorspace op

• Data kinds: rgb → lab

• Call: import fullseye as fs; fs.ledger.xyz_to_lab(xyz, white=(0.95047, 1.0, 1.08883)) (to call the implementation directly, import imgmetrics; imgmetrics.xyz_to_lab(xyz, white=(0.95047, 1.0, 1.08883)); from the registry, opsimgmetrics.get("xyz_to_lab"))

Usage

CIE XYZ → CIE 1976 L*a*b*. The default white point is D65 2°.

Formula: with t = XYZ / white, f(t) = cbrt(t) (t > (6/29)^3), otherwise t / (3 (6/29)^2) + 4/29. L = 116 f(Y) - 16, a = 500 (f(X) - f(Y)), b = 200 (f(Y) - f(Z)).

• xyz: any shape whose last axis is 3. Same as the output of rgb_to_xyz, on the scale where Y = 1 is white.

• white: white point (Xn, Yn, Zn), 3 positive numbers. Default D65_WHITE = (0.95047, 1.0, 1.08883). Change this for XYZ captured under another light source (D50 etc.).

• Return value: float64 of the same shape as the input. L is 100 for white and 0 for black. a/b are signed.

• Failure (MetricContractError): the last axis is not 3 / white does not have 3 elements or contains a value of 0 or less. The XYZ value range is not checked (negative values and values above 1 are computed as is).

delta_e_76 / delta_e_2000 take this Lab as input.

Detailed usage guide

• image_difference_metrics family guide

Background guides (the physics and conventions behind this op)

• colorimetry — Colorimetry and spectra — colour is fixed only by spectrum × illuminant × observer

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)

• image_quality_metrics — py -3.11 examples/image_quality_metrics.py

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

lab_to_rgb · delta_e_2000 · delta_e_76 · luma_limited_u8 · rank_data · tid2013_index

Same category (colorspace)

rgb_to_lab · lab_to_rgb · rgb_to_xyz

*Provenance: imgmetrics.py — IMGMETRICS 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.