colordiff op• Data kinds: lab × lab → image2d
• Call: import fullseye as fs; fs.ledger.delta_e_76(lab1, lab2) (to call the implementation directly, import imgmetrics; imgmetrics.delta_e_76(lab1, lab2); from the registry, opsimgmetrics.get("delta_e_76"))
CIE76 colour difference — the Euclidean distance in Lab space.
Simple, but it does not match perception (especially for highly saturated blues), so it is provided only as a baseline for comparison. For practical use, delta_e_2000.
Formula: sqrt((L1 - L2)^2 + (a1 - a2)^2 + (b1 - b2)^2).
• lab1, lab2: arrays whose last axis is 3. Shapes broadcast by numpy rules (a 1-colour (3,) with an image (H, W, 3) is also fine). Pass the output of rgb_to_lab.
• Return value: float64 with the last axis dropped ((H, W) for images). 0 means identical colours. As a rule of thumb about 1 is the discrimination threshold and 2.3 is said to be the JND, but with this formula, depending on hue and chroma, the same number can look different.
• Failure (MetricContractError): the last axis of either is not 3. Finiteness of values is not checked (NaN propagates as is).
• image_difference_metrics family guide
• colorimetry — Colorimetry and spectra — colour is fixed only by spectrum × illuminant × observer
• 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.
• image_quality_metrics — py -3.11 examples/image_quality_metrics.py
image2d as input)mse · rmse · psnr · ssim · ms_ssim · ssim_map · image_entropy · joint_entropy
colordiff)*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.