Uniformity and hue

WLab is meant to be a uniform space for material color: equal distances in it should be seen as about equal color differences, and a line of one WLab hue should keep one hue as its colors get lighter and grayer. This page looks at that claim beside CIELAB and CAM16-UCS, with numbers and pictures, and leaves the judging to you.

An observing condition (hereafter "condition") is an observer under a light. WLab describes the color of a material, whatever the condition; CIELAB and CAM16-UCS describe a color in one condition. Where they are compared below, theirs are the chosen condition's coordinates of the material the WLab color stands for. CAM16-UCS is the uniform color space of the CIE's color appearance model CAM16, computed here for an adapting luminance of 100 cd/m², a background of Yb 20, an average surround and full adaptation to the condition's white. The page opens on CIE 1931 (2°) under illuminant C, the condition of the Munsell renotation and of WLab's own reference.

1. Constant hue

Each row is one lightness, from gray (left) to the most vivid color your screen can show. Each chip is split: the lower-right half holds WLab hue constant; the upper-left half has the same lightness and chroma in the space it is compared with (CIELAB or CAM16-UCS, in the chosen condition) but holds that space's hue constant, at the hue of the ringed chip. Judge for yourself which half stays closer to the ringed chip's hue as the colors get lighter and grayer.

Both halves show the material each color stands for as it would look under D65, the screen's white, so that only the hue line differs between them. The observers are the CIE color-matching functions, for which CIELAB and CAM16 are defined.

2. Munsell loci

The Munsell system was built by eye so that its steps look even: its Hues are lines of constant hue, and its Chromas rings of equal color strength. Here are the corrected Munsell colors of one Value (lightness) in WLab, in CIELAB and in CAM16-UCS: each Hue's chips joined from gray outward (a spoke), and each Chroma's chips joined around the circle (a ring). A space that holds hue constant along Munsell's Hues draws the spokes straight, and one whose steps are even draws the rings round and evenly spaced.

Each dot is a chip in the color it has under D65; hover over one for its notation. The 40 Hues (2.5, 5, 7.5 and 10 of each of the ten families) at the Book's even Chromas, from the corrected Munsell colors.

Hue by Hue

For each Hue at this Value, the hue angles of its chips: their spread from the smallest to the largest, and their RMS about their mean. 0 means every chip of the Hue has one hue angle.

3. The object-color solid

No material can be more chromatic than the colors at the surface of the object-color solid: those of reflectances that are 0 or 1, jumping between them once or twice across the spectrum. Each condition has its own solid. In WLab, which describes materials whatever the condition, the solids of two conditions would be the same if the two conditions told the same set of materials apart; where they differ, one condition gives more room to some colors than the other does. Cut across at one WLab lightness, each solid gives an outline in the aw–bw plane.

Condition A

Condition B

The solid in 3D

The 6,480 colors on the solid's surface (every band of the spectrum at 0 or 1, on the 5 nm grid), in the colors they have under D65; drag to turn it. Each cross-section is found exactly in Wpt, where the cut at one lightness is flat, and drawn in WLab.

4. Published data

Still to come: checks against published sets of constant-hue loci and of color differences judged by observers, where their licenses allow. The sets in view are not yet cleared for use here.