The OKLCH Mathematical Manifesto

The coordinate is useful, not sovereign

Ottosson’s original 2020 post introduces Oklab for image-processing tasks, with aims including useful perceptual relationships and smoother color manipulation. OKLCH expresses the related coordinates as lightness, chroma and hue. It gives a designer a practical way to vary one coordinate while keeping the others fixed. That is a design advantage, not proof of perfect uniformity for every viewing condition.

The source title is A perceptual color space for image processing, not the earlier bibliography’s graphics variant. CSS Color Level 4 describes the web color representation and its conversions. A specification for coordinates does not turn a chosen palette into measured material reflectance or make every requested color reproducible on an sRGB display.

A neutral pair exposes the distinction

Consider opaque neutral OKLCH with L=0.65 and C=0. For the neutral case, the linear sRGB channels are L³=0.274625 before quantization. Against ideal black, the WCAG relative-luminance ratio is (0.274625+0.05)/0.05≈6.49:1. Against ideal white it is 1.05/(0.274625+0.05)≈3.23:1. The same color therefore supports different ratios depending on the background.

The calculation uses an ideal, unquantized neutral and opaque sRGB black and white. An 8-bit preview rounds channels and gives slightly different values. WCAG’s contrast guidance describes the ratio and relevant text criteria. Passing a ratio for one pair does not establish complete accessibility, and a lightness coordinate alone cannot select the right background.

The earlier specimen advertised a verified invariant 14.6:1 ratio without identifying a complete foreground/background pair. That number is withdrawn. So are fixed percentages presented as universal fractions of all visible colors in sRGB or Display P3, and a universal chroma threshold for exceeding sRGB. Gamut boundaries depend on the coordinates under consideration.

Read the preview’s transformation

The study pairs an HSL scale at 65% saturation with an OKLCH scale at chroma 0.15. Its slider changes hue. The labels are each model’s lightness coordinates; an HSL label of 65 and an OKLCH value of 0.65 do not mean that their samples have equal luminance or equal appearance.

The study converts OKLCH to linear sRGB and marks samples whose channels lie outside the supported interval before clipping. Its preview clamps channels into that interval and encodes an sRGB color. The dagger marks an out-of-gamut request. This is explicitly a clipped teaching preview, not a demonstration of a particular browser’s more elaborate gamut-mapping procedure.

Inspect the color the reader receives

A palette workflow should save the authored coordinate, intended output space and any mapping or clipping step. Then inspect the rendered foreground against its actual background, including opacity, overlays and states such as hover or disabled. A contrast computation for a solid pair cannot certify text placed over a moving image.

For a declared design task, use OKLCH to develop related ink candidates, convert them through the supported display path and choose among them using the actual reading pair. If a vivid candidate changes substantially after mapping, the design decision may need to change too. The color-space advantage is retained while the display result is allowed to interrupt it.

A manifesto with a checkable claim

I favor OKLCH because it makes some palette decisions easier to express and inspect. I do not favor calling its numbers an accessibility invariant. A useful color argument names a pair, its conversion and its criterion; a useful visual judgment also pays attention to the text and setting.

The study makes the conversion’s limitation visible, and the neutral example makes contrast’s dependency visible. Together they support a narrower, stronger claim than the old guarantee: coordinates help author the palette, while explicit evaluation helps decide whether a particular reader can use it.