Fix hcl/lch/oklch interpolation not reaching pure black/white exactly - #401
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Fixes gka#310. When interpolating toward (or from) an achromatic color in hcl/lch/oklch, saturation/chroma is intentionally frozen at the colored endpoint's value for intermediate steps, so shade()/tint() keep colors looking vivid instead of fading to gray. But the freeze also applied at f === 1 (or f === 0), where the result must equal the achromatic endpoint exactly. Since chroma is still meaningful at L=0 in these color spaces (unlike saturation at L=0/1 in hsl/hsv/hcg/hsi), keeping it frozen at a nonzero value there produced a slightly-off-black/white result, e.g. interpolate('#f00', '#000', 1, 'hcl') returning '#5b0000' instead of '#000000'. Stop the freeze exactly at that boundary so it falls through to the normal linear interpolation, which already resolves correctly to the target's own chroma. Intermediate steps (f between 0 and 1) are unaffected.
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Summary
Fixes #310.
interpolator/_hsx.jsintentionally freezes saturation/chroma at the colored endpoint's value while interpolating toward (or from) an achromatic color, soshade()/tint()keep colors looking vivid instead of fading to gray at intermediate steps. That freeze is correct and tested behavior forfstrictly between 0 and 1.The bug is that the freeze also applied exactly at the boundary (
f === 1interpolating toward black/white, orf === 0interpolating away from it), where the result must equal the achromatic endpoint exactly. Unlike hsl/hsv/hcg/hsi (where saturation has no visual effect at extreme lightness/value), chroma in hcl/lch/oklch is still meaningful at L=0, so freezing it at a large nonzero value there produces a slightly-off-black/white color once converted to RGB.Fix
Stop the freeze exactly at that boundary (
f < 1/f > 0guards) so it falls through to the existing linear interpolation, which already resolves correctly to the target's own chroma (0 for a fully achromatic target). Every other case is untouched.Test plan
test/mix.test.js:hcl/lch/oklchreaching pure black from a saturated color),red.shade(0.5, 'lch')to lock in that the intermediate-step "stay vivid" behavior is unchanged.'#5b0000'/'#100000'(the exact wrong values from the issue), while the shade test still passes on the reverted code — confirming the test isolates the actual bug rather than the intentional shading behavior.npx vitest run— 2522 passing (2519 existing + 3 new).npm run lint(prettier + eslint) passes, and the repo's pre-commit hook (lint + full test run) passed on commit.