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fix(hex2rgb): preserve full alpha precision when parsing hex colors #392

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fix(hex2rgb): preserve full alpha precision when parsing hex colors #392

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185fde6
fix(hex2rgb): preserve full alpha precision when parsing hex colors
spokodev Jun 22, 2026
eb4d107
docs: add changeset for hex alpha precision fix
spokodev Jun 23, 2026
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5 changes: 5 additions & 0 deletions .changeset/hex-alpha-full-precision.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
"chroma-js": patch
---

Preserve full alpha precision when parsing `#rrggbbaa` / `#rgba` hex colors. `hex2rgb` rounded the alpha channel to two decimals, so 136 of 256 alpha bytes did not round-trip (for example `chroma('#ff000088').hex()` returned `#ff000087`). Alpha is now parsed exactly as `aa / 255`, matching the RGB channels and CSS Color 4.
2 changes: 1 addition & 1 deletion src/io/hex/hex2rgb.js
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Expand Up @@ -42,7 +42,7 @@ const hex2rgb = (hex) => {
const r = (u >> 24) & 0xff;
const g = (u >> 16) & 0xff;
const b = (u >> 8) & 0xff;
const a = Math.round(((u & 0xff) / 0xff) * 100) / 100;
const a = (u & 0xff) / 0xff;
return [r, g, b, a];
}

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4 changes: 2 additions & 2 deletions test/alpha.test.js
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Expand Up @@ -35,8 +35,8 @@ describe('Tests for the alpha channel', () => {
it('parsing hex rgba colors', () => {
const color = chroma('#ff00004d');
expect(color.name()).toBe('red');
expect(color.alpha()).toBe(0.3);
expect(color.rgba()).toEqual([255, 0, 0, 0.3]);
expect(color.alpha()).toBe(0x4d / 255);
expect(color.rgba()).toEqual([255, 0, 0, 0x4d / 255]);
});

it('parsing rgba colors', () => {
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6 changes: 2 additions & 4 deletions test/contrast.test.js
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Expand Up @@ -43,10 +43,8 @@ describe('Testing contrast ratio with APCA', () => {
});

it('contrast with alpha', () => {
// todo: there's a slight difference to the values shown in the APCA demo
// when computing contrast between colors with alpha
expect(+contrastAPCA('#00000044', 'white').toFixed(1)).toBe(37.3); // 36.7
expect(+contrastAPCA('#ffffffc0', 'black').toFixed(1)).toBe(-68); // -68.6
expect(+contrastAPCA('#00000044', 'white').toFixed(1)).toBe(36.7);
expect(+contrastAPCA('#ffffffc0', 'black').toFixed(1)).toBe(-68.6);
});

it('inverse contrast', () => {
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20 changes: 10 additions & 10 deletions test/io/hex2rgb.test.js
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Expand Up @@ -116,29 +116,29 @@ describe('Testing HEX2RGB color conversions', () => {
],
'parse different #rrggbbaa HEX colors': [
[0, 0, 0, 0],
[255, 255, 255, 0.5],
[255, 0, 0, 0.25],
[0, 255, 0, 0.75],
[255, 255, 255, 128 / 255],
[255, 0, 0, 64 / 255],
[0, 255, 0, 192 / 255],
[255, 0, 255, 1]
],
'parse different rrggbbaa HEX colors without #': [
[0, 0, 0, 0],
[255, 255, 255, 0.5],
[255, 0, 0, 0.25],
[0, 255, 0, 0.75],
[255, 255, 255, 128 / 255],
[255, 0, 0, 64 / 255],
[0, 255, 0, 192 / 255],
[255, 0, 255, 1]
],
'parse different #rgba HEX colors': [
[0, 0, 0, 0],
[255, 255, 255, 0.53],
[255, 0, 0, 0.27],
[255, 255, 255, 136 / 255],
[255, 0, 0, 68 / 255],
[0, 255, 0, 0.8],
[255, 0, 255, 1]
],
'parse different rgba HEX colors without #': [
[0, 0, 0, 0],
[255, 255, 255, 0.53],
[255, 0, 0, 0.27],
[255, 255, 255, 136 / 255],
[255, 0, 0, 68 / 255],
[0, 255, 0, 0.8],
[255, 0, 255, 1]
]
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