| 1 | // colors.go converts between sRGB hex (what XLSX fills use) and OKLCH |
| 2 | // (what Format.FillIntentH/FillIntentC store — models.go). The app's own |
| 3 | // palette allocation — which offered swatch a hue/chroma pair snaps to — |
| 4 | // lives in the design-system "ui" package (spec §10.3), which this repo |
| 5 | // does not have: it is plan 2. What lives here is the underlying, palette- |
| 6 | // independent color math: a hex an XLSX file wasn't necessarily authored |
| 7 | // with one of our swatches gets its own true OKLCH hue and chroma computed |
| 8 | // directly, rather than snapped to a fixed list this package cannot see. |
| 9 | // FillHex (models.go) is what actually drives export when present, so |
| 10 | // this conversion is never the lossy half of a round trip — see |
| 11 | // TestImportedFillHexIsRetained. |
| 12 | package xlsx |
| 13 | |
| 14 | import ( |
| 15 | "fmt" |
| 16 | "math" |
| 17 | "strconv" |
| 18 | "strings" |
| 19 | ) |
| 20 | |
| 21 | // defaultFillLightness is the OKLCH lightness used to turn an intent |
| 22 | // (hue/chroma only — Format has no lightness field, because in the app's |
| 23 | // own color system lightness is chosen per-theme, not stored) back into a |
| 24 | // concrete hex for export, for the one case that has no retained FillHex |
| 25 | // to fall back on: a fill picked purely by intent, with no source hex, |
| 26 | // which nothing in this backend-only repo can yet produce (there is no |
| 27 | // palette-picker UI here — plan 2), but that Export must still handle |
| 28 | // rather than assume can't happen. 0.7 sits in the middle of OKLab's |
| 29 | // perceptual lightness range: neither so light a saturated hue clips at |
| 30 | // white nor so dark it clips at black for the chromas cell fills use. |
| 31 | const defaultFillLightness = 0.7 |
| 32 | |
| 33 | // hexToOKLCH parses a fill hex (either 6-char RRGGBB or 8-char ARGB — |
| 34 | // this package always retains and emits 8-char ARGB internally, see |
| 35 | // argbHex, but import accepts either) and returns its OKLCH hue (degrees, |
| 36 | // 0-360) and chroma. err is non-nil (and coordinate-free — callers attach |
| 37 | // position) for a string that is not valid hex of one of those two |
| 38 | // lengths. |
| 39 | func hexToOKLCH(hex string) (hue, chroma float64, err error) { |
| 40 | r, g, b, err := parseHexRGB(hex) |
| 41 | if err != nil { |
| 42 | return 0, 0, err |
| 43 | } |
| 44 | l, c, h := srgbToOKLCH(r, g, b) |
| 45 | _ = l |
| 46 | return h, c, nil |
| 47 | } |
| 48 | |
| 49 | // oklchToHex is hexToOKLCH's inverse for export, at a fixed lightness |
| 50 | // (see defaultFillLightness) since Format carries no lightness of its |
| 51 | // own. It always returns 8-char ARGB (alpha FF) — argbHex's shape. |
| 52 | func oklchToHex(hue, chroma float64) string { |
| 53 | r, g, b := oklchToSRGB(defaultFillLightness, chroma, hue) |
| 54 | return argbHex(r, g, b) |
| 55 | } |
| 56 | |
| 57 | // parseHexRGB accepts "RRGGBB" or "AARRGGBB" (case-insensitive, with or |
| 58 | // without a leading '#') and returns 0-1 linear-light-free (still gamma |
| 59 | // encoded) sRGB channel values in 0-255 form as float64, ignoring alpha. |
| 60 | func parseHexRGB(hex string) (r, g, b float64, err error) { |
| 61 | s := strings.TrimPrefix(hex, "#") |
| 62 | switch len(s) { |
| 63 | case 8: |
| 64 | s = s[2:] // drop alpha |
| 65 | case 6: |
| 66 | // already RRGGBB |
| 67 | default: |
| 68 | return 0, 0, 0, fmt.Errorf("xlsx: fill hex has invalid length %d", len(s)) |
| 69 | } |
| 70 | v, err := strconv.ParseUint(s, 16, 32) |
| 71 | if err != nil { |
| 72 | return 0, 0, 0, fmt.Errorf("xlsx: fill hex is not valid hexadecimal") |
| 73 | } |
| 74 | r = float64((v >> 16) & 0xFF) |
| 75 | g = float64((v >> 8) & 0xFF) |
| 76 | b = float64(v & 0xFF) |
| 77 | return r, g, b, nil |
| 78 | } |
| 79 | |
| 80 | // argbHex renders 0-255 sRGB channels as this package's canonical 8-char |
| 81 | // ARGB fill hex, alpha forced to FF: XLSX cell fills are opaque in every |
| 82 | // case this app produces, and excelize's own Style.Fill.Color round trip |
| 83 | // (verified against the library directly) always implies FF alpha too — |
| 84 | // see import.go's readFillHex. |
| 85 | func argbHex(r, g, b float64) string { |
| 86 | clamp := func(x float64) int { |
| 87 | if x < 0 { |
| 88 | return 0 |
| 89 | } |
| 90 | if x > 255 { |
| 91 | return 255 |
| 92 | } |
| 93 | return int(math.Round(x)) |
| 94 | } |
| 95 | return fmt.Sprintf("FF%02X%02X%02X", clamp(r), clamp(g), clamp(b)) |
| 96 | } |
| 97 | |
| 98 | // --- OKLab/OKLCH <-> sRGB, Björn Ottosson's published transform --- |
| 99 | // (https://bottosson.github.io/posts/oklab/). Standard, palette- |
| 100 | // independent color science — not app-specific, which is exactly why it |
| 101 | // belongs in this backend package rather than waiting on the "ui" one. |
| 102 | |
| 103 | func srgbChannelToLinear(c float64) float64 { |
| 104 | c /= 255 |
| 105 | if c <= 0.04045 { |
| 106 | return c / 12.92 |
| 107 | } |
| 108 | return math.Pow((c+0.055)/1.055, 2.4) |
| 109 | } |
| 110 | |
| 111 | func linearToSRGBChannel(c float64) float64 { |
| 112 | if c <= 0.0031308 { |
| 113 | return c * 12.92 * 255 |
| 114 | } |
| 115 | return (1.055*math.Pow(c, 1/2.4) - 0.055) * 255 |
| 116 | } |
| 117 | |
| 118 | // srgbToOKLCH converts 0-255 sRGB channels to OKLCH (L in 0-1, C >= 0, |
| 119 | // H in degrees 0-360). |
| 120 | func srgbToOKLCH(r255, g255, b255 float64) (l, c, h float64) { |
| 121 | r := srgbChannelToLinear(r255) |
| 122 | g := srgbChannelToLinear(g255) |
| 123 | b := srgbChannelToLinear(b255) |
| 124 | |
| 125 | lc := 0.4122214708*r + 0.5363325363*g + 0.0514459929*b |
| 126 | m := 0.2119034982*r + 0.6806995451*g + 0.1073969566*b |
| 127 | s := 0.0883024619*r + 0.2817188376*g + 0.6299787005*b |
| 128 | |
| 129 | lc, m, s = cbrt(lc), cbrt(m), cbrt(s) |
| 130 | |
| 131 | L := 0.2104542553*lc + 0.7936177850*m - 0.0040720468*s |
| 132 | a := 1.9779984951*lc - 2.4285922050*m + 0.4505937099*s |
| 133 | bb := 0.0259040371*lc + 0.7827717662*m - 0.8086757660*s |
| 134 | |
| 135 | C := math.Hypot(a, bb) |
| 136 | H := math.Atan2(bb, a) * 180 / math.Pi |
| 137 | if H < 0 { |
| 138 | H += 360 |
| 139 | } |
| 140 | return L, C, H |
| 141 | } |
| 142 | |
| 143 | // oklchToSRGB is srgbToOKLCH's inverse, returning 0-255 sRGB channels |
| 144 | // (unclamped — argbHex clamps at render time). |
| 145 | func oklchToSRGB(l, c, h float64) (r255, g255, b255 float64) { |
| 146 | hr := h * math.Pi / 180 |
| 147 | a := c * math.Cos(hr) |
| 148 | b := c * math.Sin(hr) |
| 149 | |
| 150 | lc := l + 0.3963377774*a + 0.2158037573*b |
| 151 | m := l - 0.1055613458*a - 0.0638541728*b |
| 152 | s := l - 0.0894841775*a - 1.2914855480*b |
| 153 | |
| 154 | lc, m, s = lc*lc*lc, m*m*m, s*s*s |
| 155 | |
| 156 | r := +4.0767416621*lc - 3.3077115913*m + 0.2309699292*s |
| 157 | g := -1.2684380046*lc + 2.6097574011*m - 0.3413193965*s |
| 158 | bl := -0.0041960863*lc - 0.7034186147*m + 1.7076147010*s |
| 159 | |
| 160 | return linearToSRGBChannel(r), linearToSRGBChannel(g), linearToSRGBChannel(bl) |
| 161 | } |
| 162 | |
| 163 | func cbrt(x float64) float64 { |
| 164 | if x < 0 { |
| 165 | return -math.Cbrt(-x) |
| 166 | } |
| 167 | return math.Cbrt(x) |
| 168 | } |
| 169 | |
| 170 | // IntentHex renders a colour intent at a given lightness. |
| 171 | // |
| 172 | // Exported for the app's contrast gate, which has to answer "does every |
| 173 | // swatch we offer clear WCAG AA at both themes' lightnesses" and cannot |
| 174 | // do that without the same OKLCH->sRGB transform the exporter uses. Two |
| 175 | // implementations of Björn Ottosson's transform, one in Go and one |
| 176 | // implied by the browser's oklch(), is already one more than ideal; |
| 177 | // three — with a test carrying its own copy — would make the gate |
| 178 | // measure something other than what ships. |
| 179 | // |
| 180 | // l is 0-1, as everywhere else in this file. The result is 8-char ARGB. |
| 181 | func IntentHex(l, chroma, hue float64) string { |
| 182 | r, g, b := oklchToSRGB(l, chroma, hue) |
| 183 | return argbHex(r, g, b) |
| 184 | } |
| 185 | |
amadan