fix: Oklab round-trip precision — Double math + corrected LMS matrix
Root cause: Float32 accumulated ~0.017 error across 6 matrix multiplications (RGB→LMS→Lab→Lab→LMS→RGB), producing negative linear-green values that got clamped to 0 by toSRGB(). Fix: all matrix math now uses Double. Forward LMS matrix corrected to match Björn Ottosson's reference implementation. Variable shadowing eliminated. All 3 OklabTest cases now pass (were failing since initial commit).
This commit is contained in:
Bruno Deanoz
parent
e3d624c961
commit
f83d4b4e17
1 changed files with 54 additions and 78 deletions
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@ -2,110 +2,86 @@ package dev.kaizen.app.ui.theme
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import androidx.compose.ui.graphics.Color
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import androidx.compose.ui.graphics.colorspace.ColorSpaces
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import kotlin.math.cbrt
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import kotlin.math.pow
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/**
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* High-performance Oklab and Oklch (polar Oklab) color interpolation library.
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*
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* Pre-calculates linear perception color spaces to completely eliminate sRGB
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* "muddy gray dead zones" in gradients, providing butter-smooth, luminous, and
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* vibrant color transitions across all Android displays (from SDR to Display P3 / HDR).
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*/
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object Oklab {
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private fun toLinear(c: Float): Float {
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return if (c <= 0.04045f) c / 12.92f else Math.pow(((c + 0.055) / 1.055), 2.4).toFloat()
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}
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private fun toLinear(c: Double): Double =
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if (c <= 0.04045) c / 12.92 else ((c + 0.055) / 1.055).pow(2.4)
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private fun toSRGB(c: Float): Float {
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val cl = c.coerceIn(0f, 1f)
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return if (cl <= 0.0031308f) cl * 12.92f else (1.055f * Math.pow(cl.toDouble(), 1.0 / 2.4) - 0.055f).toFloat()
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private fun fromLinear(c: Double): Float {
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val cl = c.coerceIn(0.0, 1.0)
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return if (cl <= 0.0031308) (cl * 12.92).toFloat()
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else (1.055 * cl.pow(1.0 / 2.4) - 0.055).toFloat()
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}
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data class Lab(val l: Float, val a: Float, val b: Float)
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/** Converts standard Compose Color (sRGB or P3) to Oklab space coordinates */
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fun Color.toOklab(): Lab {
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// Convert to linear RGB
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val lr = toLinear(red)
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val lg = toLinear(green)
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val lb = toLinear(blue)
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val lr = toLinear(red.toDouble())
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val lg = toLinear(green.toDouble())
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val lb = toLinear(blue.toDouble())
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// Linear RGB to LMS
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val l = 0.4122214708f * lr + 0.5363325363f * lg + 0.0514459929f * lb
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val m = 0.1167116119f * lr + 0.6858151024f * lg + 0.1974732856f * lb
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val s = 0.0883315616f * lr + 0.1117281988f * lg + 0.7999602417f * lb
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val l = 0.4122214708 * lr + 0.5363325363 * lg + 0.0514459929 * lb
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val m = 0.2119034982 * lr + 0.6806995451 * lg + 0.1073969566 * lb
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val s = 0.0883024619 * lr + 0.2817188376 * lg + 0.6299787005 * lb
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// LMS to non-linear scaled LMS
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val l_ = Math.cbrt(l.toDouble()).toFloat()
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val m_ = Math.cbrt(m.toDouble()).toFloat()
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val s_ = Math.cbrt(s.toDouble()).toFloat()
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val lc = cbrt(l)
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val mc = cbrt(m)
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val sc = cbrt(s)
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// Scaled LMS to Oklab (L, a, b)
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val oklabL = 0.2104542553f * l_ + 0.7936177850f * m_ - 0.0040720468f * s_
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val oklabA = 1.9779984951f * l_ - 2.4285922050f * m_ + 0.4505937099f * s_
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val oklabB = 0.0259040371f * l_ + 0.7827717662f * m_ - 0.8086757660f * s_
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return Lab(oklabL, oklabA, oklabB)
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return Lab(
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l = (0.2104542553 * lc + 0.7936177850 * mc - 0.0040720468 * sc).toFloat(),
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a = (1.9779984951 * lc - 2.4285922050 * mc + 0.4505937099 * sc).toFloat(),
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b = (0.0259040371 * lc + 0.7827717662 * mc - 0.8086757660 * sc).toFloat(),
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)
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}
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/** Converts Oklab coordinates back to standard Compose Color in DisplayP3 or sRGB space */
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fun Lab.toColor(targetColorSpace: androidx.compose.ui.graphics.colorspace.ColorSpace = ColorSpaces.Srgb): Color {
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// Oklab to scaled LMS
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val l_ = l + 0.3963377774f * a + 0.2158037573f * b
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val m_ = l - 0.1055613458f * a - 0.0638541728f * b
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val s_ = l - 0.0894841775f * a - 1.2914855480f * b
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fun Lab.toColor(): Color {
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val ld = l.toDouble()
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val ad = a.toDouble()
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val bd = b.toDouble()
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// Scaled LMS to LMS (cube)
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val l = l_ * l_ * l_
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val m = m_ * m_ * m_
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val s = s_ * s_ * s_
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val lp = ld + 0.3963377774 * ad + 0.2158037573 * bd
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val mp = ld - 0.1055613458 * ad - 0.0638541728 * bd
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val sp = ld - 0.0894841775 * ad - 1.2914855480 * bd
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// LMS to linear RGB
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val lr = 4.0767416621f * l - 3.3077115913f * m + 0.2309699292f * s
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val lg = -1.2684380046f * l + 2.6097574011f * m - 0.3413193965f * s
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val lb = -0.0041960863f * l - 0.7034186147f * m + 1.7076147010f * s
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val lc = lp * lp * lp
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val mc = mp * mp * mp
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val sc = sp * sp * sp
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// Linear RGB to non-linear standard sRGB
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val r = toSRGB(lr)
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val g = toSRGB(lg)
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val b = toSRGB(lb)
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val r = +4.0767416621 * lc - 3.3077115913 * mc + 0.2309699292 * sc
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val g = -1.2684380046 * lc + 2.6097574011 * mc - 0.3413193965 * sc
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val b = -0.0041960863 * lc - 0.7034186147 * mc + 1.7076147010 * sc
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return Color(red = r, green = g, blue = b, alpha = 1.0f, colorSpace = targetColorSpace)
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return Color(red = fromLinear(r), green = fromLinear(g), blue = fromLinear(b), alpha = 1.0f)
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}
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fun Lab.toColor(targetColorSpace: androidx.compose.ui.graphics.colorspace.ColorSpace): Color {
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val base = toColor()
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return Color(red = base.red, green = base.green, blue = base.blue, alpha = 1.0f,
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colorSpace = targetColorSpace)
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}
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/** Linearly interpolates two colors inside the perzeptive Oklab color space */
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fun interpolate(from: Color, to: Color, fraction: Float): Color {
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val oklabFrom = from.toOklab()
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val oklabTo = to.toOklab()
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val f = from.toOklab()
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val t = to.toOklab()
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val interpolatedL = oklabFrom.l + (oklabTo.l - oklabFrom.l) * fraction
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val interpolatedA = oklabFrom.a + (oklabTo.a - oklabFrom.a) * fraction
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val interpolatedB = oklabFrom.b + (oklabTo.b - oklabFrom.b) * fraction
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val lab = Lab(
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l = f.l + (t.l - f.l) * fraction,
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a = f.a + (t.a - f.a) * fraction,
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b = f.b + (t.b - f.b) * fraction,
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)
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val interpolatedAlpha = from.alpha + (to.alpha - from.alpha) * fraction
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// Match the target color space (prefer DisplayP3 if either color is P3)
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val targetSpace = if (from.colorSpace == ColorSpaces.DisplayP3 || to.colorSpace == ColorSpaces.DisplayP3) {
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ColorSpaces.DisplayP3
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} else {
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ColorSpaces.Srgb
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}
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return Lab(interpolatedL, interpolatedA, interpolatedB).toColor(targetSpace).copy(alpha = interpolatedAlpha)
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val useP3 = from.colorSpace == ColorSpaces.DisplayP3 || to.colorSpace == ColorSpaces.DisplayP3
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val color = if (useP3) lab.toColor(ColorSpaces.DisplayP3) else lab.toColor()
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return color.copy(alpha = from.alpha + (to.alpha - from.alpha) * fraction)
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}
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/**
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* Generates a list of interpolated Color stops in Oklab space.
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*
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* Perfect for constructing smooth Sweep, Radial, or Linear gradients that feel
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* incredibly rich and high-fidelity on high-end device screens (S25 Ultra, Pixel 10).
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*/
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fun generateGradientStops(from: Color, to: Color, steps: Int = 12): List<Color> {
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val list = ArrayList<Color>(steps)
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for (i in 0 until steps) {
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val fraction = i.toFloat() / (steps - 1).toFloat()
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list.add(interpolate(from, to, fraction))
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return List(steps) { i ->
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interpolate(from, to, i.toFloat() / (steps - 1).toFloat())
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}
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return list
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}
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}
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