Initial commit.

lrgba/lrgba.go

@@ -0,0 +1,93 @@
+// Provides a [color.Color] type for dealing with premultiplied linear RGBA colours.
+package lrgba
+
+import (
+	"image/color"
+	"math"
+
+	"smariot.com/color/internal/helper"
+)
+
+// Color is a pre-multiplied linear RGBA [color.Color].
+type Color struct {
+	R, G, B, A float64
+}
+
+func sqr(a float64) float64 {
+	return a * a
+}
+
+// DistanceSqr returns the maximum possible euclidean distance squared between two colours,
+// accounting for the possible backgrounds they might be composited over.
+func DistanceSqr(a, b Color) float64 {
+	dR := a.R - b.R
+	dG := a.G - b.G
+	dB := a.B - b.B
+	dA := a.A - b.A
+	return max(sqr(dR), sqr(dR+dA)) + max(sqr(dG), sqr(dG+dA)) + max(sqr(dB), sqr(dB+dA))
+}
+
+// Distance returns the maximum possible euclidean distance between two colours,
+// accounting for the possible backgrounds they might be composited over.
+//
+// If you just want to compare relative distances, use [DistanceSqr] instead.
+func Distance(a, b Color) float64 {
+	return math.Sqrt(DistanceSqr(a, b))
+}
+
+// RGBA converts to premultiplied RGBA, implementing the [color.Color] interface.
+func (c Color) RGBA() (r, g, b, a uint32) {
+	return helper.NLRGBAtoRGBA(c.NLRGBA())
+}
+
+// NRGBA converts to non-premultiplied RGBA.
+func (c Color) NRGBA() (r, g, b, a uint32) {
+	return helper.NLRGBAtoNRGBA(c.NLRGBA())
+}
+
+// NLRGBA converts to non-premultiplied linear RGBA.
+func (c Color) NLRGBA() (r, g, b, a float64) {
+	if c.A <= 0 {
+		return 0, 0, 0, 0
+	}
+
+	return c.R / c.A, c.G / c.A, c.B / c.A, c.A
+}
+
+// NXYZA converts to non-premultiplied XYZ+Alpha.
+func (c Color) NXYZA() (x, y, z, a float64) {
+	if c.A <= 0 {
+		return 0, 0, 0, 0
+	}
+
+	x, y, z = helper.LRGBtoXYZ(c.R/c.A, c.G/c.A, c.B/c.A)
+	return x, y, z, c.A
+}
+
+// NOkLabA converts to non-premultiplied OkLab+Alpha.
+func (c Color) NOkLabA() (lightness, chromaA, chromaB, a float64) {
+	if c.A <= 0 {
+		return 0, 0, 0, 0
+	}
+
+	lightness, chromaA, chromaB = helper.LMStoOkLab(helper.LRGBtoLMS(c.R/c.A, c.G/c.A, c.B/c.A))
+	return lightness, chromaA, chromaB, c.A
+}
+
+// Convert converts an arbitrary colour type to a premultiplied linear RGBA [Color].
+func Convert(c color.Color) Color {
+	if c, ok := c.(Color); ok {
+		return c
+	}
+
+	r, g, b, a := helper.ColorToNLRGBA(c)
+	return Color{r * a, g * a, b * a, a}
+}
+
+// A [color.Model] for converting arbitrary colours to a premultiplied linear RGBA [Color].
+//
+// Wraps the [Convert] function, returning a [color.Color] interface rather than the [Color] type.
+var Model = helper.Model(Convert)
+
+// Type assertion.
+var _ color.Color = Color{}

lrgba/lrgba_test.go

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+package lrgba
+
+import (
+	"math"
+	"testing"
+
+	"smariot.com/color/internal/helper"
+)
+
+func eq(c0, c1 Color) bool {
+	return helper.EqFloat64SliceFuzzy(
+		[]float64{c0.R, c0.G, c0.B, c0.A},
+		[]float64{c1.R, c1.G, c1.B, c1.A},
+	)
+}
+
+func midpoint(c0, c1 Color) Color {
+	return Color{(c0.R + c1.R) / 2, (c0.G + c1.G) / 2, (c0.B + c1.B) / 2, (c0.A + c1.A) / 2}
+}
+
+func TestModel(t *testing.T) {
+	helper.TestModel(t, true, Model, eq, []helper.ConvertTest[Color]{
+		{
+			Name: "passthrough",
+			// These is a very illegal colour. If it makes it through
+			// unchanged, we can be reasonably confident no colour space conversions were
+			// attempted.
+			In:  Color{math.Inf(1), math.Inf(-1), math.NaN(), 0},
+			Out: Color{math.Inf(1), math.Inf(-1), math.NaN(), 0},
+		},
+	})
+}
+
+func TestDistance(t *testing.T) {
+	helper.TestDistance(t, true, midpoint, Distance, Model)
+}