color/lgraya/lgraya.go

// Provides a [color.Color] type for dealing with premultiplied linear grayscale+alpha colours.
package lgraya

import (
	"image/color"
	"math"

	"smariot.com/color/internal/helper"
)

// Color is a pre-multiplied linear grayscale+alpha [color.Color].
type Color struct {
	Y, 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 {
	dY := a.Y - b.Y
	dA := a.A - b.A
	return max(sqr(dY), sqr(dY+dA))
}

// Distance returns the maximum possible euclidean distance between two colours,
// accounting for the possible backgrounds they might be composited over.
func Distance(a, b Color) float64 {
	dY := a.Y - b.Y
	dA := a.A - b.A
	return max(math.Abs(dY), math.Abs(dY+dA))
}

// YA converts to premultiplied sRGB grayscale+alpha.
func (c Color) YA() (y, a uint32) {
	y, a = c.NYA()
	y = y * a / 0xffff
	return
}

// NYA converts to non-premultiplied sRGB grayscale+alpha.
func (c Color) NYA() (y, a uint32) {
	_y, _a := c.NLYA()
	return helper.Delinearize(_y), uint32(_a*0xffff + 0.5)
}

// NLYA converts to non-premultiplied linear grayscale+alpha.
func (c Color) NLYA() (y, a float64) {
	if c.A <= 0 {
		return 0, 0
	}

	return c.Y / c.A, c.A
}

// RGBA converts to premultiplied RGBA, implementing the [color.Color] interface.
func (c Color) RGBA() (r, g, b, a uint32) {
	y, a := c.YA()
	return y, y, y, a
}

// NRGBA converts to non-premultiplied RGBA.
func (c Color) NRGBA() (r, g, b, a uint32) {
	y, a := c.NYA()
	return y, y, y, a
}

// NLRGBA converts to non-premultiplied linear RGBA.
func (c Color) NLRGBA() (r, g, b, a float64) {
	y, a := c.NLYA()
	return y, y, y, 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.Y/c.A, c.Y/c.A, c.Y/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.Y/c.A, c.Y/c.A, c.Y/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)

	// the color.Gray16Model documents that it uses the coefficients 0.299, 0.5867, and 0.114.
	// however, it does this using integer arithmetic, so the actual coefficients are effectively rounded to the nearest 1/0x10000.

	return Color{
		Y: helper.DelinearizeF(
			helper.LinearizeF(r)*0x0.4c8bp0+
				helper.LinearizeF(g)*0x0.9646p0+
				helper.LinearizeF(b)*0x0.1d2fp0,
		) * a,
		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{}