pigo-wasm-demos/draw/gradient.go

package draw

import (
	"image/color"
	"math"
	"sort"
)

type Gradient interface {
	AddColorStop(offset float64, color color.Color)
	ColorAt(x, y int) color.Color
}

type stop struct {
	pos   float64
	color color.Color
}

type stops []stop

// Len satisfies the Sort interface.
func (s stops) Len() int           { return len(s) }
func (s stops) Less(i, j int) bool { return s[i].pos < s[j].pos }
func (s stops) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

type circle struct {
	x, y, r float64
}

type radialGradient struct {
	c0, c1, cd circle
	a, inva    float64
	mindr      float64
	stops      stops
}

func dot3(x0, y0, z0, x1, y1, z1 float64) float64 {
	return x0*x1 + y0*y1 + z0*z1
}

func (g *radialGradient) ColorAt(x, y int) color.Color {
	if len(g.stops) == 0 {
		return color.Transparent
	}

	// copy from pixman's pixman-radial-gradient.c
	dx, dy := float64(x)+0.5-g.c0.x, float64(y)+0.5-g.c0.y
	b := dot3(dx, dy, g.c0.r, g.cd.x, g.cd.y, g.cd.r)
	c := dot3(dx, dy, -g.c0.r, dx, dy, g.c0.r)

	if g.a == 0 {
		if b == 0 {
			return color.Transparent
		}
		t := 0.5 * c / b
		if t*g.cd.r >= g.mindr {
			return getColor(t, g.stops)
		}
		return color.Transparent
	}

	discr := dot3(b, g.a, 0, b, -c, 0)
	if discr >= 0 {
		sqrtdiscr := math.Sqrt(discr)
		t0 := (b + sqrtdiscr) * g.inva
		t1 := (b - sqrtdiscr) * g.inva

		if t0*g.cd.r >= g.mindr {
			return getColor(t0, g.stops)
		} else if t1*g.cd.r >= g.mindr {
			return getColor(t1, g.stops)
		}
	}

	return color.Transparent
}

func (g *radialGradient) AddColorStop(offset float64, color color.Color) {
	g.stops = append(g.stops, stop{pos: offset, color: color})
	sort.Sort(g.stops)
}

func NewRadialGradient(x0, y0, r0, x1, y1, r1 float64) Gradient {
	c0 := circle{x0, y0, r0}
	c1 := circle{x1, y1, r1}
	cd := circle{x1 - x0, y1 - y0, r1 - r0}
	a := dot3(cd.x, cd.y, -cd.r, cd.x, cd.y, cd.r)
	var inva float64
	if a != 0 {
		inva = 1.0 / a
	}
	mindr := -c0.r
	g := &radialGradient{
		c0:    c0,
		c1:    c1,
		cd:    cd,
		a:     a,
		inva:  inva,
		mindr: mindr,
	}
	return g
}

func getColor(pos float64, stops stops) color.Color {
	if pos <= 0.0 || len(stops) == 1 {
		return stops[0].color
	}

	last := stops[len(stops)-1]

	if pos >= last.pos {
		return last.color
	}

	for i, stop := range stops[1:] {
		if pos < stop.pos {
			pos = (pos - stops[i].pos) / (stop.pos - stops[i].pos)
			return colorLerp(stops[i].color, stop.color, pos)
		}
	}

	return last.color
}

func colorLerp(c0, c1 color.Color, t float64) color.Color {
	r0, g0, b0, a0 := c0.RGBA()
	r1, g1, b1, a1 := c1.RGBA()

	return color.RGBA{
		lerp(r0, r1, t),
		lerp(g0, g1, t),
		lerp(b0, b1, t),
		lerp(a0, a1, t),
	}
}

func lerp(a, b uint32, t float64) uint8 {
	return uint8(int32(float64(a)*(1.0-t)+float64(b)*t) >> 8)
}