Add cubehelix (#10)

* cubehelix

* Remove warm, cool, & rainbow

Now using cubehelix

* Add files via upload

* Update README.md

README.md

@@ -135,6 +135,9 @@ grad.Colors(15)
 `grad := colorgrad.Rainbow()`
 ![img](img/gradient-rainbow.png)
 
+`grad := colorgrad.CubehelixDefault()`
+![img](img/gradient-cubehelix-default.png)
+
 `grad := colorgrad.Sinebow()`
 ![img](img/gradient-sinebow.png)
 

gradfn.go

@@ -8,6 +8,7 @@ import (
 
 // Algorithms adapted from: https://github.com/d3/d3-scale-chromatic
 
+const deg2rad = math.Pi / 180
 const pi_1_3 = math.Pi / 3
 const pi_2_3 = math.Pi * 2 / 3
 
@@ -95,6 +96,99 @@ func (self cividisGradient) Colors(count uint) []colorful.Color {
 	return colors
 }
 
+// Cubehelix
+
+type cubehelix struct {
+	h, s, l float64
+}
+
+func (c cubehelix) toColorful() colorful.Color {
+	h := (c.h + 120) * deg2rad
+	l := c.l
+	a := c.s * l * (1 - l)
+	cosh := math.Cos(h)
+	sinh := math.Sin(h)
+	r := (l - a*math.Min(0.14861*cosh-1.78277*sinh, 1.0))
+	g := (l - a*math.Min(0.29227*cosh+0.90649*sinh, 1.0))
+	b := l + a*(1.97294*cosh)
+	return colorful.Color{
+		R: clamp01(r),
+		G: clamp01(g),
+		B: clamp01(b),
+	}
+}
+
+func (c cubehelix) interpolate(c2 cubehelix, t float64) cubehelix {
+	return cubehelix{
+		h: c.h + t*(c2.h-c.h),
+		s: c.s + t*(c2.s-c.s),
+		l: c.l + t*(c2.l-c.l),
+	}
+}
+
+type cubehelixGradient struct {
+	start, end cubehelix
+}
+
+func CubehelixDefault() Gradient {
+	return cubehelixGradient{
+		start: cubehelix{300, 0.5, 0.0},
+		end:   cubehelix{-240, 0.5, 1.0},
+	}
+}
+
+func Warm() Gradient {
+	return cubehelixGradient{
+		start: cubehelix{-100, 0.75, 0.35},
+		end:   cubehelix{80, 1.50, 0.8},
+	}
+}
+
+func Cool() Gradient {
+	return cubehelixGradient{
+		start: cubehelix{260, 0.75, 0.35},
+		end:   cubehelix{80, 1.50, 0.8},
+	}
+}
+
+func (self cubehelixGradient) At(t float64) colorful.Color {
+	return self.start.interpolate(self.end, t).toColorful()
+}
+
+func (self cubehelixGradient) Colors(count uint) []colorful.Color {
+	l := float64(count - 1)
+	colors := make([]colorful.Color, count)
+	for i := range colors {
+		colors[i] = self.At(float64(i) / l)
+	}
+	return colors
+}
+
+type rainbow struct{}
+
+func Rainbow() Gradient {
+	return rainbow{}
+}
+
+func (self rainbow) At(t float64) colorful.Color {
+	t = math.Max(0, math.Min(1, t))
+	ts := math.Abs(t - 0.5)
+	return cubehelix{
+		h: 360*t - 100,
+		s: 1.5 - 1.5*ts,
+		l: 0.8 - 0.9*ts,
+	}.toColorful()
+}
+
+func (self rainbow) Colors(count uint) []colorful.Color {
+	l := float64(count - 1)
+	colors := make([]colorful.Color, count)
+	for i := range colors {
+		colors[i] = self.At(float64(i) / l)
+	}
+	return colors
+}
+
 func clamp01(t float64) float64 {
 	return math.Max(0, math.Min(1, t))
 }

gradset.go

@@ -1,29 +1,5 @@
 package colorgrad
 
-func Warm() Gradient {
-	colors := []string{"#6e40aa", "#8a3eb2", "#a83cb3", "#c53dad", "#df40a1", "#f4468f", "#ff507a", "#ff5e63", "#ff704e", "#ff843d", "#f89b31", "#e6b32e", "#d2c934", "#bfde43", "#aff05b"}
-	grad, _ := NewGradient().
-		HexColors(colors...).
-		Build()
-	return grad
-}
-
-func Cool() Gradient {
-	colors := []string{"#6e40aa", "#654ec0", "#585fd2", "#4973dd", "#3988e1", "#2b9ede", "#1fb3d3", "#1ac7c2", "#1bd9ac", "#24e695", "#34f07e", "#4df56a", "#6bf75c", "#8cf457", "#aff05b"}
-	grad, _ := NewGradient().
-		HexColors(colors...).
-		Build()
-	return grad
-}
-
-func Rainbow() Gradient {
-	colors := []string{"#6e40aa", "#a83cb3", "#df40a1", "#ff507a", "#ff704e", "#f89b31", "#d2c934", "#aff05b", "#6bf75c", "#34f07e", "#1bd9ac", "#1fb3d3", "#3988e1", "#585fd2", "#6e40aa"}
-	grad, _ := NewGradient().
-		HexColors(colors...).
-		Build()
-	return grad
-}
-
 func Viridis() Gradient {
 	colors := []string{"#440154", "#482777", "#3f4a8a", "#31678e", "#26838f", "#1f9d8a", "#6cce5a", "#b6de2b", "#fee825"}
 	grad, _ := NewGradient().