Add HSV, HSVA

image/color/hsv.go

@@ -0,0 +1,177 @@
+package color
+
+import "fmt"
+import "image/color"
+
+// HSV represents a color with hue, saturation, and value components. Each
+// component C is in range 0 <= C <= 1.
+type HSV struct {
+	H float64
+	S float64
+	V float64
+}
+
+// HSVA is an HSV color with an added 8-bit alpha component. The alpha component
+// ranges from 0x0000 (fully transparent) to 0xFFFF (opaque), and has no bearing
+// on the other components.
+type HSVA struct {
+	H float64
+	S float64
+	V float64
+	A uint16
+}
+
+var (
+	HSVModel  color.Model = color.ModelFunc(hsvModel)
+	HSVAModel color.Model = color.ModelFunc(hsvaModel)
+)
+
+func (hsv HSV) RGBA () (r, g, b, a uint32) {
+	// Adapted from:
+	// https://www.cs.rit.edu/~ncs/color/t_convert.html
+	
+	component := func (x float64) uint32 {
+		return uint32(float64(0xFFFF) * x)
+	}
+
+	s := clamp01(hsv.S)
+	v := clamp01(hsv.V)
+	if s == 0 {
+		light := component(v)
+		return light, light, light, 0xFFFF
+	}
+	
+	h := clamp01(hsv.H) * 360
+	sector := int(h / 60)
+	// otherwise when given 1.0 for H, sector would overflow to 6
+	if sector > 5 { sector = 5 }
+	offset := (h / 60) - float64(sector)
+	
+	p  := component(v * (1 - s))
+	q  := component(v * (1 - s * offset))
+	t  := component(v * (1 - s * (1 - offset)))
+	va := component(v)
+	
+	switch sector {
+	case 0:  return va, t,  p,  0xFFFF
+	case 1:  return q,  va, p,  0xFFFF
+	case 2:  return p,  va, t,  0xFFFF
+	case 3:  return p,  q,  va, 0xFFFF
+	case 4:  return t,  p,  va, 0xFFFF
+	default: return va, p,  q,  0xFFFF
+	}
+}
+
+func (hsva HSVA) RGBA () (r, g, b, a uint32) {
+	r, g, b, a = HSV {
+		H: hsva.H,
+		S: hsva.S,
+		V: hsva.V,
+	}.RGBA()
+	a = uint32(hsva.A)
+	// alpha premultiplication
+	r = (r * a) / 0xFFFF
+	g = (g * a) / 0xFFFF
+	b = (b * a) / 0xFFFF
+	return
+}
+
+// Canon returns the color but with the H, S, and V fields are constrained to
+// the range 0.0-1.0
+func (hsv HSV) Canon () HSV {
+	hsv.H = clamp01(hsv.H)
+	hsv.S = clamp01(hsv.S)
+	hsv.V = clamp01(hsv.V)
+	return hsv
+}
+
+// Canon returns the color but with the H, S, and V fields are constrained to
+// the range 0.0-1.0
+func (hsva HSVA) Canon () HSVA {
+	hsva.H = clamp01(hsva.H)
+	hsva.S = clamp01(hsva.S)
+	hsva.V = clamp01(hsva.V)
+	return hsva
+}
+
+func clamp01 (x float64) float64 {
+	if x > 1.0 { return 1.0 }
+	if x < 0.0 { return 0.0 }
+	return x
+}
+
+func hsvModel (c color.Color) color.Color {
+	switch c := c.(type) {
+	case HSV:  return c
+	case HSVA: return HSV { H: c.H, S: c.S, V: c.V }
+	default:
+		r, g, b, a := c.RGBA()
+		// alpha unpremultiplication
+		r = (r / a) * 0xFFFF
+		g = (g / a) * 0xFFFF
+		b = (b / a) * 0xFFFF
+		return rgbToHSV(r, g, b)
+	}
+}
+
+func hsvaModel (c color.Color) color.Color {
+	switch c := c.(type) {
+	case HSV:  return HSVA { H: c.H, S: c.S, V: c.V, A: 0xFFFF }
+	case HSVA: return c
+	default:
+		r, g, b, a := c.RGBA()
+		hsv := rgbToHSV(r, g, b)
+
+		return HSVA {
+			H: hsv.H,
+			S: hsv.S,
+			V: hsv.V,
+			A: uint16(a),
+		}
+	}
+}
+
+func rgbToHSV (r, g, b uint32) HSV {
+	// Adapted from:
+	// https://www.cs.rit.edu/~ncs/color/t_convert.html
+	
+	component := func (x uint32) float64 {
+		return clamp01(float64(x) / 0xFFFF)
+	}
+	cr := component(r)
+	cg := component(g)
+	cb := component(b)
+	
+	var maxComponent float64
+	if cr > maxComponent { maxComponent = cr }
+	if cg > maxComponent { maxComponent = cg }
+	if cb > maxComponent { maxComponent = cb }
+	var minComponent = 1.0
+	if cr < minComponent { minComponent = cr }
+	if cg < minComponent { minComponent = cg }
+	if cb < minComponent { minComponent = cb }
+	
+	hsv := HSV {
+		V: maxComponent,
+	}
+	
+	delta := maxComponent - minComponent
+	if delta == 0 {
+		// hsva.S is undefined, so hue doesn't matter
+		return hsv
+	}
+	hsv.S = delta / maxComponent
+	
+	switch {
+	case cr == maxComponent: hsv.H =     (cg - cb) / delta
+	case cg == maxComponent: hsv.H = 2 + (cb - cr) / delta
+	case cb == maxComponent: hsv.H = 4 + (cr - cg) / delta
+	}
+	
+	hsv.H *= 60
+	if hsv.H < 0 { hsv.H += 360 }
+	hsv.H /= 360
+	
+	return hsv
+}
+