Add an HSVA color implementation

internal/color.go

@@ -0,0 +1,106 @@
+package internal
+
+type HSVA struct {
+	H float64
+	S float64
+	V float64
+	A uint8
+}
+
+func (hsva HSVA) 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)
+	}
+
+	ca := uint32(hsva.A) << 8
+	s := clamp01(hsva.S)
+	v := clamp01(hsva.V)
+	if s == 0 {
+		light := component(v)
+		return light, light, light, ca
+	}
+	
+	h := clamp01(hsva.H) * 360
+	sector := int(h / 60)
+	offset := (h / 60) - float64(sector)
+	
+	fac := float64(hsva.A) / 255
+	p  := component(fac * v * (1 - s))
+	q  := component(fac * v * (1 - s * offset))
+	t  := component(fac * v * (1 - s * (1 - offset)))
+	va := component(v)
+	
+	switch sector {
+	case 0:  return va, t,  p,  ca
+	case 1:  return q,  va, p,  ca
+	case 2:  return p,  va, t,  ca
+	case 3:  return p,  q,  va, ca
+	case 4:  return t,  p,  va, ca
+	default: return va, p,  q,  ca
+	}
+}
+
+// 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 RGBAToHSVA (r, g, b, a uint32) HSVA {
+	// Adapted from:
+	// https://www.cs.rit.edu/~ncs/color/t_convert.html
+	
+	// FIXME: this does not always work!
+
+	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 }
+	
+	hsva := HSVA {
+		V: maxComponent,
+		A: uint8(a >> 8),
+	}
+	
+	delta := maxComponent - minComponent
+	if maxComponent == 0 {
+		// hsva.S is undefined, so hue doesn't matter
+		return hsva
+	}
+	hsva.S = delta / maxComponent
+	
+	switch {
+	case cr == maxComponent: hsva.H =     (cg - cb) / delta
+	case cg == maxComponent: hsva.H = 2 + (cb - cr) / delta
+	case cb == maxComponent: hsva.H = 4 + (cr - cg) / delta
+	}
+	
+	hsva.H *= 60
+	if hsva.H < 0 { hsva.H += 360 }
+	hsva.H /= 360
+	
+	return hsva
+}