objects/internal/color.go

package internal

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
}

// FormatNRGBA formats an NRGBA value into a hex string.
func FormatNRGBA (nrgba color.NRGBA) string {
	return fmt.Sprintf("%02X%02X%02X%02X", nrgba.R, nrgba.G, nrgba.B, nrgba.A)
}

// ParseNRGBA parses an NRGBA value from a hex string. It can be of the format:
//   - RGB
//   - RGBA
//   - RRGGBB
//   - RRGGBBAA
// If none of these are specified, this function will return an opaque black
// color. Hex digits may either be upper case or lower case.
func ParseNRGBA (str string) color.NRGBA {
	runes := []rune(str)
	c := color.NRGBA { A: 255 }
	switch len(runes) {
	case 3:
		c.R = fillOctet(hexDigit(runes[0]))
		c.G = fillOctet(hexDigit(runes[1]))
		c.B = fillOctet(hexDigit(runes[2]))
	case 4:
		c.R = fillOctet(hexDigit(runes[0]))
		c.G = fillOctet(hexDigit(runes[1]))
		c.B = fillOctet(hexDigit(runes[2]))
		c.A = fillOctet(hexDigit(runes[3]))
	case 6:
		c.R = hexOctet(runes[0], runes[1])
		c.G = hexOctet(runes[2], runes[3])
		c.B = hexOctet(runes[4], runes[5])
	case 8:
		c.R = hexOctet(runes[0], runes[1])
		c.G = hexOctet(runes[2], runes[3])
		c.B = hexOctet(runes[4], runes[5])
		c.A = hexOctet(runes[6], runes[7])
	}
	return c
}

func hexDigit (r rune) uint8 {
	switch {
	case r >= '0' && r <= '9': return uint8(r - '0')
	case r >= 'A' && r <= 'F': return uint8(r - 'A') + 10
	case r >= 'a' && r <= 'f': return uint8(r - 'a') + 10
	default: return 0
	}
}

func fillOctet (low uint8) uint8 {
	return low << 4 | low
}

func hexOctet (high, low rune) uint8 {
	return hexDigit(high) << 4 | hexDigit(low)
}