Separate HSVA color into HSV, HSVA, fix alpha premultiplication
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@ -1,13 +1,31 @@
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package internal
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import "image/color"
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// HSV represents a color with hue, saturation, and value components. Each
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// component C is in range 0 <= C <= 1.
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type HSV struct {
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H float64
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S float64
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V float64
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}
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// HSVA is an HSV color with an added 8-bit alpha component. The alpha component
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// ranges from 0x0000 (fully transparent) to 0xFFFF (opaque), and has no bearing
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// on the other components.
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type HSVA struct {
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H float64
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S float64
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V float64
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A uint8
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A uint16
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}
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func (hsva HSVA) RGBA () (r, g, b, a uint32) {
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var (
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HSVModel color.Model = color.ModelFunc(hsvModel)
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HSVAModel color.Model = color.ModelFunc(hsvaModel)
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)
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func (hsv HSV) RGBA () (r, g, b, a uint32) {
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// Adapted from:
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// https://www.cs.rit.edu/~ncs/color/t_convert.html
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@ -15,34 +33,55 @@ func (hsva HSVA) RGBA () (r, g, b, a uint32) {
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return uint32(float64(0xFFFF) * x)
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}
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ca := uint32(hsva.A) << 8
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s := clamp01(hsva.S)
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v := clamp01(hsva.V)
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s := clamp01(hsv.S)
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v := clamp01(hsv.V)
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if s == 0 {
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light := component(v)
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return light, light, light, ca
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return light, light, light, 0xFFFF
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}
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h := clamp01(hsva.H) * 360
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h := clamp01(hsv.H) * 360
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sector := int(h / 60)
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offset := (h / 60) - float64(sector)
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fac := float64(hsva.A) / 255
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p := component(fac * v * (1 - s))
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q := component(fac * v * (1 - s * offset))
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t := component(fac * v * (1 - s * (1 - offset)))
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p := component(v * (1 - s))
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q := component(v * (1 - s * offset))
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t := component(v * (1 - s * (1 - offset)))
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va := component(v)
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switch sector {
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case 0: return va, t, p, ca
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case 1: return q, va, p, ca
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case 2: return p, va, t, ca
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case 3: return p, q, va, ca
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case 4: return t, p, va, ca
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default: return va, p, q, ca
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case 0: return va, t, p, 0xFFFF
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case 1: return q, va, p, 0xFFFF
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case 2: return p, va, t, 0xFFFF
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case 3: return p, q, va, 0xFFFF
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case 4: return t, p, va, 0xFFFF
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default: return va, p, q, 0xFFFF
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}
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}
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func (hsva HSVA) RGBA () (r, g, b, a uint32) {
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r, g, b, a = HSV {
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H: hsva.H,
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S: hsva.S,
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V: hsva.V,
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}.RGBA()
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a = uint32(hsva.A)
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// alpha premultiplication
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r = (r * a) / 0xFFFF
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g = (g * a) / 0xFFFF
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b = (b * a) / 0xFFFF
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return
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}
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// Canon returns the color but with the H, S, and V fields are constrained to
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// the range 0.0-1.0
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func (hsv HSV) Canon () HSV {
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hsv.H = clamp01(hsv.H)
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hsv.S = clamp01(hsv.S)
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hsv.V = clamp01(hsv.V)
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return hsv
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}
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// Canon returns the color but with the H, S, and V fields are constrained to
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// the range 0.0-1.0
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func (hsva HSVA) Canon () HSVA {
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@ -58,7 +97,38 @@ func clamp01 (x float64) float64 {
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return x
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}
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func RGBAToHSVA (r, g, b, a uint32) HSVA {
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func hsvModel (c color.Color) color.Color {
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switch c := c.(type) {
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case HSV: return c
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case HSVA: return HSV { H: c.H, S: c.S, V: c.V }
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default:
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r, g, b, a := c.RGBA()
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// alpha unpremultiplication
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r = (r / a) * 0xFFFF
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g = (g / a) * 0xFFFF
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b = (b / a) * 0xFFFF
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return rgbToHSV(r, g, b)
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}
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}
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func hsvaModel (c color.Color) color.Color {
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switch c := c.(type) {
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case HSV: return HSVA { H: c.H, S: c.S, V: c.V, A: 0xFFFF }
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case HSVA: return c
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default:
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r, g, b, a := c.RGBA()
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hsv := rgbToHSV(r, g, b)
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return HSVA {
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H: hsv.H,
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S: hsv.S,
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V: hsv.V,
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A: uint16(a),
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}
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}
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}
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func rgbToHSV (r, g, b uint32) HSV {
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// Adapted from:
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// https://www.cs.rit.edu/~ncs/color/t_convert.html
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@ -78,27 +148,26 @@ func RGBAToHSVA (r, g, b, a uint32) HSVA {
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if cg < minComponent { minComponent = cg }
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if cb < minComponent { minComponent = cb }
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hsva := HSVA {
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hsv := HSV {
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V: maxComponent,
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A: uint8(a >> 8),
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}
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delta := maxComponent - minComponent
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if delta == 0 {
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// hsva.S is undefined, so hue doesn't matter
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return hsva
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return hsv
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}
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hsva.S = delta / maxComponent
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hsv.S = delta / maxComponent
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switch {
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case cr == maxComponent: hsva.H = (cg - cb) / delta
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case cg == maxComponent: hsva.H = 2 + (cb - cr) / delta
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case cb == maxComponent: hsva.H = 4 + (cr - cg) / delta
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case cr == maxComponent: hsv.H = (cg - cb) / delta
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case cg == maxComponent: hsv.H = 2 + (cb - cr) / delta
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case cb == maxComponent: hsv.H = 4 + (cr - cg) / delta
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}
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hsva.H *= 60
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if hsva.H < 0 { hsva.H += 360 }
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hsva.H /= 360
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hsv.H *= 60
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if hsv.H < 0 { hsv.H += 360 }
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hsv.H /= 360
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return hsva
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return hsv
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}
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