// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package color implements a basic color library.
package color
// Color can convert itself to alpha-premultiplied 16-bits per channel RGBA.
// The conversion may be lossy.
type Color interface {
// RGBA returns the alpha-premultiplied red, green, blue and alpha values
// for the color. Each value ranges within [0, 0xffff], but is represented
// by a uint32 so that multiplying by a blend factor up to 0xffff will not
// overflow.
//
// An alpha-premultiplied color component c has been scaled by alpha (a),
// so has valid values 0 <= c <= a.
RGBA() (r, g, b, a uint32)
}
// RGBA represents a traditional 32-bit alpha-premultiplied color, having 8
// bits for each of red, green, blue and alpha.
//
// An alpha-premultiplied color component C has been scaled by alpha (A), so
// has valid values 0 <= C <= A.
type RGBA struct {
R, G, B, A uint8
}
func (c RGBA) RGBA() (r, g, b, a uint32) {
r = uint32(c.R)
r |= r << 8
g = uint32(c.G)
g |= g << 8
b = uint32(c.B)
b |= b << 8
a = uint32(c.A)
a |= a << 8
return
}
// RGBA64 represents a 64-bit alpha-premultiplied color, having 16 bits for
// each of red, green, blue and alpha.
//
// An alpha-premultiplied color component C has been scaled by alpha (A), so
// has valid values 0 <= C <= A.
type RGBA64 struct {
R, G, B, A uint16
}
func (c RGBA64) RGBA() (r, g, b, a uint32) {
return uint32(c.R), uint32(c.G), uint32(c.B), uint32(c.A)
}
// NRGBA represents a non-alpha-premultiplied 32-bit color.
type NRGBA struct {
R, G, B, A uint8
}
func (c NRGBA) RGBA() (r, g, b, a uint32) {
r = uint32(c.R)
r |= r << 8
r *= uint32(c.A)
r /= 0xff
g = uint32(c.G)
g |= g << 8
g *= uint32(c.A)
g /= 0xff
b = uint32(c.B)
b |= b << 8
b *= uint32(c.A)
b /= 0xff
a = uint32(c.A)
a |= a << 8
return
}
// NRGBA64 represents a non-alpha-premultiplied 64-bit color,
// having 16 bits for each of red, green, blue and alpha.
type NRGBA64 struct {
R, G, B, A uint16
}
func (c NRGBA64) RGBA() (r, g, b, a uint32) {
r = uint32(c.R)
r *= uint32(c.A)
r /= 0xffff
g = uint32(c.G)
g *= uint32(c.A)
g /= 0xffff
b = uint32(c.B)
b *= uint32(c.A)
b /= 0xffff
a = uint32(c.A)
return
}
// Alpha represents an 8-bit alpha color.
type Alpha struct {
A uint8
}
func (c Alpha) RGBA() (r, g, b, a uint32) {
a = uint32(c.A)
a |= a << 8
return a, a, a, a
}
// Alpha16 represents a 16-bit alpha color.
type Alpha16 struct {
A uint16
}
func (c Alpha16) RGBA() (r, g, b, a uint32) {
a = uint32(c.A)
return a, a, a, a
}
// Gray represents an 8-bit grayscale color.
type Gray struct {
Y uint8
}
func (c Gray) RGBA() (r, g, b, a uint32) {
y := uint32(c.Y)
y |= y << 8
return y, y, y, 0xffff
}
// Gray16 represents a 16-bit grayscale color.
type Gray16 struct {
Y uint16
}
func (c Gray16) RGBA() (r, g, b, a uint32) {
y := uint32(c.Y)
return y, y, y, 0xffff
}
// Model can convert any Color to one from its own color model. The conversion
// may be lossy.
type Model interface {
Convert(c Color) Color
}
// ModelFunc returns a Model that invokes f to implement the conversion.
func ModelFunc(f func(Color) Color) Model {
// Note: using *modelFunc as the implementation
// means that callers can still use comparisons
// like m == RGBAModel. This is not possible if
// we use the func value directly, because funcs
// are no longer comparable.
return &modelFunc{f}
}
type modelFunc struct {
f func(Color) Color
}
func (m *modelFunc) Convert(c Color) Color {
return m.f(c)
}
// Models for the standard color types.
var (
RGBAModel Model = ModelFunc(rgbaModel)
RGBA64Model Model = ModelFunc(rgba64Model)
NRGBAModel Model = ModelFunc(nrgbaModel)
NRGBA64Model Model = ModelFunc(nrgba64Model)
AlphaModel Model = ModelFunc(alphaModel)
Alpha16Model Model = ModelFunc(alpha16Model)
GrayModel Model = ModelFunc(grayModel)
Gray16Model Model = ModelFunc(gray16Model)
)
func rgbaModel(c Color) Color {
if _, ok := c.(RGBA); ok {
return c
}
r, g, b, a := c.RGBA()
return RGBA{uint8(r >> 8), uint8(g >> 8), uint8(b >> 8), uint8(a >> 8)}
}
func rgba64Model(c Color) Color {
if _, ok := c.(RGBA64); ok {
return c
}
r, g, b, a := c.RGBA()
return RGBA64{uint16(r), uint16(g), uint16(b), uint16(a)}
}
func nrgbaModel(c Color) Color {
if _, ok := c.(NRGBA); ok {
return c
}
r, g, b, a := c.RGBA()
if a == 0xffff {
return NRGBA{uint8(r >> 8), uint8(g >> 8), uint8(b >> 8), 0xff}
}
if a == 0 {
return NRGBA{0, 0, 0, 0}
}
// Since Color.RGBA returns an alpha-premultiplied color, we should have r <= a && g <= a && b <= a.
r = (r * 0xffff) / a
g = (g * 0xffff) / a
b = (b * 0xffff) / a
return NRGBA{uint8(r >> 8), uint8(g >> 8), uint8(b >> 8), uint8(a >> 8)}
}
func nrgba64Model(c Color) Color {
if _, ok := c.(NRGBA64); ok {
return c
}
r, g, b, a := c.RGBA()
if a == 0xffff {
return NRGBA64{uint16(r), uint16(g), uint16(b), 0xffff}
}
if a == 0 {
return NRGBA64{0, 0, 0, 0}
}
// Since Color.RGBA returns an alpha-premultiplied color, we should have r <= a && g <= a && b <= a.
r = (r * 0xffff) / a
g = (g * 0xffff) / a
b = (b * 0xffff) / a
return NRGBA64{uint16(r), uint16(g), uint16(b), uint16(a)}
}
func alphaModel(c Color) Color {
if _, ok := c.(Alpha); ok {
return c
}
_, _, _, a := c.RGBA()
return Alpha{uint8(a >> 8)}
}
func alpha16Model(c Color) Color {
if _, ok := c.(Alpha16); ok {
return c
}
_, _, _, a := c.RGBA()
return Alpha16{uint16(a)}
}
func grayModel(c Color) Color {
if _, ok := c.(Gray); ok {
return c
}
r, g, b, _ := c.RGBA()
// These coefficients (the fractions 0.299, 0.587 and 0.114) are the same
// as those given by the JFIF specification and used by func RGBToYCbCr in
// ycbcr.go.
//
// Note that 19595 + 38470 + 7471 equals 65536.
//
// The 24 is 16 + 8. The 16 is the same as used in RGBToYCbCr. The 8 is
// because the return value is 8 bit color, not 16 bit color.
y := (19595*r + 38470*g + 7471*b + 1<<15) >> 24
return Gray{uint8(y)}
}
func gray16Model(c Color) Color {
if _, ok := c.(Gray16); ok {
return c
}
r, g, b, _ := c.RGBA()
// These coefficients (the fractions 0.299, 0.587 and 0.114) are the same
// as those given by the JFIF specification and used by func RGBToYCbCr in
// ycbcr.go.
//
// Note that 19595 + 38470 + 7471 equals 65536.
y := (19595*r + 38470*g + 7471*b + 1<<15) >> 16
return Gray16{uint16(y)}
}
// Palette is a palette of colors.
type Palette []Color
// Convert returns the palette color closest to c in Euclidean R,G,B space.
func (p Palette) Convert(c Color) Color {
if len(p) == 0 {
return nil
}
return p[p.Index(c)]
}
// Index returns the index of the palette color closest to c in Euclidean
// R,G,B,A space.
func (p Palette) Index(c Color) int {
// A batch version of this computation is in image/draw/draw.go.
cr, cg, cb, ca := c.RGBA()
ret, bestSum := 0, uint32(1<<32-1)
for i, v := range p {
vr, vg, vb, va := v.RGBA()
sum := sqDiff(cr, vr) + sqDiff(cg, vg) + sqDiff(cb, vb) + sqDiff(ca, va)
if sum < bestSum {
if sum == 0 {
return i
}
ret, bestSum = i, sum
}
}
return ret
}
// sqDiff returns the squared-difference of x and y, shifted by 2 so that
// adding four of those won't overflow a uint32.
//
// x and y are both assumed to be in the range [0, 0xffff].
func sqDiff(x, y uint32) uint32 {
// The canonical code of this function looks as follows:
//
// var d uint32
// if x > y {
// d = x - y
// } else {
// d = y - x
// }
// return (d * d) >> 2
//
// Language spec guarantees the following properties of unsigned integer
// values operations with respect to overflow/wrap around:
//
// > For unsigned integer values, the operations +, -, *, and << are
// > computed modulo 2n, where n is the bit width of the unsigned
// > integer's type. Loosely speaking, these unsigned integer operations
// > discard high bits upon overflow, and programs may rely on ``wrap
// > around''.
//
// Considering these properties and the fact that this function is
// called in the hot paths (x,y loops), it is reduced to the below code
// which is slightly faster. See TestSqDiff for correctness check.
d := x - y
return (d * d) >> 2
}
// Standard colors.
var (
Black = Gray16{0}
White = Gray16{0xffff}
Transparent = Alpha16{0}
Opaque = Alpha16{0xffff}
)
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