all: use color.NRGBA in public API

color.RGBA has two problems with regards to using it.

First the color values need to be premultiplied, whereas most APIs
have non-premultiplied values. This is mainly to preserve color components
with low alpha values.

Second there are two ways to premultiply with sRGB. One is to premultiply
after sRGB conversion, the other is before. This makes using the API more
confusing.

Using color.NRGBA in sRGB makes it align with CSS.e

Signed-off-by: Egon Elbre <egonelbre@gmail.com>

widget/border.go

@@ -15,7 +15,7 @@ import (
 
 // Border lays out a widget and draws a border inside it.
 type Border struct {
-	Color        color.RGBA
+	Color        color.NRGBA
 	CornerRadius unit.Value
 	Width        unit.Value
 }

widget/icon.go

@@ -7,6 +7,7 @@ import (
 	"image/color"
 	"image/draw"
 
+	"gioui.org/internal/f32color"
 	"gioui.org/layout"
 	"gioui.org/op/paint"
 	"gioui.org/unit"
@@ -14,12 +15,12 @@ import (
 )
 
 type Icon struct {
-	Color color.RGBA
+	Color color.NRGBA
 	src   []byte
 	// Cached values.
 	op       paint.ImageOp
 	imgSize  int
-	imgColor color.RGBA
+	imgColor color.NRGBA
 }
 
 // NewIcon returns a new Icon from IconVG data.
@@ -28,7 +29,7 @@ func NewIcon(data []byte) (*Icon, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &Icon{src: data, Color: color.RGBA{A: 0xff}}, nil
+	return &Icon{src: data, Color: color.NRGBA{A: 0xff}}, nil
 }
 
 func (ic *Icon) Layout(gtx layout.Context, sz unit.Value) layout.Dimensions {
@@ -49,7 +50,7 @@ func (ic *Icon) image(sz int) paint.ImageOp {
 	img := image.NewRGBA(image.Rectangle{Max: image.Point{X: sz, Y: int(float32(sz) * dy / dx)}})
 	var ico iconvg.Rasterizer
 	ico.SetDstImage(img, img.Bounds(), draw.Src)
-	m.Palette[0] = ic.Color
+	m.Palette[0] = f32color.NRGBAToRGBA(ic.Color)
 	iconvg.Decode(&ico, ic.src, &iconvg.DecodeOptions{
 		Palette: &m.Palette,
 	})

widget/material/button.go

@@ -22,10 +22,10 @@ import (
 type ButtonStyle struct {
 	Text string
 	// Color is the text color.
-	Color        color.RGBA
+	Color        color.NRGBA
 	Font         text.Font
 	TextSize     unit.Value
-	Background   color.RGBA
+	Background   color.NRGBA
 	CornerRadius unit.Value
 	Inset        layout.Inset
 	Button       *widget.Clickable
@@ -33,15 +33,15 @@ type ButtonStyle struct {
 }
 
 type ButtonLayoutStyle struct {
-	Background   color.RGBA
+	Background   color.NRGBA
 	CornerRadius unit.Value
 	Button       *widget.Clickable
 }
 
 type IconButtonStyle struct {
-	Background color.RGBA
+	Background color.NRGBA
 	// Color is the icon color.
-	Color color.RGBA
+	Color color.NRGBA
 	Icon  *widget.Icon
 	// Size is the icon size.
 	Size   unit.Value
@@ -272,7 +272,7 @@ func drawInk(gtx layout.Context, c widget.Press) {
 	const col = 0.8
 	ba, bc := byte(alpha*0xff), byte(col*0xff)
 	defer op.Push(gtx.Ops).Pop()
-	rgba := f32color.MulAlpha(color.RGBA{A: 0xff, R: bc, G: bc, B: bc}, ba)
+	rgba := f32color.MulAlpha(color.NRGBA{A: 0xff, R: bc, G: bc, B: bc}, ba)
 	ink := paint.ColorOp{Color: rgba}
 	ink.Add(gtx.Ops)
 	rr := size * .5

widget/material/checkable.go

@@ -17,10 +17,10 @@ import (
 
 type checkable struct {
 	Label              string
-	Color              color.RGBA
+	Color              color.NRGBA
 	Font               text.Font
 	TextSize           unit.Value
-	IconColor          color.RGBA
+	IconColor          color.NRGBA
 	Size               unit.Value
 	shaper             text.Shaper
 	checkedStateIcon   *widget.Icon

widget/material/doc.go

@@ -36,7 +36,7 @@
 // Theme-global parameters: For changing the look of all widgets drawn with a
 // particular theme, adjust the `Theme` fields:
 //
-//     theme.Color.Primary = color.RGBA{...}
+//     theme.Color.Primary = color.NRGBA{...}
 //
 // Widget-local parameters: For changing the look of a particular widget,
 // adjust the widget specific theme object:

widget/material/editor.go

@@ -18,11 +18,11 @@ type EditorStyle struct {
 	Font     text.Font
 	TextSize unit.Value
 	// Color is the text color.
-	Color color.RGBA
+	Color color.NRGBA
 	// Hint contains the text displayed when the editor is empty.
 	Hint string
 	// HintColor is the color of hint text.
-	HintColor color.RGBA
+	HintColor color.NRGBA
 	Editor    *widget.Editor
 
 	shaper text.Shaper

widget/material/label.go

@@ -16,7 +16,7 @@ type LabelStyle struct {
 	// Face defines the text style.
 	Font text.Font
 	// Color is the text color.
-	Color color.RGBA
+	Color color.NRGBA
 	// Alignment specify the text alignment.
 	Alignment text.Alignment
 	// MaxLines limits the number of lines. Zero means no limit.

widget/material/loader.go

@@ -17,7 +17,7 @@ import (
 )
 
 type LoaderStyle struct {
-	Color color.RGBA
+	Color color.NRGBA
 }
 
 func Loader(th *Theme) LoaderStyle {

widget/material/progressbar.go

@@ -15,7 +15,7 @@ import (
 )
 
 type ProgressBarStyle struct {
-	Color    color.RGBA
+	Color    color.NRGBA
 	Progress int
 }
 
@@ -27,7 +27,7 @@ func ProgressBar(th *Theme, progress int) ProgressBarStyle {
 }
 
 func (p ProgressBarStyle) Layout(gtx layout.Context) layout.Dimensions {
-	shader := func(width float32, color color.RGBA) layout.Dimensions {
+	shader := func(width float32, color color.NRGBA) layout.Dimensions {
 		maxHeight := unit.Dp(4)
 		rr := float32(gtx.Px(unit.Dp(2)))
 

widget/material/slider.go

@@ -28,7 +28,7 @@ func Slider(th *Theme, float *widget.Float, min, max float32) SliderStyle {
 
 type SliderStyle struct {
 	Min, Max float32
-	Color    color.RGBA
+	Color    color.NRGBA
 	Float    *widget.Float
 }
 

widget/material/switch.go

@@ -19,8 +19,8 @@ import (
 
 type SwitchStyle struct {
 	Color struct {
-		Enabled  color.RGBA
-		Disabled color.RGBA
+		Enabled  color.NRGBA
+		Disabled color.NRGBA
 	}
 	Switch *widget.Bool
 }
@@ -124,7 +124,7 @@ func (s SwitchStyle) Layout(gtx layout.Context) layout.Dimensions {
 	return layout.Dimensions{Size: dims}
 }
 
-func drawDisc(ops *op.Ops, sz float32, col color.RGBA) {
+func drawDisc(ops *op.Ops, sz float32, col color.NRGBA) {
 	defer op.Push(ops).Pop()
 	rr := sz / 2
 	r := f32.Rectangle{Max: f32.Point{X: sz, Y: sz}}

widget/material/theme.go

@@ -14,10 +14,10 @@ import (
 type Theme struct {
 	Shaper text.Shaper
 	Color  struct {
-		Primary color.RGBA
-		Text    color.RGBA
-		Hint    color.RGBA
-		InvText color.RGBA
+		Primary color.NRGBA
+		Text    color.NRGBA
+		Hint    color.NRGBA
+		InvText color.NRGBA
 	}
 	TextSize unit.Value
 	Icon     struct {
@@ -53,10 +53,10 @@ func mustIcon(ic *widget.Icon, err error) *widget.Icon {
 	return ic
 }
 
-func rgb(c uint32) color.RGBA {
+func rgb(c uint32) color.NRGBA {
 	return argb(0xff000000 | c)
 }
 
-func argb(c uint32) color.RGBA {
-	return color.RGBA{A: uint8(c >> 24), R: uint8(c >> 16), G: uint8(c >> 8), B: uint8(c)}
+func argb(c uint32) color.NRGBA {
+	return color.NRGBA{A: uint8(c >> 24), R: uint8(c >> 16), G: uint8(c >> 8), B: uint8(c)}
 }