widget/material: better disabled color calculation

Use desaturation in combination with alpha multiplication.

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

internal/f32color/rgba.go

@@ -35,6 +35,14 @@ func (col RGBA) SRGB() color.NRGBA {
 	}
 }
 
+// Luminance calculates the relative luminance of a linear RGBA color.
+// Normalized to 0 for black and 1 for white.
+//
+// See https://www.w3.org/TR/WCAG20/#relativeluminancedef for more details
+func (col RGBA) Luminance() float32 {
+	return 0.2126*col.R + 0.7152*col.G + 0.0722*col.B
+}
+
 // Opaque returns the color without alpha component.
 func (col RGBA) Opaque() RGBA {
 	col.A = 1.0
@@ -115,3 +123,28 @@ func MulAlpha(c color.NRGBA, alpha uint8) color.NRGBA {
 	c.A = uint8(uint32(c.A) * uint32(alpha) / 0xFF)
 	return c
 }
+
+// Disabled blends color towards the luminance and multiplies alpha.
+// Blending towards luminance will desaturate the color.
+// Multiplying alpha blends the color together more with the background.
+func Disabled(c color.NRGBA) (d color.NRGBA) {
+	const r = 80 // blend ratio
+	lum := approxLuminance(c)
+	return color.NRGBA{
+		R: byte((int(c.R)*r + int(lum)*(256-r)) / 256),
+		G: byte((int(c.G)*r + int(lum)*(256-r)) / 256),
+		B: byte((int(c.B)*r + int(lum)*(256-r)) / 256),
+		A: byte(int(c.A) * (128 + 32) / 256),
+	}
+}
+
+// approxLuminance is a fast approximate version of RGBA.Luminance.
+func approxLuminance(c color.NRGBA) byte {
+	const (
+		r = 13933 // 0.2126 * 256 * 256
+		g = 46871 // 0.7152 * 256 * 256
+		b = 4732  // 0.0722 * 256 * 256
+		t = r + g + b
+	)
+	return byte((r*int(c.R) + g*int(c.G) + b*int(c.B)) / t)
+}

widget/material/button.go

@@ -129,7 +129,7 @@ func (b ButtonLayoutStyle) Layout(gtx layout.Context, w layout.Widget) layout.Di
 			}}, rr).Add(gtx.Ops)
 			background := b.Background
 			if gtx.Queue == nil {
-				background = f32color.MulAlpha(b.Background, 150)
+				background = f32color.Disabled(b.Background)
 			}
 			paint.Fill(gtx.Ops, background)
 			for _, c := range b.Button.History() {
@@ -156,7 +156,7 @@ func (b IconButtonStyle) Layout(gtx layout.Context) layout.Dimensions {
 			}, rr).Add(gtx.Ops)
 			background := b.Background
 			if gtx.Queue == nil {
-				background = f32color.MulAlpha(b.Background, 150)
+				background = f32color.Disabled(b.Background)
 			}
 			paint.Fill(gtx.Ops, background)
 			for _, c := range b.Button.History() {

widget/material/checkable.go

@@ -43,7 +43,7 @@ func (c *checkable) layout(gtx layout.Context, checked bool) layout.Dimensions {
 					size := gtx.Px(c.Size)
 					icon.Color = c.IconColor
 					if gtx.Queue == nil {
-						icon.Color = f32color.MulAlpha(icon.Color, 150)
+						icon.Color = f32color.Disabled(icon.Color)
 					}
 					icon.Layout(gtx, unit.Px(float32(size)))
 					return layout.Dimensions{

widget/material/editor.go

@@ -61,7 +61,7 @@ func (e EditorStyle) Layout(gtx layout.Context) layout.Dimensions {
 	if e.Editor.Len() > 0 {
 		textColor := e.Color
 		if disabled {
-			textColor = f32color.MulAlpha(textColor, 150)
+			textColor = f32color.Disabled(textColor)
 		}
 		paint.ColorOp{Color: textColor}.Add(gtx.Ops)
 		e.Editor.PaintText(gtx)

widget/material/progressbar.go

@@ -55,7 +55,7 @@ func (p ProgressBarStyle) Layout(gtx layout.Context) layout.Dimensions {
 			fillWidth := progressBarWidth * clamp1(p.Progress)
 			fillColor := p.Color
 			if gtx.Queue == nil {
-				fillColor = f32color.MulAlpha(fillColor, 200)
+				fillColor = f32color.Disabled(fillColor)
 			}
 			return shader(fillWidth, fillColor)
 		}),

widget/material/slider.go

@@ -68,7 +68,7 @@ func (s SliderStyle) Layout(gtx layout.Context) layout.Dimensions {
 
 	color := s.Color
 	if gtx.Queue == nil {
-		color = f32color.MulAlpha(color, 150)
+		color = f32color.Disabled(color)
 	}
 
 	// Draw track before thumb.

widget/material/switch.go

@@ -55,7 +55,7 @@ func (s SwitchStyle) Layout(gtx layout.Context) layout.Dimensions {
 		col = s.Color.Enabled
 	}
 	if gtx.Queue == nil {
-		col = f32color.MulAlpha(col, 150)
+		col = f32color.Disabled(col)
 	}
 	trackColor := s.Color.Track
 	op.Offset(f32.Point{Y: trackOff}).Add(gtx.Ops)