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)
+}