widget: use correct color in Icon

iconvg seems to expect a linear premultiplied color.

Fixes gio#132

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

internal/f32color/rgba.go

@@ -77,6 +77,40 @@ func NRGBAToRGBA(col color.NRGBA) color.RGBA {
 	}
 }
 
+// NRGBAToLinearRGBA converts from non-premultiplied sRGB color to premultiplied linear RGBA color.
+//
+// Each component in the result is `c * alpha`, where `c` is the linear color.
+func NRGBAToLinearRGBA(col color.NRGBA) color.RGBA {
+	if col.A == 0xFF {
+		return color.RGBA(col)
+	}
+	c := LinearFromSRGB(col)
+	return color.RGBA{
+		R: uint8(c.R*255 + .5),
+		G: uint8(c.G*255 + .5),
+		B: uint8(c.B*255 + .5),
+		A: col.A,
+	}
+}
+
+// NRGBAToRGBA_PostAlpha converts from non-premultiplied sRGB color to premultiplied sRGB color.
+//
+// Each component in the result is `sRGBToLinear(c) * alpha`, where `c`
+// is the linear color.
+func NRGBAToRGBA_PostAlpha(col color.NRGBA) color.RGBA {
+	if col.A == 0xFF {
+		return color.RGBA(col)
+	} else if col.A == 0x00 {
+		return color.RGBA{}
+	}
+	return color.RGBA{
+		R: uint8(uint32(col.R) * uint32(col.A) / 0xFF),
+		G: uint8(uint32(col.G) * uint32(col.A) / 0xFF),
+		B: uint8(uint32(col.B) * uint32(col.A) / 0xFF),
+		A: col.A,
+	}
+}
+
 // RGBAToNRGBA converts from premultiplied sRGB color to non-premultiplied sRGB color.
 func RGBAToNRGBA(col color.RGBA) color.NRGBA {
 	if col.A == 0xFF {

internal/f32color/rgba_test.go

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package f32color
+
+import (
+	"image/color"
+	"testing"
+)
+
+func TestNRGBAToRGBA_PostAlpha_Boundary(t *testing.T) {
+	for col := 0; col <= 0xFF; col++ {
+		for alpha := 0; alpha <= 0xFF; alpha++ {
+			in := color.NRGBA{R: uint8(col), A: uint8(alpha)}
+			premul := NRGBAToRGBA_PostAlpha(in)
+			if premul.A != uint8(alpha) {
+				t.Errorf("%v: got %v expected %v", in, premul.A, alpha)
+			}
+			if premul.R > premul.A {
+				t.Errorf("%v: R=%v > A=%v", in, premul.R, premul.A)
+			}
+		}
+	}
+}
+
+func TestNRGBAToLinearRGBA_Boundary(t *testing.T) {
+	for col := 0; col <= 0xFF; col++ {
+		for alpha := 0; alpha <= 0xFF; alpha++ {
+			in := color.NRGBA{R: uint8(col), A: uint8(alpha)}
+			premul := NRGBAToLinearRGBA(in)
+			if premul.A != uint8(alpha) {
+				t.Errorf("%v: got %v expected %v", in, premul.A, alpha)
+			}
+			if premul.R > premul.A {
+				t.Errorf("%v: R=%v > A=%v", in, premul.R, premul.A)
+			}
+		}
+	}
+}

widget/icon.go

@@ -51,7 +51,7 @@ 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] = f32color.NRGBAToRGBA(ic.Color)
+	m.Palette[0] = f32color.NRGBAToLinearRGBA(ic.Color)
 	iconvg.Decode(&ico, ic.src, &iconvg.DecodeOptions{
 		Palette: &m.Palette,
 	})

widget/icon_test.go

@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package widget
+
+import (
+	"image"
+	"image/color"
+	"testing"
+
+	"gioui.org/layout"
+	"gioui.org/op"
+	"gioui.org/unit"
+	"golang.org/x/exp/shiny/materialdesign/icons"
+)
+
+func TestIcon_Alpha(t *testing.T) {
+	icon, err := NewIcon(icons.ToggleCheckBox)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	icon.Color = color.NRGBA{B: 0xff, A: 0x40}
+
+	gtx := layout.Context{
+		Ops:         new(op.Ops),
+		Constraints: layout.Exact(image.Pt(100, 100)),
+	}
+
+	_ = icon.Layout(gtx, unit.Sp(18))
+}