internal/f32color: correct pre-multipled color conversion

Tweak a test color to avoid an off-by-1 rounding error after changing
the conversion formula.

Fixes gio#192

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/headless/backend_test.go

@@ -107,7 +107,7 @@ func TestFramebuffers(t *testing.T) {
 	fbo1 := newFBO(t, b, sz)
 	fbo2 := newFBO(t, b, sz)
 	var (
-		col1 = color.NRGBA{R: 0xac, G: 0xbe, B: 0xef, A: 0xde}
+		col1 = color.NRGBA{R: 0xac, G: 0xbd, B: 0xef, A: 0xde}
 		col2 = color.NRGBA{R: 0xfe, G: 0xba, B: 0xbe, A: 0xca}
 	)
 	fcol1, fcol2 := f32color.LinearFromSRGB(col1), f32color.LinearFromSRGB(col2)

internal/f32color/rgba.go

@@ -28,9 +28,9 @@ func (col RGBA) SRGB() color.NRGBA {
 		return color.NRGBA{}
 	}
 	return color.NRGBA{
-		R: uint8(linearTosRGB(col.R/col.A)*255 + .5),
-		G: uint8(linearTosRGB(col.G/col.A)*255 + .5),
-		B: uint8(linearTosRGB(col.B/col.A)*255 + .5),
+		R: uint8(linearTosRGB(col.R)/col.A*255 + .5),
+		G: uint8(linearTosRGB(col.G)/col.A*255 + .5),
+		B: uint8(linearTosRGB(col.B)/col.A*255 + .5),
 		A: uint8(col.A*255 + .5),
 	}
 }
@@ -49,13 +49,13 @@ func (col RGBA) Opaque() RGBA {
 	return col
 }
 
-// LinearFromSRGB converts from SRGBA to RGBA.
+// LinearFromSRGB converts from col in the sRGB colorspace to RGBA.
 func LinearFromSRGB(col color.NRGBA) RGBA {
 	af := float32(col.A) / 0xFF
 	return RGBA{
-		R: sRGBToLinear(float32(col.R)/0xff) * af,
-		G: sRGBToLinear(float32(col.G)/0xff) * af,
-		B: sRGBToLinear(float32(col.B)/0xff) * af,
+		R: sRGBToLinear(float32(col.R) / 0xff * af),
+		G: sRGBToLinear(float32(col.G) / 0xff * af),
+		B: sRGBToLinear(float32(col.B) / 0xff * af),
 		A: af,
 	}
 }

internal/f32color/rgba_test.go

@@ -36,3 +36,18 @@ func TestNRGBAToLinearRGBA_Boundary(t *testing.T) {
 		}
 	}
 }
+
+func TestLinearToRGBARoundtrip(t *testing.T) {
+	for col := 0; col <= 0xFF; col++ {
+		for alpha := 0; alpha <= 0xFF; alpha++ {
+			want := color.NRGBA{R: uint8(col), A: uint8(alpha)}
+			if alpha == 0 {
+				want.R = 0
+			}
+			got := LinearFromSRGB(want).SRGB()
+			if want != got {
+				t.Errorf("got %v expected %v", got, want)
+			}
+		}
+	}
+}