Clamp colors.

internal/zenutil/color.go

@@ -35,7 +35,14 @@ func ParseColor(s string) color.Color {
 
 		var a float32
 		if _, err := fmt.Sscanf(s, "rgba(%d,%d,%d,%f)", &c.R, &c.G, &c.B, &a); err == nil {
+			switch {
+			case a <= 0:
+				c.A = 0
+			case a >= 1:
+				c.A = 255
+			default:
 				c.A = uint8(255*a + 0.5)
+			}
 			return c
 		}
 	}

internal/zenutil/color_test.go

@@ -0,0 +1,36 @@
+package zenutil
+
+import (
+	"image/color"
+	"testing"
+
+	"golang.org/x/image/colornames"
+)
+
+func TestColor(t *testing.T) {
+	tests := []string{
+		"chocolate",
+		"lime",
+		"olive",
+		"orange",
+		"plum",
+		"salmon",
+		"tomato",
+	}
+	eq := func(c1, c2 color.Color) bool {
+		r1, g1, b1, a1 := c1.RGBA()
+		r2, g2, b2, a2 := c2.RGBA()
+		return r1 == r2 && g1 == g2 && b1 == b2 && a1 == a2
+	}
+	for _, test := range tests {
+		c1 := colornames.Map[test]
+		c2 := ParseColor(test)
+		c3 := ParseColor(UnparseColor(c2))
+		if !eq(c1, c2) {
+			t.Errorf("ParseColor(%s) = %v, want %v", test, c2, c1)
+		}
+		if !eq(c1, c3) {
+			t.Errorf("ParseColor(UnparseColor(%s)) = %v, want %v", test, c3, c1)
+		}
+	}
+}