gx: implement C1.over(C2), add gx.cyan and gx.magenta, fix + and - ops, add more tests

vlib/gx/color.v

@@ -1,9 +1,19 @@
 module gx
 
 pub const (
-	blue = Color{
+	black = Color{
 		r: 0
 		g: 0
+		b: 0
+	}
+	gray = Color{
+		r: 128
+		g: 128
+		b: 128
+	}
+	white = Color{
+		r: 255
+		g: 255
 		b: 255
 	}
 	red = Color{
@@ -16,11 +26,26 @@ pub const (
 		g: 255
 		b: 0
 	}
+	blue = Color{
+		r: 0
+		g: 0
+		b: 255
+	}
 	yellow = Color{
 		r: 255
 		g: 255
 		b: 0
 	}
+	magenta = Color{
+		r: 255
+		g: 0
+		b: 255
+	}
+	cyan = Color{
+		r: 0
+		g: 255
+		b: 255
+	}
 	orange = Color{
 		r: 255
 		g: 165
@@ -31,16 +56,6 @@ pub const (
 		g: 0
 		b: 128
 	}
-	black = Color{
-		r: 0
-		g: 0
-		b: 0
-	}
-	gray = Color{
-		r: 128
-		g: 128
-		b: 128
-	}
 	indigo = Color{
 		r: 75
 		g: 0
@@ -56,11 +71,6 @@ pub const (
 		g: 130
 		b: 238
 	}
-	white = Color{
-		r: 255
-		g: 255
-		b: 255
-	}
 	dark_blue = Color{
 		r: 0
 		g: 0
@@ -139,21 +149,55 @@ pub fn rgba(r byte, g byte, b byte, a byte) Color {
 	}
 }
 
-pub fn (c Color) + (c2 Color) Color {
+// + adds `b` to `a`, with a maximum value of 255 for each channel
+pub fn (a Color) + (b Color) Color {
+	mut na := int(a.a) + b.a
+	mut nr := int(a.r) + b.r
+	mut ng := int(a.g) + b.g
+	mut nb := int(a.b) + b.b
+	if na > 255 {
+		na = 255
+	}
+	if nr > 255 {
+		nr = 255
+	}
+	if ng > 255 {
+		ng = 255
+	}
+	if nb > 255 {
+		nb = 255
+	}
 	return Color{
-		r: c.r + c2.r
-		g: c.g + c2.g
-		b: c.b + c2.b
-		a: c.a + c2.a
+		r: byte(nr)
+		g: byte(ng)
+		b: byte(nb)
+		a: byte(na)
 	}
 }
 
-pub fn (c Color) - (c2 Color) Color {
+// - subtracts `b` from `a`, with a minimum value of 0 for each channel
+pub fn (a Color) - (b Color) Color {
+	mut na := if a.a > b.a { a.a } else { b.a }
+	mut nr := int(a.r) - b.r
+	mut ng := int(a.g) - b.g
+	mut nb := int(a.b) - b.b
+	if na < 0 {
+		na = 0
+	}
+	if nr < 0 {
+		nr = 0
+	}
+	if ng < 0 {
+		ng = 0
+	}
+	if nb < 0 {
+		nb = 0
+	}
 	return Color{
-		r: c.r - c2.r
-		g: c.g - c2.g
-		b: c.b - c2.b
-		a: c.a - c2.a
+		r: byte(nr)
+		g: byte(ng)
+		b: byte(nb)
+		a: byte(na)
 	}
 }
 
@@ -175,6 +219,24 @@ pub fn (c Color) / (c2 Color) Color {
 	}
 }
 
+// over - implements an `a` over `b` operation.
+// see https://keithp.com/~keithp/porterduff/p253-porter.pdf
+pub fn (a Color) over(b Color) Color {
+	aa := f32(a.a) / 255
+	ab := f32(b.a) / 255
+	ar := aa + ab * (1 - aa)
+	//
+	rr := (f32(a.r) * aa + f32(b.r) * ab * (1 - aa)) / ar
+	gr := (f32(a.g) * aa + f32(b.g) * ab * (1 - aa)) / ar
+	br := (f32(a.b) * aa + f32(b.b) * ab * (1 - aa)) / ar
+	return Color{
+		r: byte(rr)
+		g: byte(gr)
+		b: byte(br)
+		a: byte(ar * 255)
+	}
+}
+
 pub fn (c Color) eq(c2 Color) bool {
 	return c.r == c2.r && c.g == c2.g && c.b == c2.b && c.a == c2.a
 }