xgb-window: add comparison with correct blending

prototypes/xgb-window.go

@@ -4,6 +4,7 @@ import (
 	"github.com/BurntSushi/xgb"
 	"github.com/BurntSushi/xgb/xproto"
 	"log"
+	"math"
 	"math/rand"
 )
 
@@ -73,6 +74,17 @@ func main() {
 	_ = xproto.CreateGC(X, cid, xproto.Drawable(wid),
 		xproto.GcGraphicsExposures, []uint32{0})
 
+	blend := func(a, b uint32, ratio, gamma float64) uint32 {
+		iratio := 1 - ratio
+
+		fa := math.Pow(float64(a)/255, gamma)
+		fb := math.Pow(float64(b)/255, gamma)
+
+		return uint32(math.Pow(ratio*fa+iratio*fb, 1/gamma)*255) & 0xff
+	}
+
+	// TODO: We could show some text just like we intend to with xgb-xrender.go.
+
 	var w, h uint16
 	var start, end uint32 = 0xabcdef, 0x32ab54
 	gradient := func() {
@@ -86,17 +98,28 @@ func main() {
 
 		for y := low; y < high; y++ {
 			ratio := float64(y-low) / (float64(high) - float64(low))
-			iratio := 1 - ratio
 
-			rR := uint32(ratio*float64(ra)+iratio*float64(rb)) & 0xff
-			gG := uint32(ratio*float64(ga)+iratio*float64(gb)) & 0xff
-			bB := uint32(ratio*float64(ba)+iratio*float64(bb)) & 0xff
+			rR := blend(ra, rb, ratio, 2.2)
+			gG := blend(ga, gb, ratio, 2.2)
+			bB := blend(ba, bb, ratio, 2.2)
 
 			_ = xproto.ChangeGC(X, cid, xproto.GcForeground,
 				[]uint32{0xff000000 | rR<<16 | gG<<8 | bB})
 			_ = xproto.PolyLine(X, xproto.CoordModeOrigin, xproto.Drawable(wid),
 				cid, []xproto.Point{
 					{X: 50, Y: int16(y)},
+					{X: int16(w / 2), Y: int16(y)},
+				})
+
+			rR = blend(ra, rb, ratio, 1)
+			gG = blend(ga, gb, ratio, 1)
+			bB = blend(ba, bb, ratio, 1)
+
+			_ = xproto.ChangeGC(X, cid, xproto.GcForeground,
+				[]uint32{0xff000000 | rR<<16 | gG<<8 | bB})
+			_ = xproto.PolyLine(X, xproto.CoordModeOrigin, xproto.Drawable(wid),
+				cid, []xproto.Point{
+					{X: int16(w / 2), Y: int16(y)},
 					{X: int16(w - 50), Y: int16(y)},
 				})
 		}