transparent border optimization should alter the command array

context.go

@@ -315,6 +315,8 @@ func (ctx *context) Draw(wimg *image.RGBA) int {
 		ctx.cmdstim = ctx.cmdstim[:0]
 	}
 
+	transparentBorderOptimization := false
+
 	for i := range ctx.cmds {
 		if perfUpdate {
 			t0 = time.Now()
@@ -372,6 +374,17 @@ func (ctx *context) Draw(wimg *image.RGBA) int {
 				// first command draws the background, insure that it's always fully opaque
 				cmd.Color.A = 0xff
 			}
+			if transparentBorderOptimization {
+				transparentBorderOptimization = false
+				prevcmd := ctx.cmds[i-1].RectFilled
+				const m = 1<<16 - 1
+				sr, sg, sb, sa := cmd.Color.RGBA()
+				a := (m - sa) * 0x101
+				cmd.Color.R = uint8((uint32(prevcmd.Color.R)*a/m + sr) >> 8)
+				cmd.Color.G = uint8((uint32(prevcmd.Color.G)*a/m + sg) >> 8)
+				cmd.Color.B = uint8((uint32(prevcmd.Color.B)*a/m + sb) >> 8)
+				cmd.Color.A = uint8((uint32(prevcmd.Color.A)*a/m + sa) >> 8)
+			}
 			colimg := image.NewUniform(cmd.Color)
 			op := draw.Over
 			if cmd.Color.A == 0xff {
@@ -399,19 +412,12 @@ func (ctx *context) Draw(wimg *image.RGBA) int {
 				// only draw parts of body if this command can be optimized to a border with the next command
 
 				bordopt = true
-				cmd2 := &ctx.cmds[i+1]
-
-				if cmd2.RectFilled.Color.A != 0xff {
-					const m = 1<<16 - 1
-					sr, sg, sb, sa := cmd2.RectFilled.Color.RGBA()
-					a := (m - sa) * 0x101
-					cmd2.RectFilled.Color.R = uint8((uint32(cmd.Color.R)*a/m + sr) >> 8)
-					cmd2.RectFilled.Color.G = uint8((uint32(cmd.Color.G)*a/m + sg) >> 8)
-					cmd2.RectFilled.Color.B = uint8((uint32(cmd.Color.B)*a/m + sb) >> 8)
-					cmd2.RectFilled.Color.A = uint8((uint32(cmd.Color.A)*a/m + sa) >> 8)
+
+				if ctx.cmds[i+1].RectFilled.Color.A != 0xff {
+					transparentBorderOptimization = true
 				}
 
-				border += int(cmd2.RectFilled.Rounding)
+				border += int(ctx.cmds[i+1].RectFilled.Rounding)
 
 				top := image.Rect(body.Min.X, body.Min.Y, body.Max.X, body.Min.Y+border)
 				bot := image.Rect(body.Min.X, body.Max.Y-border, body.Max.X, body.Max.Y)