Fix boundary detection for polygons

x/canvas/draw.go

@@ -1,6 +1,7 @@
 package xcanvas
 
 import "sort"
+import "math"
 import "image"
 import "github.com/jezek/xgbutil/xgraphics"
 
@@ -158,12 +159,12 @@ func (this *pen) fillPolygon (c xgraphics.BGRA, points ...image.Point) {
 	area = this.image.Bounds().Intersect(area)
 	if area.Empty() { return }
 
+	boundaries := make([]int, len(points))
 	context := fillingContext {
 		image:      this.image,
 		color:      this.fill,
 		min:        area.Min.X,
 		max:        area.Max.X,
-		boundaries: make([]int, len(points)),
 		points:     points,
 	}
 
@@ -181,19 +182,19 @@ func (this *pen) fillPolygon (c xgraphics.BGRA, points ...image.Point) {
 				(fPointY < fy && fPrevY  >= fy) ||
 				(fPrevY  < fy && fPointY >= fy)
 			if addboundary {
-				context.boundaries[boundaryCount] = int (
+				boundaries[boundaryCount] = int(math.Round (
 					fPointX +
 					(fy - fPointY) /
 					(fPrevY - fPointY) *
-					(fPrevX - fPointX))
+					(fPrevX - fPointX)))
 				boundaryCount ++
 			}
 			prevPoint = point
 		}
 
 		// sort boundary list
-		cutBoundaries := context.boundaries[:boundaryCount]
-		sort.Ints(cutBoundaries)
+		context.boundaries = boundaries[:boundaryCount]
+		sort.Ints(context.boundaries)
 
 		// fill pixels between boundary pairs
 		if c.A == 255 {