From 5ca69d90e6849881a87bac110c5ba7ebb7be410d Mon Sep 17 00:00:00 2001
From: Sasha Koshka <sashakoshka@tebibyte.media>
Date: Sat, 1 Jul 2023 03:44:36 -0400
Subject: [PATCH] Fixed polygon filling

---
 examples/glaggle/main.go | 15 +++++++++++++--
 polygon.go               | 34 ++++++++++++++++++++--------------
 2 files changed, 33 insertions(+), 16 deletions(-)

diff --git a/examples/glaggle/main.go b/examples/glaggle/main.go
index a8816cd..fcd187a 100644
--- a/examples/glaggle/main.go
+++ b/examples/glaggle/main.go
@@ -6,14 +6,25 @@ import "image/png"
 import "git.tebibyte.media/tomo/ggfx"
 
 func main () {
-	img := image.NewRGBA(image.Rect(0, 0, 800, 600))
+	img := image.NewRGBA(image.Rect(0, 0, 300, 300))
 	canvas := ggfx.Image[uint8] {
 		Pix:    img.Pix,
 		Stride: img.Stride,
 		Bounds: img.Rect,
 		Width:  4,
 	}
-	canvas.FillRectangle([]uint8 { 255, 0, 0, 255 }, image.Rect(7, 4, 34, 78))
+	black := []uint8 { 0,   0, 0, 255 }
+	red   := []uint8 { 255, 0, 0, 255 }
+	canvas.FillPolygon (
+		black,
+		image.Pt(10, 10),
+		image.Pt(70, 50),
+		image.Pt(20, 70))
+	canvas.StrokePolygon (
+		red, 1,
+		image.Pt(10, 10),
+		image.Pt(70, 50),
+		image.Pt(20, 70))
 	
 	file, err := os.Create("TEST.png")
 	if err != nil { panic(err.Error()) }
diff --git a/polygon.go b/polygon.go
index 02e5f5e..2ddcf19 100644
--- a/polygon.go
+++ b/polygon.go
@@ -14,10 +14,11 @@ func (this Image[T]) FillPolygon (fill []T, points ...image.Point) {
 	for _, point := range points[1:] {
 		if point.X < area.Min.X { area.Min.X = point.X }
 		if point.Y < area.Min.Y { area.Min.Y = point.Y }
-		if point.X < area.Max.X { area.Max.X = point.X }
-		if point.Y < area.Max.Y { area.Max.Y = point.Y }
+		if point.X > area.Max.X { area.Max.X = point.X }
+		if point.Y > area.Max.Y { area.Max.Y = point.Y }
 	}
 	area = this.Bounds.Intersect(area)
+	if area.Empty() { return }
 	
 	// this algorithm is adapted from:
 	// https://www.alienryderflex.com/polygon_fill/
@@ -28,30 +29,35 @@ func (this Image[T]) FillPolygon (fill []T, points ...image.Point) {
 		boundaryCount := 0
 		prevPoint := points[len(points) - 1]
 		for _, point := range points {
+			fy      := float64(y)
+			fPointX := float64(point.X)
+			fPointY := float64(point.Y)
+			fPrevX  := float64(prevPoint.X)
+			fPrevY  := float64(prevPoint.Y)
 			addboundary :=
-				point.Y     < y && prevPoint.Y >= y ||
-				prevPoint.Y < y && point.Y     >= y
+				(fPointY < fy && fPrevY  >= fy) ||
+				(fPrevY  < fy && fPointY >= fy)
 			if addboundary {
-				boundaries[boundaryCount] =
-					point.X +
-					(y - point.Y) /
-					(prevPoint.Y - point.Y) *
-					(prevPoint.X - point.X)
+				boundaries[boundaryCount] = int (
+					fPointX +
+					(fy - fPointY) /
+					(fPrevY - fPointY) *
+					(fPrevX - fPointX))
 				boundaryCount ++
 			}
 			prevPoint = point
 		}
 
 		// sort boundary list
-		boundaries = boundaries[:boundaryCount]
-		sort.Ints(boundaries)
+		cutBoundaries := boundaries[:boundaryCount]
+		sort.Ints(cutBoundaries)
 
 		// fill pixels between boundary pairs
 		min := area.Min.X
 		max := area.Max.X
-		for index := 0; index < len(boundaries); index ++ {
-			left  := boundaries[index]
-			right := boundaries[index + 1]
+		for index := 0; index < len(cutBoundaries); index += 2 {
+			left  := cutBoundaries[index]
+			right := cutBoundaries[index + 1]
 
 			// stop if we have exited the polygon
 			if left >= max { break }