vector: Implement miter limit

Fixes #843

examples/vector/main.go

@@ -19,7 +19,6 @@ package main
 import (
 	"image/color"
 	"log"
-	"math"
 
 	"github.com/hajimehoshi/ebiten"
 	"github.com/hajimehoshi/ebiten/vector"
@@ -84,79 +83,28 @@ func drawEbitenText(screen *ebiten.Image) {
 	path.Draw(screen, op)
 }
 
-type roundingPoint struct {
+func drawLines(screen *ebiten.Image) {
-	cx     float32
-	cy     float32
-	r      float32
-	degree int
-}
-
-func (r *roundingPoint) Update() {
-	r.degree++
-	r.degree %= 360
-}
-
-func (r *roundingPoint) Position() (float32, float32) {
-	s, c := math.Sincos(float64(r.degree) / 360 * 2 * math.Pi)
-	return r.cx + r.r*float32(c), r.cy + r.r*float32(s)
-}
-
-func drawLinesByRoundingPoints(screen *ebiten.Image, points []*roundingPoint) {
-	if len(points) == 0 {
-		return
-	}
-
 	var path vector.Path
-
+	path.MoveTo(20, 80+float32(counter%320)/4)
-	path.MoveTo(points[0].Position())
+	path.LineTo(60, 120)
-	for i := 1; i < len(points); i++ {
+	path.LineTo(20, 160-float32(counter%320)/4)
-		path.LineTo(points[i].Position())
-	}
 
 	op := &vector.DrawPathOptions{}
-	op.LineWidth = 4
+	op.LineWidth = 16
 	op.StrokeColor = color.White
 	path.Draw(screen, op)
 }
 
-var points = []*roundingPoint{
+var counter = 0
-	{
-		cx:     100,
-		cy:     120,
-		r:      10,
-		degree: 0,
-	},
-	{
-		cx:     120,
-		cy:     120,
-		r:      10,
-		degree: 90,
-	},
-	{
-		cx:     100,
-		cy:     140,
-		r:      10,
-		degree: 180,
-	},
-	{
-		cx:     120,
-		cy:     140,
-		r:      10,
-		degree: 270,
-	},
-}
 
 func update(screen *ebiten.Image) error {
-	for _, p := range points {
+	counter++
-		p.Update()
-	}
-
 	if ebiten.IsDrawingSkipped() {
 		return nil
 	}
 
 	drawEbitenText(screen)
-	drawLinesByRoundingPoints(screen, points)
+	drawLines(screen)
 	return nil
 }
 

vector/path.go

@@ -63,6 +63,8 @@ func (p *Path) LineTo(x, y float32) {
 }
 
 func (p *Path) strokeVertices(lineWidth float32, clr color.Color) (vertices []ebiten.Vertex, indices []uint16) {
+	const miterLimit = 10
+
 	if len(p.segs) == 0 {
 		return nil, nil
 	}
@@ -103,11 +105,69 @@ func (p *Path) strokeVertices(lineWidth float32, clr color.Color) (vertices []eb
 
 			v2 := s0.P1
 			v3 := s1.P1
+			cut := false
 			if i != len(ss)-1 {
 				ns := ss[i+1]
-				v2 = ns.Translate(-lineWidth / 2).IntersectionAsLines(s0)
+				nv2 := ns.Translate(-lineWidth / 2).IntersectionAsLines(s0)
-				v3 = ns.Translate(lineWidth / 2).IntersectionAsLines(s1)
+				nv3 := ns.Translate(lineWidth / 2).IntersectionAsLines(s1)
+				l := lineWidth / 2 * miterLimit
+				if (nv2.X-nv3.X)*(nv2.X-nv3.X)+(nv2.Y-nv3.Y)*(nv2.Y-nv3.Y) < l*l {
+					v2 = nv2
+					v3 = nv3
+				} else {
+					cut = true
 				}
+			}
+
+			if cut {
+				ns := ss[i+1]
+				s2 := ns.Translate(-lineWidth / 2)
+				s3 := ns.Translate(lineWidth / 2)
+				vertices = append(vertices,
+					ebiten.Vertex{
+						DstX:   s0.P1.X,
+						DstY:   s0.P1.Y,
+						SrcX:   s0.P1.X,
+						SrcY:   s0.P1.Y,
+						ColorR: rf,
+						ColorG: gf,
+						ColorB: bf,
+						ColorA: af,
+					},
+					ebiten.Vertex{
+						DstX:   s1.P1.X,
+						DstY:   s1.P1.Y,
+						SrcX:   s1.P1.X,
+						SrcY:   s1.P1.Y,
+						ColorR: rf,
+						ColorG: gf,
+						ColorB: bf,
+						ColorA: af,
+					},
+					ebiten.Vertex{
+						DstX:   s2.P0.X,
+						DstY:   s2.P0.Y,
+						SrcX:   s2.P0.X,
+						SrcY:   s2.P0.Y,
+						ColorR: rf,
+						ColorG: gf,
+						ColorB: bf,
+						ColorA: af,
+					},
+					ebiten.Vertex{
+						DstX:   s3.P0.X,
+						DstY:   s3.P0.Y,
+						SrcX:   s3.P0.X,
+						SrcY:   s3.P0.Y,
+						ColorR: rf,
+						ColorG: gf,
+						ColorB: bf,
+						ColorA: af,
+					})
+				indices = append(indices, idx, idx+1, idx+2, idx+1, idx+2, idx+3,
+					idx+2, idx+3, idx+4, idx+3, idx+4, idx+5)
+				idx += 4
+			} else {
 				vertices = append(vertices,
 					ebiten.Vertex{
 						DstX:   v2.X,
@@ -129,11 +189,11 @@ func (p *Path) strokeVertices(lineWidth float32, clr color.Color) (vertices []eb
 						ColorB: bf,
 						ColorA: af,
 					})
-
 				indices = append(indices, idx, idx+1, idx+2, idx+1, idx+2, idx+3)
 				idx += 2
 			}
 		}
+	}
 
 	return
 }