vector: add LineJoin

Updates #1843

examples/lines/main.go

@@ -0,0 +1,176 @@
+// Copyright 2022 The Ebitengine Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"image"
+	"image/color"
+	"log"
+	"math"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
+	"github.com/hajimehoshi/ebiten/v2/inpututil"
+	"github.com/hajimehoshi/ebiten/v2/vector"
+)
+
+func min(x, y int) int {
+	if x < y {
+		return x
+	}
+	return y
+}
+
+var (
+	emptyImage = ebiten.NewImage(3, 3)
+
+	// emptySubImage is an internal sub image of emptyImage.
+	// Use emptySubImage at DrawTriangles instead of emptyImage in order to avoid bleeding edges.
+	emptySubImage = emptyImage.SubImage(image.Rect(1, 1, 2, 2)).(*ebiten.Image)
+)
+
+func init() {
+	emptyImage.Fill(color.White)
+}
+
+const (
+	screenWidth  = 640
+	screenHeight = 480
+)
+
+type Game struct {
+	counter int
+
+	vertices []ebiten.Vertex
+	indices  []uint16
+
+	offscreen *ebiten.Image
+
+	aa         bool
+	showCenter bool
+}
+
+func (g *Game) Update() error {
+	g.counter++
+	if inpututil.IsKeyJustPressed(ebiten.KeyA) {
+		g.aa = !g.aa
+	}
+	if inpututil.IsKeyJustPressed(ebiten.KeyC) {
+		g.showCenter = !g.showCenter
+	}
+	return nil
+}
+
+func (g *Game) Draw(screen *ebiten.Image) {
+	target := screen
+	if g.aa {
+		// Prepare the double-sized offscreen.
+		// This is for anti-aliasing by a pseudo MSAA (multisample anti-aliasing).
+		if g.offscreen != nil {
+			sw, sh := screen.Size()
+			ow, oh := g.offscreen.Size()
+			if ow != sw*2 || oh != sh*2 {
+				g.offscreen.Dispose()
+				g.offscreen = nil
+			}
+		}
+		if g.offscreen == nil {
+			sw, sh := screen.Size()
+			g.offscreen = ebiten.NewImage(sw*2, sh*2)
+		}
+		g.offscreen.Clear()
+		target = g.offscreen
+	}
+
+	ow, oh := target.Size()
+	size := min(ow/4, oh/4)
+	offsetX, offsetY := (ow-size*3)/2, (oh-size*3)/2
+
+	// Render the lines on the target.
+	for j := 0; j < 3; j++ {
+		for i, join := range []vector.LineJoin{vector.LineJoinMiter, vector.LineJoinBevel, vector.LineJoinRound} {
+			r := image.Rect(i*size+offsetX, j*size+offsetY, (i+1)*size+offsetX, (j+1)*size+offsetY)
+			g.drawLine(target, r, join)
+		}
+	}
+
+	if g.aa {
+		// Render the offscreen to the screen.
+		op := &ebiten.DrawImageOptions{}
+		op.GeoM.Scale(0.5, 0.5)
+		op.Filter = ebiten.FilterLinear
+		screen.DrawImage(g.offscreen, op)
+	}
+
+	msg := `Press A to switch anti-aliasing.
+Press C to switch to draw the center lines.`
+	ebitenutil.DebugPrint(screen, msg)
+}
+
+func (g *Game) drawLine(screen *ebiten.Image, region image.Rectangle, join vector.LineJoin) {
+	c0x := float64(region.Min.X + region.Dx()/4)
+	c0y := float64(region.Min.Y + region.Dy()/4)
+	c1x := float64(region.Max.X - region.Dx()/4)
+	c1y := float64(region.Max.Y - region.Dy()/4)
+	r := float64(min(region.Dx(), region.Dy()) / 4)
+	a := 2 * math.Pi * float64(g.counter) / (10 * ebiten.DefaultTPS)
+
+	var path vector.Path
+	sin, cos := math.Sincos(a)
+	path.MoveTo(float32(r*cos+c0x), float32(r*sin+c0y))
+	path.LineTo(float32(-r*cos+c0x), float32(-r*sin+c0y))
+	path.LineTo(float32(r*cos+c1x), float32(r*sin+c1y))
+	path.LineTo(float32(-r*cos+c1x), float32(-r*sin+c1y))
+
+	// Draw the main line in white.
+	op := &vector.StrokeOptions{}
+	op.LineJoin = join
+	op.Width = float32(r / 2)
+	vs, is := path.AppendVerticesAndIndicesForStroke(g.vertices[:0], g.indices[:0], op)
+	for i := range vs {
+		vs[i].SrcX = 1
+		vs[i].SrcY = 1
+	}
+	screen.DrawTriangles(vs, is, emptySubImage, nil)
+
+	// Draw the center line in red.
+	if g.showCenter {
+		op.Width = 1
+		vs, is := path.AppendVerticesAndIndicesForStroke(g.vertices[:0], g.indices[:0], op)
+		for i := range vs {
+			vs[i].SrcX = 1
+			vs[i].SrcY = 1
+			vs[i].SrcX = 1
+			vs[i].SrcY = 1
+			vs[i].ColorR = 1
+			vs[i].ColorG = 0
+			vs[i].ColorB = 0
+		}
+		screen.DrawTriangles(vs, is, emptySubImage, nil)
+	}
+}
+
+func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {
+	return screenWidth, screenHeight
+}
+
+func main() {
+	var g Game
+	ebiten.SetWindowSize(screenWidth, screenHeight)
+	ebiten.SetWindowTitle("Lines (Ebitengine Demo)")
+	if err := ebiten.RunGame(&g); err != nil {
+		log.Fatal(err)
+	}
+}

examples/vector/main.go

@@ -118,6 +118,7 @@ func drawEbitenText(screen *ebiten.Image, x, y int, scale float32, line bool) {
 	if line {
 		op := &vector.StrokeOptions{}
 		op.Width = 5
+		op.LineJoin = vector.LineJoinRound
 		vs, is = path.AppendVerticesAndIndicesForStroke(nil, nil, op)
 	} else {
 		vs, is = path.AppendVerticesAndIndicesForFilling(nil, nil)
@@ -170,6 +171,7 @@ func drawEbitenLogo(screen *ebiten.Image, x, y int, scale float32, line bool) {
 	if line {
 		op := &vector.StrokeOptions{}
 		op.Width = 5
+		op.LineJoin = vector.LineJoinRound
 		vs, is = path.AppendVerticesAndIndicesForStroke(nil, nil, op)
 	} else {
 		vs, is = path.AppendVerticesAndIndicesForFilling(nil, nil)
@@ -211,6 +213,7 @@ func drawArc(screen *ebiten.Image, count int, scale float32, line bool) {
 	if line {
 		op := &vector.StrokeOptions{}
 		op.Width = 5
+		op.LineJoin = vector.LineJoinRound
 		vs, is = path.AppendVerticesAndIndicesForStroke(nil, nil, op)
 	} else {
 		vs, is = path.AppendVerticesAndIndicesForFilling(nil, nil)
@@ -267,6 +270,7 @@ func drawWave(screen *ebiten.Image, counter int, scale float32, line bool) {
 	if line {
 		op := &vector.StrokeOptions{}
 		op.Width = 5
+		op.LineJoin = vector.LineJoinRound
 		vs, is = path.AppendVerticesAndIndicesForStroke(nil, nil, op)
 	} else {
 		vs, is = path.AppendVerticesAndIndicesForFilling(nil, nil)

vector/path.go

@@ -72,18 +72,35 @@ func (p *Path) QuadTo(x1, y1, x2, y2 float32) {
 	p.quadTo(x1, y1, x2, y2, 0)
 }
 
+// lineForTwoPoints returns parameters for a line passing through p0 and p1.
+func lineForTwoPoints(p0, p1 point) (a, b, c float32) {
+	// Line passing through p0 and p1 in the form of ax + by + c = 0
+	a = p1.y - p0.y
+	b = -(p1.x - p0.x)
+	c = (p1.x-p0.x)*p0.y - (p1.y-p0.y)*p0.x
+	return
+}
+
 // isPointCloseToSegment detects the distance between a segment (x0, y0)-(x1, y1) and a point (x, y) is less than allow.
 func isPointCloseToSegment(x, y, x0, y0, x1, y1 float32, allow float32) bool {
-	// Line passing through (x0, y0) and (x1, y1) in the form of ax + by + c = 0
-	a := y1 - y0
-	b := -(x1 - x0)
-	c := (x1-x0)*y0 - (y1-y0)*x0
+	a, b, c := lineForTwoPoints(point{x: x0, y: y0}, point{x: x1, y: y1})
 
 	// The distance between a line ax+by+c=0 and (x0, y0) is
 	//     |ax0 + by0 + c| / √(a² + b²)
 	return allow*allow*(a*a+b*b) > (a*x+b*y+c)*(a*x+b*y+c)
 }
 
+// crossingPointForTwoLines returns a crossing point for two lines.
+func crossingPointForTwoLines(p00, p01, p10, p11 point) point {
+	a0, b0, c0 := lineForTwoPoints(p00, p01)
+	a1, b1, c1 := lineForTwoPoints(p10, p11)
+	det := a0*b1 - a1*b0
+	return point{
+		x: (b0*c1 - b1*c0) / det,
+		y: (a1*c0 - a0*c1) / det,
+	}
+}
+
 func (p *Path) quadTo(x1, y1, x2, y2 float32, level int) {
 	if level > 10 {
 		return
@@ -313,10 +330,23 @@ func (p *Path) AppendVerticesAndIndicesForFilling(vertices []ebiten.Vertex, indi
 	return vertices, indices
 }
 
+// LineJoin represents the way in which how two segments are joined.
+type LineJoin int
+
+const (
+	LineJoinMiter LineJoin = iota
+	LineJoinBevel
+	LineJoinRound
+)
+
 // StokeOptions is options to render a stroke.
 type StrokeOptions struct {
 	// Width is the stroke width in pixels.
 	Width float32
+
+	// LineJoin is the way in which how two segments are joined.
+	// The default (zero) value is LineJoiMiter.
+	LineJoin LineJoin
 }
 
 // AppendVerticesAndIndicesForStroke appends vertices and indices to render a stroke of this path and returns them.
@@ -327,6 +357,10 @@ type StrokeOptions struct {
 //
 // The returned values are intended to be passed to DrawTriangles or DrawTrianglesShader with FillAll fill mode, not EvenOdd fill mode.
 func (p *Path) AppendVerticesAndIndicesForStroke(vertices []ebiten.Vertex, indices []uint16, op *StrokeOptions) ([]ebiten.Vertex, []uint16) {
+	if op == nil {
+		return vertices, indices
+	}
+
 	for _, seg := range p.segs {
 		if len(seg) < 2 {
 			continue
@@ -406,14 +440,53 @@ func (p *Path) AppendVerticesAndIndicesForStroke(vertices []ebiten.Vertex, indic
 				continue
 			}
 
-			var arc Path
-			arc.MoveTo(c.x, c.y)
-			if da < math.Pi {
-				arc.Arc(c.x, c.y, op.Width/2, a0, a1, Clockwise)
-			} else {
-				arc.Arc(c.x, c.y, op.Width/2, a0+math.Pi, a1+math.Pi, CounterClockwise)
+			switch op.LineJoin {
+			case LineJoinMiter:
+				// TODO: Enable to configure this.
+				const miterLimit = 10
+				delta := math.Pi - da
+				exceed := math.Abs(1/math.Sin(float64(delta/2))) > miterLimit
+				var quad Path
+				quad.MoveTo(c.x, c.y)
+				if da < math.Pi {
+					quad.LineTo(rect[1].x, rect[1].y)
+					if !exceed {
+						pt := crossingPointForTwoLines(rect[0], rect[1], nextRect[0], nextRect[1])
+						quad.LineTo(pt.x, pt.y)
+					}
+					quad.LineTo(nextRect[0].x, nextRect[0].y)
+				} else {
+					quad.LineTo(rect[3].x, rect[3].y)
+					if !exceed {
+						pt := crossingPointForTwoLines(rect[2], rect[3], nextRect[2], nextRect[3])
+						quad.LineTo(pt.x, pt.y)
+					}
+					quad.LineTo(nextRect[2].x, nextRect[2].y)
+				}
+				vertices, indices = quad.AppendVerticesAndIndicesForFilling(vertices, indices)
+
+			case LineJoinBevel:
+				var tri Path
+				tri.MoveTo(c.x, c.y)
+				if da < math.Pi {
+					tri.LineTo(rect[1].x, rect[1].y)
+					tri.LineTo(nextRect[0].x, nextRect[0].y)
+				} else {
+					tri.LineTo(rect[3].x, rect[3].y)
+					tri.LineTo(nextRect[2].x, nextRect[2].y)
+				}
+				vertices, indices = tri.AppendVerticesAndIndicesForFilling(vertices, indices)
+
+			case LineJoinRound:
+				var arc Path
+				arc.MoveTo(c.x, c.y)
+				if da < math.Pi {
+					arc.Arc(c.x, c.y, op.Width/2, a0, a1, Clockwise)
+				} else {
+					arc.Arc(c.x, c.y, op.Width/2, a0+math.Pi, a1+math.Pi, CounterClockwise)
+				}
+				vertices, indices = arc.AppendVerticesAndIndicesForFilling(vertices, indices)
 			}
-			vertices, indices = arc.AppendVerticesAndIndicesForFilling(vertices, indices)
 		}
 	}
 	return vertices, indices