vector: add LineCap

Closes #1843

examples/lines/main.go

@@ -94,23 +94,31 @@ func (g *Game) Draw(screen *ebiten.Image) {
 		target = g.offscreen
 	}
 
+	joins := []vector.LineJoin{
+		vector.LineJoinMiter,
+		vector.LineJoinMiter,
+		vector.LineJoinBevel,
+		vector.LineJoinRound,
+	}
+	caps := []vector.LineCap{
+		vector.LineCapButt,
+		vector.LineCapRound,
+		vector.LineCapSquare,
+	}
+
 	ow, oh := target.Size()
-	size := min(ow/5, oh/4)
+	size := min(ow/(len(joins)+1), oh/(len(caps)+1))
-	offsetX, offsetY := (ow-size*4)/2, (oh-size*3)/2
+	offsetX, offsetY := (ow-size*len(joins))/2, (oh-size*len(caps))/2
 
 	// Render the lines on the target.
-	for j := 0; j < 3; j++ {
+	for j, cap := range caps {
-		for i, join := range []vector.LineJoin{
+		for i, join := range joins {
-			vector.LineJoinMiter,
-			vector.LineJoinMiter,
-			vector.LineJoinBevel,
-			vector.LineJoinRound} {
 			r := image.Rect(i*size+offsetX, j*size+offsetY, (i+1)*size+offsetX, (j+1)*size+offsetY)
 			miterLimit := float32(5)
 			if i == 1 {
 				miterLimit = 10
 			}
-			g.drawLine(target, r, join, miterLimit)
+			g.drawLine(target, r, cap, join, miterLimit)
 		}
 	}
 
@@ -127,7 +135,7 @@ 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, miterLimit float32) {
+func (g *Game) drawLine(screen *ebiten.Image, region image.Rectangle, cap vector.LineCap, join vector.LineJoin, miterLimit float32) {
 	c0x := float64(region.Min.X + region.Dx()/4)
 	c0y := float64(region.Min.Y + region.Dy()/4)
 	c1x := float64(region.Max.X - region.Dx()/4)
@@ -146,6 +154,7 @@ func (g *Game) drawLine(screen *ebiten.Image, region image.Rectangle, join vecto
 
 	// Draw the main line in white.
 	op := &vector.StrokeOptions{}
+	op.LineCap = cap
 	op.LineJoin = join
 	op.MiterLimit = miterLimit
 	op.Width = float32(r / 2)

vector/path.go

@@ -348,6 +348,15 @@ func (p *Path) AppendVerticesAndIndicesForFilling(vertices []ebiten.Vertex, indi
 	return vertices, indices
 }
 
+// LineCap represents the way in which how the ends of the stroke are rendered.
+type LineCap int
+
+const (
+	LineCapButt LineCap = iota
+	LineCapRound
+	LineCapSquare
+)
+
 // LineJoin represents the way in which how two segments are joined.
 type LineJoin int
 
@@ -362,6 +371,10 @@ type StrokeOptions struct {
 	// Width is the stroke width in pixels.
 	Width float32
 
+	// LineCap is the way in which how the ends of the stroke are rendered.
+	// The default (zero) value is LineCapButt.
+	LineCap LineCap
+
 	// LineJoin is the way in which how two segments are joined.
 	// The default (zero) value is LineJoiMiter.
 	LineJoin LineJoin
@@ -511,6 +524,68 @@ func (p *Path) AppendVerticesAndIndicesForStroke(vertices []ebiten.Vertex, indic
 				vertices, indices = arc.AppendVerticesAndIndicesForFilling(vertices, indices)
 			}
 		}
+
+		if len(rects) == 0 {
+			continue
+		}
+
+		switch op.LineCap {
+		case LineCapButt:
+			// Do nothing.
+
+		case LineCapRound:
+			startR, endR := rects[0], rects[len(rects)-1]
+			{
+				c := point{
+					x: (startR[0].x + startR[2].x) / 2,
+					y: (startR[0].y + startR[2].y) / 2,
+				}
+				a := float32(math.Atan2(float64(startR[0].y-startR[2].y), float64(startR[0].x-startR[2].x)))
+				var arc Path
+				arc.MoveTo(startR[0].x, startR[0].y)
+				arc.Arc(c.x, c.y, op.Width/2, a, a+math.Pi, CounterClockwise)
+				vertices, indices = arc.AppendVerticesAndIndicesForFilling(vertices, indices)
+			}
+			{
+				c := point{
+					x: (endR[1].x + endR[3].x) / 2,
+					y: (endR[1].y + endR[3].y) / 2,
+				}
+				a := float32(math.Atan2(float64(endR[1].y-endR[3].y), float64(endR[1].x-endR[3].x)))
+				var arc Path
+				arc.MoveTo(endR[1].x, endR[1].y)
+				arc.Arc(c.x, c.y, op.Width/2, a, a+math.Pi, Clockwise)
+				vertices, indices = arc.AppendVerticesAndIndicesForFilling(vertices, indices)
+			}
+
+		case LineCapSquare:
+			startR, endR := rects[0], rects[len(rects)-1]
+			{
+				a := math.Atan2(float64(startR[0].y-startR[1].y), float64(startR[0].x-startR[1].x))
+				s, c := math.Sincos(a)
+				dx, dy := float32(c)*op.Width/2, float32(s)*op.Width/2
+
+				var quad Path
+				quad.MoveTo(startR[0].x, startR[0].y)
+				quad.LineTo(startR[0].x+dx, startR[0].y+dy)
+				quad.LineTo(startR[2].x+dx, startR[2].y+dy)
+				quad.LineTo(startR[2].x, startR[2].y)
+				vertices, indices = quad.AppendVerticesAndIndicesForFilling(vertices, indices)
+			}
+			{
+				a := math.Atan2(float64(endR[1].y-endR[0].y), float64(endR[1].x-endR[0].x))
+				s, c := math.Sincos(a)
+				dx, dy := float32(c)*op.Width/2, float32(s)*op.Width/2
+
+				var quad Path
+				quad.MoveTo(endR[1].x, endR[1].y)
+				quad.LineTo(endR[1].x+dx, endR[1].y+dy)
+				quad.LineTo(endR[3].x+dx, endR[3].y+dy)
+				quad.LineTo(endR[3].x, endR[3].y)
+				vertices, indices = quad.AppendVerticesAndIndicesForFilling(vertices, indices)
+			}
+		}
 	}
+
 	return vertices, indices
 }