From 97615b75957a73bcafcb33eeeb39beb0d22c4aa7 Mon Sep 17 00:00:00 2001
From: Thomas Friedel <tomee6@gmail.com>
Date: Wed, 24 Jan 2018 16:59:07 +0100
Subject: [PATCH] added polygon filling

---
 canvas.go        | 46 +++++++++++++++++++----
 triangulation.go | 97 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 136 insertions(+), 7 deletions(-)
 create mode 100644 triangulation.go

diff --git a/canvas.go b/canvas.go
index c5a37dc..c587883 100644
--- a/canvas.go
+++ b/canvas.go
@@ -155,13 +155,14 @@ func (cv *Canvas) Stroke() {
 		}
 		p1 := p.pos
 
-		v := p1.Sub(p0).Norm()
-		v = lm.Vec2{v[1], -v[0]}.MulF(cv.stroke.lineWidth * 0.5)
+		v1 := p1.Sub(p0).Norm()
+		v2 := lm.Vec2{v1[1], -v1[0]}.MulF(cv.stroke.lineWidth * 0.5)
+		v1 = v1.MulF(cv.stroke.lineWidth * 0.5)
 
-		x0f, y0f := cv.vecToGL(p0.Add(v))
-		x1f, y1f := cv.vecToGL(p1.Add(v))
-		x2f, y2f := cv.vecToGL(p1.Sub(v))
-		x3f, y3f := cv.vecToGL(p0.Sub(v))
+		x0f, y0f := cv.vecToGL(p0.Sub(v1).Add(v2))
+		x1f, y1f := cv.vecToGL(p1.Add(v1).Add(v2))
+		x2f, y2f := cv.vecToGL(p1.Add(v1).Sub(v2))
+		x3f, y3f := cv.vecToGL(p0.Sub(v1).Sub(v2))
 
 		tris = append(tris, x0f, y0f, x1f, y1f, x2f, y2f, x0f, y0f, x2f, y2f, x3f, y3f)
 
@@ -170,7 +171,7 @@ func (cv *Canvas) Stroke() {
 
 	gli.BufferData(gl_ARRAY_BUFFER, len(tris)*4, unsafe.Pointer(&tris[0]), gl_STREAM_DRAW)
 	gli.VertexAttribPointer(sr.vertex, 2, gl_FLOAT, false, 0, nil)
-	gli.DrawArrays(gl_TRIANGLES, 6, int32(len(tris)/2))
+	gli.DrawArrays(gl_TRIANGLES, 6, int32(len(tris)/2-6))
 
 	gli.ColorMask(true, true, true, true)
 	gli.StencilFunc(gl_EQUAL, 1, 0xFF)
@@ -182,3 +183,34 @@ func (cv *Canvas) Stroke() {
 
 	gli.Disable(gl_STENCIL_TEST)
 }
+
+func (cv *Canvas) Fill() {
+	if len(cv.path) < 3 {
+		return
+	}
+
+	cv.activate()
+
+	gli.UseProgram(sr.id)
+	gli.Uniform4f(sr.color, cv.fill.r, cv.fill.g, cv.fill.b, cv.fill.a)
+	gli.EnableVertexAttribArray(sr.vertex)
+
+	gli.BindBuffer(gl_ARRAY_BUFFER, buf)
+
+	var buf [1000]float32
+	tris := buf[:0]
+	tris = append(tris, -1, -1, -1, 1, 1, 1, -1, -1, 1, 1, 1, -1)
+
+	tris = triangulatePath(cv.path, tris)
+	total := len(tris)
+	for i := 12; i < total; i += 2 {
+		x, y := tris[i], tris[i+1]
+		tris[i], tris[i+1] = cv.ptToGL(x, y)
+	}
+
+	gli.BufferData(gl_ARRAY_BUFFER, len(tris)*4, unsafe.Pointer(&tris[0]), gl_STREAM_DRAW)
+	gli.VertexAttribPointer(sr.vertex, 2, gl_FLOAT, false, 0, nil)
+	gli.DrawArrays(gl_TRIANGLES, 6, int32(len(tris)/2-6))
+
+	gli.DisableVertexAttribArray(sr.vertex)
+}
diff --git a/triangulation.go b/triangulation.go
new file mode 100644
index 0000000..39b109c
--- /dev/null
+++ b/triangulation.go
@@ -0,0 +1,97 @@
+package canvas
+
+import (
+	"github.com/void6/lm"
+)
+
+func pointIsRightOfLine(a, b, p lm.Vec2) bool {
+	if a[1] == b[1] {
+		return false
+	}
+	if a[1] > b[1] {
+		a, b = b, a
+	}
+	if p[1] < a[1] || p[1] > b[1] {
+		return false
+	}
+	v := b.Sub(a)
+	r := (p[1] - a[1]) / v[1]
+	x := a[0] + r*v[0]
+	return p[0] > x
+}
+
+func triangleContainsPoint(a, b, c, p lm.Vec2) bool {
+	// if point is outside triangle bounds, return false
+	if p[0] < a[0] && p[0] < b[0] && p[0] < c[0] {
+		return false
+	}
+	if p[0] > a[0] && p[0] > b[0] && p[0] > c[0] {
+		return false
+	}
+	if p[1] < a[1] && p[1] < b[1] && p[1] < c[1] {
+		return false
+	}
+	if p[1] > a[1] && p[1] > b[1] && p[1] > c[1] {
+		return false
+	}
+	// check whether the point is to the right of each triangle line.
+	// if the total is 1, it is inside the triangle
+	count := 0
+	if pointIsRightOfLine(a, b, p) {
+		count++
+	}
+	if pointIsRightOfLine(b, c, p) {
+		count++
+	}
+	if pointIsRightOfLine(c, a, p) {
+		count++
+	}
+	return count == 1
+}
+
+func polygonContainsPoint(polygon []lm.Vec2, p lm.Vec2) bool {
+	a := polygon[len(polygon)-1]
+	count := 0
+	for _, b := range polygon {
+		if pointIsRightOfLine(a, b, p) {
+			count++
+		}
+		a = b
+	}
+	return count%2 == 1
+}
+
+func triangulatePath(path []pathPoint, target []float32) []float32 {
+	var buf [500]lm.Vec2
+	polygon := buf[:0]
+	for _, p := range path {
+		polygon = append(polygon, p.pos)
+	}
+
+	for len(polygon) > 2 {
+		var i int
+	triangles:
+		for i = range polygon {
+			a := polygon[i]
+			b := polygon[(i+1)%len(polygon)]
+			c := polygon[(i+2)%len(polygon)]
+			for i2, p := range polygon {
+				if i2 >= i && i2 <= i+2 {
+					continue
+				}
+				if triangleContainsPoint(a, b, c, p) {
+					continue triangles
+				}
+				center := a.Add(b).Add(c).DivF(3)
+				if !polygonContainsPoint(polygon, center) {
+					continue triangles
+				}
+			}
+			target = append(target, a[0], a[1], b[0], b[1], c[0], c[1])
+			break
+		}
+		remove := (i + 1) % len(polygon)
+		polygon = append(polygon[:remove], polygon[remove+1:]...)
+	}
+	return target
+}