Matrix, WritePNG, Ellipse example

context.go

@@ -46,6 +46,7 @@ type Context struct {
 	lineJoin  LineJoin
 	fillRule  FillRule
 	fontFace  font.Face
+	matrix    Matrix
 }
 
 func NewContext(width, height int) *Context {
@@ -65,6 +66,7 @@ func NewContextForRGBA(im *image.RGBA) *Context {
 		lineWidth: 1,
 		fillRule:  FillRuleWinding,
 		fontFace:  basicfont.Face7x13,
+		matrix:    Identity(),
 	}
 }
 
@@ -80,8 +82,8 @@ func (dc *Context) Height() int {
 	return dc.height
 }
 
-func (dc *Context) WriteToPNG(path string) error {
-	return writeToPNG(path, dc.im)
+func (dc *Context) WritePNG(path string) error {
+	return writePNG(path, dc.im)
 }
 
 func (dc *Context) SetLineWidth(lineWidth float64) {
@@ -163,11 +165,13 @@ func (dc *Context) SetRGB(r, g, b float64) {
 // Path Manipulation
 
 func (dc *Context) MoveTo(x, y float64) {
+	x, y = dc.TransformPoint(x, y)
 	dc.start = fp(x, y)
 	dc.path.Start(dc.start)
 }
 
 func (dc *Context) LineTo(x, y float64) {
+	x, y = dc.TransformPoint(x, y)
 	if len(dc.path) == 0 {
 		dc.MoveTo(x, y)
 	} else {
@@ -176,6 +180,8 @@ func (dc *Context) LineTo(x, y float64) {
 }
 
 func (dc *Context) QuadraticTo(x1, y1, x2, y2 float64) {
+	x1, y1 = dc.TransformPoint(x1, y1)
+	x2, y2 = dc.TransformPoint(x2, y2)
 	if len(dc.path) == 0 {
 		dc.MoveTo(x1, y1)
 	} else {
@@ -307,6 +313,7 @@ func (dc *Context) LoadFontFace(path string, size float64) {
 }
 
 func (dc *Context) DrawString(x, y float64, s string) {
+	x, y = dc.TransformPoint(x, y)
 	d := &font.Drawer{
 		Dst:  dc.im,
 		Src:  image.NewUniform(dc.color),
@@ -325,3 +332,33 @@ func (dc *Context) MeasureString(s string) float64 {
 	a := d.MeasureString(s)
 	return float64(a >> 6)
 }
+
+// Transformation Matrix Operations
+
+func (dc *Context) Identity() {
+	dc.matrix = Identity()
+}
+
+func (dc *Context) Translate(x, y float64) {
+	dc.matrix = dc.matrix.Translate(x, y)
+}
+
+func (dc *Context) Scale(x, y float64) {
+	dc.matrix = dc.matrix.Scale(x, y)
+}
+
+func (dc *Context) Rotate(angle float64) {
+	dc.matrix = dc.matrix.Rotate(angle)
+}
+
+func (dc *Context) RotateAbout(angle, x, y float64) {
+	dc.matrix = dc.matrix.RotateAbout(angle, x, y)
+}
+
+func (dc *Context) Shear(x, y float64) {
+	dc.matrix = dc.matrix.Shear(x, y)
+}
+
+func (dc *Context) TransformPoint(x, y float64) (tx, ty float64) {
+	return dc.matrix.TransformPoint(x, y)
+}

examples/circle.go

@@ -12,5 +12,5 @@ func main() {
 	dc.SetRGB(0, 0, 0.5)
 	dc.SetLineWidth(8)
 	dc.Stroke()
-	dc.WriteToPNG("out.png")
+	dc.WritePNG("out.png")
 }

examples/ellipse.go

@@ -0,0 +1,18 @@
+package main
+
+import "github.com/fogleman/gg"
+
+func main() {
+	const S = 1024
+	dc := gg.NewContext(S, S)
+	dc.SetRGB(1, 1, 1)
+	dc.Clear()
+	for i := 0; i < 360; i += 15 {
+		dc.Identity()
+		dc.RotateAbout(gg.Radians(float64(i)), S/2, S/2)
+		dc.DrawEllipse(S/2, S/2, S*7/16, S/8)
+		dc.SetRGBA(0, 0, 0, 0.1)
+		dc.Fill()
+	}
+	dc.WritePNG("out.png")
+}

examples/example0.go

@@ -16,5 +16,5 @@ func main() {
 	dc.SetRGB(0, 0, 0)
 	dc.SetLineWidth(8)
 	dc.Stroke()
-	dc.WriteToPNG("out.png")
+	dc.WritePNG("out.png")
 }

examples/lines.go

@@ -27,5 +27,5 @@ func main() {
 		dc.DrawLine(x1, y1, x2, y2)
 		dc.Stroke()
 	}
-	dc.WriteToPNG("out.png")
+	dc.WritePNG("out.png")
 }

examples/star.go

@@ -36,5 +36,5 @@ func main() {
 	dc.SetRGBA(0, 1, 0, 0.5)
 	dc.SetLineWidth(16)
 	dc.Stroke()
-	dc.WriteToPNG("out.png")
+	dc.WritePNG("out.png")
 }

examples/text.go

@@ -11,5 +11,5 @@ func main() {
 	s := "Hello, world!"
 	w := dc.MeasureString(s)
 	dc.DrawString(500-w/2, 500, s)
-	dc.WriteToPNG("out.png")
+	dc.WritePNG("out.png")
 }

matrix.go

@@ -0,0 +1,99 @@
+package gg
+
+import "math"
+
+type Matrix struct {
+	XX, YX, XY, YY, X0, Y0 float64
+}
+
+func Identity() Matrix {
+	return Matrix{
+		1, 0,
+		0, 1,
+		0, 0,
+	}
+}
+
+func Translate(x, y float64) Matrix {
+	return Matrix{
+		1, 0,
+		0, 1,
+		x, y,
+	}
+}
+
+func Scale(x, y float64) Matrix {
+	return Matrix{
+		x, 0,
+		0, y,
+		0, 0,
+	}
+}
+
+func Rotate(angle float64) Matrix {
+	c := math.Cos(angle)
+	s := math.Sin(angle)
+	return Matrix{
+		c, s,
+		-s, c,
+		0, 0,
+	}
+}
+
+func RotateAbout(angle, x, y float64) Matrix {
+	a := Translate(x, y)
+	b := Rotate(angle)
+	c := Translate(-x, -y)
+	return c.Multiply(b).Multiply(a)
+}
+
+func Shear(x, y float64) Matrix {
+	return Matrix{
+		1, x,
+		y, 1,
+		0, 0,
+	}
+}
+
+func (a Matrix) Multiply(b Matrix) Matrix {
+	return Matrix{
+		a.XX*b.XX + a.YX*b.XY,
+		a.XX*b.YX + a.YX*b.YY,
+		a.XY*b.XX + a.YY*b.XY,
+		a.XY*b.YX + a.YY*b.YY,
+		a.X0*b.XX + a.Y0*b.XY + b.X0,
+		a.X0*b.YX + a.Y0*b.YY + b.Y0,
+	}
+}
+
+func (a Matrix) TransformVector(x, y float64) (tx, ty float64) {
+	tx = a.XX*x + a.XY*y
+	ty = a.YX*x + a.YY*y
+	return
+}
+
+func (a Matrix) TransformPoint(x, y float64) (tx, ty float64) {
+	tx = a.XX*x + a.XY*y + a.X0
+	ty = a.YX*x + a.YY*y + a.Y0
+	return
+}
+
+func (a Matrix) Translate(x, y float64) Matrix {
+	return Translate(x, y).Multiply(a)
+}
+
+func (a Matrix) Scale(x, y float64) Matrix {
+	return Scale(x, y).Multiply(a)
+}
+
+func (a Matrix) Rotate(angle float64) Matrix {
+	return Rotate(angle).Multiply(a)
+}
+
+func (a Matrix) RotateAbout(angle, x, y float64) Matrix {
+	return RotateAbout(angle, x, y).Multiply(a)
+}
+
+func (a Matrix) Shear(x, y float64) Matrix {
+	return Shear(x, y).Multiply(a)
+}

util.go

@@ -6,6 +6,7 @@ import (
 	"image/draw"
 	"image/png"
 	"io/ioutil"
+	"math"
 	"os"
 	"strings"
 
@@ -15,7 +16,15 @@ import (
 	"golang.org/x/image/math/fixed"
 )
 
-func writeToPNG(path string, im image.Image) error {
+func Radians(degrees float64) float64 {
+	return degrees * math.Pi / 180
+}
+
+func Degrees(radians float64) float64 {
+	return radians * 180 / math.Pi
+}
+
+func writePNG(path string, im image.Image) error {
 	file, err := os.Create(path)
 	if err != nil {
 		return err