Add starfield experiment to community examples

community/starfield/README.md

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+# starfield
+
+Classic starfield… with [supposedly accurate stellar colors](http://www.vendian.org/mncharity/dir3/starcolor/)
+
+Made by [Peter Hellberg](https://github.com/peterhellberg/) as part of his [pixel-experiments](https://github.com/peterhellberg/pixel-experiments)
+
+## Controls
+
+Arrow up and down to change speed. Space bar to almost stop.
+
+## Screenshots
+
+![starfield animation](https://user-images.githubusercontent.com/565124/32411599-a5fcba72-c1df-11e7-8730-a570470a4eee.gif)
+
+![starfield screenshot](screenshot.png)
+
+## Links
+
+ - https://github.com/peterhellberg/pixel-experiments/tree/master/starfield
+ - https://gist.github.com/peterhellberg/4018e228cced61a0bb26991e49299c96

community/starfield/starfield.go

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+package main
+
+import (
+	"image/color"
+	"math/rand"
+	"time"
+
+	"github.com/faiface/pixel"
+	"github.com/faiface/pixel/imdraw"
+	"github.com/faiface/pixel/pixelgl"
+)
+
+const w, h = float64(1024), float64(512)
+
+var speed = float64(200)
+
+var stars [1024]*star
+
+func init() {
+	rand.Seed(4)
+
+	for i := 0; i < len(stars); i++ {
+		stars[i] = newStar()
+	}
+}
+
+type star struct {
+	pixel.Vec
+	Z float64
+	P float64
+	C color.RGBA
+}
+
+func newStar() *star {
+	return &star{
+		pixel.V(random(-w, w), random(-h, h)),
+		random(0, w), 0, Colors[rand.Intn(len(Colors))],
+	}
+}
+
+func (s *star) update(d float64) {
+	s.P = s.Z
+	s.Z -= d * speed
+
+	if s.Z < 0 {
+		s.X = random(-w, w)
+		s.Y = random(-h, h)
+		s.Z = w
+		s.P = s.Z
+	}
+}
+
+func (s *star) draw(imd *imdraw.IMDraw) {
+	p := pixel.V(
+		scale(s.X/s.Z, 0, 1, 0, w),
+		scale(s.Y/s.Z, 0, 1, 0, h),
+	)
+
+	o := pixel.V(
+		scale(s.X/s.P, 0, 1, 0, w),
+		scale(s.Y/s.P, 0, 1, 0, h),
+	)
+
+	r := scale(s.Z, 0, w, 11, 0)
+
+	imd.Color = s.C
+
+	if p.Sub(o).Len() > 6 {
+		imd.Push(p, o)
+		imd.Line(r)
+	}
+
+	imd.Push(p)
+	imd.Circle(r, 0)
+}
+
+func run() {
+	win, err := pixelgl.NewWindow(pixelgl.WindowConfig{
+		Bounds:      pixel.R(0, 0, w, h),
+		VSync:       true,
+		Undecorated: true,
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	imd := imdraw.New(nil)
+
+	imd.Precision = 7
+
+	imd.SetMatrix(pixel.IM.Moved(win.Bounds().Center()))
+
+	last := time.Now()
+
+	for !win.Closed() {
+		win.SetClosed(win.JustPressed(pixelgl.KeyEscape) || win.JustPressed(pixelgl.KeyQ))
+
+		if win.Pressed(pixelgl.KeyUp) {
+			speed += 10
+		}
+
+		if win.Pressed(pixelgl.KeyDown) {
+			if speed > 10 {
+				speed -= 10
+			}
+		}
+
+		if win.Pressed(pixelgl.KeySpace) {
+			speed = 100
+		}
+
+		d := time.Since(last).Seconds()
+
+		last = time.Now()
+
+		imd.Clear()
+
+		for _, s := range stars {
+			s.update(d)
+			s.draw(imd)
+		}
+
+		win.Clear(color.Black)
+		imd.Draw(win)
+		win.Update()
+	}
+}
+
+func main() {
+	pixelgl.Run(run)
+}
+
+func random(min, max float64) float64 {
+	return rand.Float64()*(max-min) + min
+}
+
+func scale(unscaledNum, min, max, minAllowed, maxAllowed float64) float64 {
+	return (maxAllowed-minAllowed)*(unscaledNum-min)/(max-min) + minAllowed
+}
+
+// Colors based on stellar types listed at
+// http://www.vendian.org/mncharity/dir3/starcolor/
+var Colors = []color.RGBA{
+	color.RGBA{157, 180, 255, 255},
+	color.RGBA{162, 185, 255, 255},
+	color.RGBA{167, 188, 255, 255},
+	color.RGBA{170, 191, 255, 255},
+	color.RGBA{175, 195, 255, 255},
+	color.RGBA{186, 204, 255, 255},
+	color.RGBA{192, 209, 255, 255},
+	color.RGBA{202, 216, 255, 255},
+	color.RGBA{228, 232, 255, 255},
+	color.RGBA{237, 238, 255, 255},
+	color.RGBA{251, 248, 255, 255},
+	color.RGBA{255, 249, 249, 255},
+	color.RGBA{255, 245, 236, 255},
+	color.RGBA{255, 244, 232, 255},
+	color.RGBA{255, 241, 223, 255},
+	color.RGBA{255, 235, 209, 255},
+	color.RGBA{255, 215, 174, 255},
+	color.RGBA{255, 198, 144, 255},
+	color.RGBA{255, 190, 127, 255},
+	color.RGBA{255, 187, 123, 255},
+	color.RGBA{255, 187, 123, 255},
+}