examples: add Doppler Stereo Room

examples/doppler-stereo-room/README.md

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+# Doppler Stereo Room
+
+**Use headphones for this one!**
+
+There are two speakers on the screen: green and blue. The green speaker plays the left stereo channel and the blue speaker plays the right.
+
+The black square is your head.
+
+You can move the speakers around as you like.
+
+The 3D effect is achieved merely by [delaying the sound](https://en.wikipedia.org/wiki/Sound_localization) in one of the ears. Volume is always the same in both ears.
+
+Things for you to try:
+
+- Move both speakers to the same side of your head.
+- Move one of the speakers past your head at a fast speed.
+
+![Screenshot](screenshot.png)

examples/doppler-stereo-room/main.go

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+package main
+
+import (
+	"fmt"
+	"math"
+	"os"
+	"time"
+	"unicode"
+
+	"github.com/faiface/beep"
+	"github.com/faiface/beep/effects"
+	"github.com/faiface/beep/mp3"
+	"github.com/faiface/beep/speaker"
+	"github.com/gdamore/tcell"
+)
+
+func multiplyChannels(left, right float64, s beep.Streamer) beep.Streamer {
+	return beep.StreamerFunc(func(samples [][2]float64) (n int, ok bool) {
+		n, ok = s.Stream(samples)
+		for i := range samples[:n] {
+			samples[i][0] *= left
+			samples[i][1] *= right
+		}
+		return n, ok
+	})
+}
+
+type movingStreamer struct {
+	x, y         float64
+	velX, velY   float64
+	leftDoppler  beep.Streamer
+	rightDoppler beep.Streamer
+}
+
+func newMovingStreamer(sr beep.SampleRate, x, y float64, streamer beep.Streamer) *movingStreamer {
+	ms := &movingStreamer{x: x, y: y}
+
+	const metersPerSecond = 343
+	samplesPerSecond := float64(sr)
+	samplesPerMeter := samplesPerSecond / metersPerSecond
+
+	leftEar, rightEar := beep.Dup(streamer)
+	leftEar = multiplyChannels(1, 0, leftEar)
+	rightEar = multiplyChannels(0, 1, rightEar)
+
+	const earDistance = 0.16
+	ms.leftDoppler = effects.Doppler(2, samplesPerMeter, leftEar, func(delta int) float64 {
+		dt := sr.D(delta).Seconds()
+		ms.x += ms.velX * dt
+		ms.y += ms.velY * dt
+		return math.Max(0.25, math.Hypot(ms.x+earDistance/2, ms.y))
+	})
+	ms.rightDoppler = effects.Doppler(2, samplesPerMeter, rightEar, func(delta int) float64 {
+		return math.Max(0.25, math.Hypot(ms.x-earDistance/2, ms.y))
+	})
+
+	return ms
+}
+
+func (ms *movingStreamer) play() {
+	speaker.Play(ms.leftDoppler, ms.rightDoppler)
+}
+
+func drawCircle(screen tcell.Screen, x, y float64, style tcell.Style) {
+	width, height := screen.Size()
+	centerX, centerY := float64(width)/2, float64(height)/2
+
+	lx, ly := int(centerX+(x-0.25)*2), int(centerY+y)
+	screen.SetContent(lx, ly, tcell.RuneBlock, nil, style)
+
+	rx, ry := int(centerX+(x+0.25)*2), int(centerY+y)
+	screen.SetContent(rx, ry, tcell.RuneBlock, nil, style)
+}
+
+func drawTextLine(screen tcell.Screen, x, y int, s string, style tcell.Style) {
+	for _, r := range s {
+		screen.SetContent(x, y, r, nil, style)
+		x++
+	}
+}
+
+func drawHelp(screen tcell.Screen, style tcell.Style) {
+	drawTextLine(screen, 0, 0, "Welcome to the Doppler Stereo Room!", style)
+	drawTextLine(screen, 0, 1, "Press [ESC] to quit.", style)
+
+	drawTextLine(screen, 0, 2, "Move the", style)
+	drawTextLine(screen, 9, 2, "LEFT", style.Background(tcell.ColorGreen).Foreground(tcell.ColorWhiteSmoke))
+	drawTextLine(screen, 14, 2, "speaker with WASD.", style)
+
+	drawTextLine(screen, 0, 3, "Move the", style)
+	drawTextLine(screen, 9, 3, "RIGHT", style.Background(tcell.ColorBlue).Foreground(tcell.ColorWhiteSmoke))
+	drawTextLine(screen, 15, 3, "speaker with IJKL.", style)
+
+	drawTextLine(screen, 0, 4, "Press to start moving, press again to stop. Use [SHIFT] to move fast.", style)
+}
+
+var directions = map[rune]struct{ lx, ly, rx, ry float64 }{
+	'a': {-1, 0, 0, 0},
+	'd': {+1, 0, 0, 0},
+	'w': {0, -1, 0, 0},
+	's': {0, +1, 0, 0},
+	'j': {0, 0, -1, 0},
+	'l': {0, 0, +1, 0},
+	'i': {0, 0, 0, -1},
+	'k': {0, 0, 0, +1},
+}
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Fprintf(os.Stderr, "Usage: %s song.mp3\n", os.Args[0])
+		os.Exit(1)
+	}
+	f, err := os.Open(os.Args[1])
+	if err != nil {
+		report(err)
+	}
+	streamer, format, err := mp3.Decode(f)
+	if err != nil {
+		report(err)
+	}
+	defer streamer.Close()
+
+	speaker.Init(format.SampleRate, format.SampleRate.N(time.Second/30))
+
+	leftCh, rightCh := beep.Dup(streamer)
+
+	leftCh = effects.Mono(multiplyChannels(1, 0, leftCh))
+	rightCh = effects.Mono(multiplyChannels(0, 1, rightCh))
+
+	leftMS := newMovingStreamer(format.SampleRate, -2, 0, leftCh)
+	rightMS := newMovingStreamer(format.SampleRate, +2, 0, rightCh)
+
+	leftMS.play()
+	rightMS.play()
+
+	screen, err := tcell.NewScreen()
+	if err != nil {
+		report(err)
+	}
+	err = screen.Init()
+	if err != nil {
+		report(err)
+	}
+	defer screen.Fini()
+
+	frames := time.Tick(time.Second / 30)
+	events := make(chan tcell.Event)
+	go func() {
+		for {
+			events <- screen.PollEvent()
+		}
+	}()
+
+loop:
+	for {
+		select {
+		case <-frames:
+			speaker.Lock()
+			lx, ly := leftMS.x, leftMS.y
+			rx, ry := rightMS.x, rightMS.y
+			speaker.Unlock()
+
+			style := tcell.StyleDefault.
+				Background(tcell.ColorWhiteSmoke).
+				Foreground(tcell.ColorBlack)
+
+			screen.Clear()
+			screen.Fill(' ', style)
+			drawHelp(screen, style)
+			drawCircle(screen, 0, 0, style.Foreground(tcell.ColorBlack))
+			drawCircle(screen, lx*2, ly*2, style.Foreground(tcell.ColorGreen))
+			drawCircle(screen, rx*2, ry*2, style.Foreground(tcell.ColorBlue))
+			screen.Show()
+
+		case event := <-events:
+			switch event := event.(type) {
+			case *tcell.EventKey:
+				if event.Key() == tcell.KeyESC {
+					break loop
+				}
+
+				if event.Key() != tcell.KeyRune {
+					break
+				}
+
+				const (
+					slowSpeed = 2.0
+					fastSpeed = 16.0
+				)
+
+				speaker.Lock()
+
+				speed := slowSpeed
+				if unicode.ToLower(event.Rune()) != event.Rune() {
+					speed = fastSpeed
+				}
+
+				dir := directions[unicode.ToLower(event.Rune())]
+
+				if dir.lx != 0 {
+					if leftMS.velX == dir.lx*speed {
+						leftMS.velX = 0
+					} else {
+						leftMS.velX = dir.lx * speed
+					}
+				}
+				if dir.ly != 0 {
+					if leftMS.velY == dir.ly*speed {
+						leftMS.velY = 0
+					} else {
+						leftMS.velY = dir.ly * speed
+					}
+				}
+				if dir.rx != 0 {
+					if rightMS.velX == dir.rx*speed {
+						rightMS.velX = 0
+					} else {
+						rightMS.velX = dir.rx * speed
+					}
+				}
+				if dir.ry != 0 {
+					if rightMS.velY == dir.ry*speed {
+						rightMS.velY = 0
+					} else {
+						rightMS.velY = dir.ry * speed
+					}
+				}
+
+				speaker.Unlock()
+			}
+		}
+	}
+}
+
+func report(err error) {
+	fmt.Fprintln(os.Stderr, err)
+	os.Exit(1)
+}