Refactored code

community/parallax-scrolling-background/main.go

@@ -33,11 +33,6 @@ const (
 	foregroundSpeed = 120
 )
 
-type scrollingBackground struct {
-	Pic   pixel.Picture
-	speed int
-}
-
 func run() {
 	cfg := pixelgl.WindowConfig{
 		Title:  "Scrolling background demo",
@@ -59,45 +54,15 @@ func run() {
 		panic(err)
 	}
 
-	// Backgrounds are made taking the left and right halves of the image
-	background1 := pixel.NewSprite(picBackground, pixel.R(0, 0, windowWidth, windowHeight))
-	background2 := pixel.NewSprite(picBackground, pixel.R(windowWidth, 0, windowWidth*2, windowHeight))
-	foreground1 := pixel.NewSprite(picForeground, pixel.R(0, 0, windowWidth, foregroundHeight))
-	foreground2 := pixel.NewSprite(picForeground, pixel.R(windowWidth, 0, windowWidth*2, foregroundHeight))
+	background := newScrollingBackground(picBackground, windowWidth, windowHeight, windowWidth)
+	foreground := newScrollingBackground(picForeground, windowWidth, foregroundHeight, windowWidth)
 
-	// In the beginning, vector1 will put background1 filling the whole window, while vector2 will
-	// put background2 just at the right side of the window, out of view
-	backgroundVector1 := pixel.V(windowWidth/2, (windowHeight/2)+1)
-	backgroundVector2 := pixel.V(windowWidth+(windowWidth/2), (windowHeight/2)+1)
-
-	foregroundVector1 := pixel.V(windowWidth/2, (foregroundHeight/2)+1)
-	foregroundVector2 := pixel.V(windowWidth+(windowWidth/2), (foregroundHeight/2)+1)
-
-	bi, fi := 0., 0.
 	last := time.Now()
 	for !win.Closed() {
 		dt := time.Since(last).Seconds()
 		last = time.Now()
-		// When one of the backgrounds has completely scrolled, we swap displacement vectors,
-		// so the backgrounds will swap positions too regarding the previous iteration,
-		// thus making the background endless.
-		if bi <= -windowWidth {
-			bi = 0
-			backgroundVector1, backgroundVector2 = backgroundVector2, backgroundVector1
-		}
-		if fi <= -windowWidth {
-			fi = 0
-			foregroundVector1, foregroundVector2 = foregroundVector2, foregroundVector1
-		}
-		// This delta vector will move the backgrounds to the left
-		db := pixel.V(-bi, 0)
-		df := pixel.V(-fi, 0)
-		background1.Draw(win, pixel.IM.Moved(backgroundVector1.Sub(db)))
-		background2.Draw(win, pixel.IM.Moved(backgroundVector2.Sub(db)))
-		foreground1.Draw(win, pixel.IM.Moved(foregroundVector1.Sub(df)))
-		foreground2.Draw(win, pixel.IM.Moved(foregroundVector2.Sub(df)))
-		bi -= backgroundSpeed * dt
-		fi -= foregroundSpeed * dt
+		background.update(win, backgroundSpeed, dt)
+		foreground.update(win, foregroundSpeed, dt)
 		win.Update()
 	}
 }

community/parallax-scrolling-background/scrolling_background.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"github.com/faiface/pixel"
+	"github.com/faiface/pixel/pixelgl"
+)
+
+type scrollingBackground struct {
+	width               float64
+	height              float64
+	windowWidth         float64
+	displacementCounter float64
+	backgrounds         [2]*pixel.Sprite
+	positions           [2]pixel.Vec
+}
+
+func newScrollingBackground(pic pixel.Picture, width, height, windowWidth float64) *scrollingBackground {
+	return &scrollingBackground{
+		width:       width,
+		height:      height,
+		windowWidth: windowWidth,
+		backgrounds: [2]*pixel.Sprite{
+			pixel.NewSprite(pic, pixel.R(0, 0, width, height)),
+			pixel.NewSprite(pic, pixel.R(width, 0, width*2, height)),
+		},
+		positions: [2]pixel.Vec{
+			pixel.V(width/2, (height/2)+1),
+			pixel.V(width+(width/2), (height/2)+1),
+		},
+	}
+}
+
+func (sb *scrollingBackground) update(win *pixelgl.Window, speed, dt float64) {
+	if sb.displacementCounter <= -sb.windowWidth {
+		sb.displacementCounter = 0
+		sb.positions[0], sb.positions[1] = sb.positions[1], sb.positions[0]
+	}
+	d := pixel.V(-sb.displacementCounter, 0)
+	sb.backgrounds[0].Draw(win, pixel.IM.Moved(sb.positions[0].Sub(d)))
+	sb.backgrounds[1].Draw(win, pixel.IM.Moved(sb.positions[1].Sub(d)))
+	sb.displacementCounter -= speed * dt
+}