improved parallax implementation

engine/billboard.go

@@ -11,7 +11,6 @@ import (
 var _ interface {
 	Identifier
 	Drawer
-	ParallaxScaler
 	Scanner
 	Transformer
 } = &Billboard{}
@@ -24,7 +23,6 @@ func init() {
 type Billboard struct {
 	ID
 	Hidden
-	Parallax
 	Pos image.Point
 	Src ImageRef
 	ZOrder

engine/interface.go

@@ -21,7 +21,6 @@ var (
 	HiderType       = reflect.TypeOf((*Hider)(nil)).Elem()
 	IdentifierType  = reflect.TypeOf((*Identifier)(nil)).Elem()
 	LoaderType      = reflect.TypeOf((*Loader)(nil)).Elem()
-	ParallaxScalerType = reflect.TypeOf((*ParallaxScaler)(nil)).Elem()
 	PrepperType     = reflect.TypeOf((*Prepper)(nil)).Elem()
 	ScannerType     = reflect.TypeOf((*Scanner)(nil)).Elem()
 	ScenerType      = reflect.TypeOf((*Scener)(nil)).Elem()
@@ -40,7 +39,6 @@ var (
 		HiderType,
 		IdentifierType,
 		LoaderType,
-		ParallaxScalerType,
 		PrepperType,
 		ScannerType,
 		ScenerType,
@@ -109,12 +107,6 @@ type Loader interface {
 	Load(fs.FS) error
 }
 
-// ParallaxScaler components have a scaling factor. This is used for
-// parallax layers in a scene, and can be thought of as 1/distance.
-type ParallaxScaler interface {
-	ParallaxFactor() float64
-}
-
 // Prepper components can be prepared. It is called after the component
 // database has been populated but before the game is run. The component can
 // store the reference to game, if needed, and also query the component database.

engine/misc.go

@@ -38,12 +38,6 @@ func (h *Hidden) Hide() { *h = true }
 // Show sets h to false.
 func (h *Hidden) Show() { *h = false }
 
-// Parallax implements ParallaxScaler directly (as a float64 value).
-type Parallax float64
-
-// ParallaxFactor returns s as a float64.
-func (s Parallax) ParallaxFactor() float64 { return float64(s) }
-
 // ZOrder implements DrawOrder (in Drawer) directly (as a float64 value).
 type ZOrder float64
 

engine/parallax.go

@@ -0,0 +1,45 @@
+package engine
+
+import (
+	"encoding/gob"
+	"fmt"
+
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+var _ interface {
+	Prepper
+	Scanner
+	Transformer
+} = &Parallax{}
+
+func init() {
+	gob.Register(&Parallax{})
+}
+
+// Parallax is a container that changes its transform based on the position of a
+// camera, intended to produce a "parallax" like effect.
+type Parallax struct {
+	CameraID string
+	Factor   float64 // how much to change in response to the camera
+	Child    interface{}
+
+	camera *Camera
+}
+
+func (p *Parallax) Prepare(game *Game) error {
+	c, ok := game.Component(p.CameraID).(*Camera)
+	if !ok {
+		return fmt.Errorf("component %q type != *Camera", p.CameraID)
+	}
+	p.camera = c
+	return nil
+}
+
+func (p *Parallax) Scan() []interface{} { return []interface{}{p.Child} }
+
+func (p *Parallax) Transform() (opts ebiten.DrawImageOptions) {
+	x, y := float2(p.camera.Centre)
+	opts.GeoM.Translate(x*p.Factor, y*p.Factor)
+	return opts
+}

main.go

@@ -16,7 +16,7 @@ import (
 
 func main() {
 	// Change to true to enable cpu profile
-	if true && runtime.GOOS != "js" {
+	if false && runtime.GOOS != "js" {
 		f, err := os.Create("cpuprofile.pprof")
 		if err != nil {
 			log.Fatal("could not create CPU profile: ", err)
@@ -33,7 +33,7 @@ func main() {
 	ebiten.SetWindowTitle("TODO")
 
 	// Change to true to rewrite level1.gobz
-	if false && runtime.GOOS != "js" {
+	if true && runtime.GOOS != "js" {
 		writeLevel1()
 	}
 
@@ -116,13 +116,16 @@ func writeLevel1() {
 				Color:  color.Gray{100},
 				ZOrder: 0,
 			},
-			&engine.Billboard{
+			&engine.Parallax{
+				CameraID: "game_camera",
+				Child: &engine.Billboard{
 					ID:     "bg_image",
-				Parallax: 0.5,
 					ZOrder: 1,
 					Pos:    image.Pt(-160, -120),
 					Src:    engine.ImageRef{Path: "assets/space.png"},
 				},
+				Factor: 0.5,
+			},
 			&engine.Tilemap{
 				ID:     "terrain",
 				ZOrder: 2,