parallax and interfaces

engine/actor.go

@@ -5,6 +5,9 @@ import (
 	"image"
 )
 
+// Ensure Actor satisfies interfaces.
+var _ Prepper = &Actor{}
+
 func init() {
 	gob.Register(Actor{})
 }

engine/camera.go

@@ -1,11 +1,25 @@
 package engine
 
 import (
+	"encoding/gob"
 	"image"
 
 	"github.com/hajimehoshi/ebiten/v2"
 )
 
+// Ensure Camera satisfies interfaces.
+var (
+	_ Identifier = &Camera{}
+	_ Drawer     = &Camera{}
+	_ Prepper    = &Camera{}
+	_ Scanner    = &Camera{}
+	_ Updater    = &Camera{}
+)
+
+func init() {
+	gob.Register(Camera{})
+}
+
 // Camera models a camera that is viewing a scene.
 // Changes to the configuration take effect immediately.
 // Camera ignores Scene.Draw and calls Scene's children's Draw.
@@ -74,8 +88,8 @@ func (c *Camera) Draw(screen *ebiten.Image, opts ebiten.DrawImageOptions) {
 	for _, i := range c.Scene.Components {
 		if d, ok := i.(Drawer); ok {
 			cs := 1.0
-			if s, ok := i.(CoordScaler); ok {
-				cs = s.CoordScale()
+			if s, ok := i.(ParallaxScaler); ok {
+				cs = s.ParallaxFactor()
 			}
 			var geom ebiten.GeoM
 			// 1. Move centre to the origin, subject to CoordScale

engine/debug.go

@@ -13,6 +13,14 @@ import (
 	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
 )
 
+var (
+	_ Drawer      = PerfDisplay{}
+	_ DrawOrderer = PerfDisplay{}
+
+	_ Prepper = &GobDumper{}
+	_ Updater = &GobDumper{}
+)
+
 func init() {
 	gob.Register(GobDumper{})
 	gob.Register(PerfDisplay{})

engine/fill.go

@@ -6,10 +6,13 @@ import (
 	"github.com/hajimehoshi/ebiten/v2"
 )
 
+// Ensure Fill satisfies Drawer.
+var _ Drawer = &Fill{}
+
 // Fill fills the screen with a colour.
 type Fill struct {
 	Color color.Color
-	DrawOrder
+	ZOrder
 	Hidden bool
 	ID
 }

engine/image.go

@@ -0,0 +1,38 @@
+package engine
+
+import (
+	"encoding/gob"
+	"image"
+
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+// Ensure Image satisfies interfaces.
+var (
+	_ Identifier     = &Image{}
+	_ Drawer         = &Image{}
+	_ DrawOrderer    = &Image{}
+	_ ParallaxScaler = &Image{}
+)
+
+func init() {
+	gob.Register(Image{})
+}
+
+// Image draws an image at a position.
+type Image struct {
+	ID
+	Parallax
+	ZOrder
+	Src ImageRef
+	Pos image.Point
+}
+
+// Draw draws the image.
+func (i *Image) Draw(screen *ebiten.Image, opts ebiten.DrawImageOptions) {
+	var geom ebiten.GeoM
+	geom.Translate(float64(i.Pos.X), float64(i.Pos.Y))
+	geom.Concat(opts.GeoM)
+	opts.GeoM = geom
+	screen.DrawImage(i.Src.Image(), &opts)
+}

engine/interface.go

@@ -11,10 +11,10 @@ type Collider interface {
 	CollidesWith(image.Rectangle) bool
 }
 
-// CoordScaler components have a scaling factor. This is used for
-// e.g. parallax layers in a scene, and can be thought of as 1/distance.
-type CoordScaler interface {
-	CoordScale() float64
+// 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
 }
 
 // Drawer components can draw themselves. Draw is called often.

engine/misc.go

@@ -6,8 +6,14 @@ type ID string
 // Ident returns id as a string.
 func (id ID) Ident() string { return string(id) }
 
-// DrawOrder implements DrawOrderer directly (as a float64 value).
-type DrawOrder float64
+// 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 DrawOrderer directly (as a float64 value).
+type ZOrder float64
 
 // DrawOrder returns z as a float64.
-func (z DrawOrder) DrawOrder() float64 { return float64(z) }
+func (z ZOrder) DrawOrder() float64 { return float64(z) }

engine/scene.go

@@ -8,6 +8,16 @@ import (
 	"github.com/hajimehoshi/ebiten/v2"
 )
 
+// Ensure Scene satisfies interfaces.
+var (
+	_ Identifier  = &Scene{}
+	_ Drawer      = &Scene{}
+	_ DrawOrderer = &Scene{}
+	_ Prepper     = &Scene{}
+	_ Scanner     = &Scene{}
+	_ Updater     = &Scene{}
+)
+
 func init() {
 	gob.Register(Scene{})
 }
@@ -16,7 +26,7 @@ func init() {
 type Scene struct {
 	Components []interface{}
 	Disabled   bool
-	DrawOrder
+	ZOrder
 	Hidden bool
 	ID
 }

engine/solid.go

@@ -5,6 +5,8 @@ import (
 	"image"
 )
 
+var _ Collider = SolidRect{}
+
 func init() {
 	gob.Register(SolidRect{})
 }

engine/sprite.go

@@ -7,6 +7,15 @@ import (
 	"github.com/hajimehoshi/ebiten/v2"
 )
 
+// Ensure Sprite satisfies interfaces.
+var (
+	_ Identifier  = &Sprite{}
+	_ Drawer      = &Sprite{}
+	_ DrawOrderer = &Sprite{}
+	_ Scanner     = &Sprite{}
+	_ Updater     = &Sprite{}
+)
+
 func init() {
 	gob.Register(Sprite{})
 }
@@ -14,7 +23,7 @@ func init() {
 // Sprite combines an Actor with the ability to Draw from a single spritesheet.
 type Sprite struct {
 	Actor
-	DrawOrder
+	ZOrder
 	FrameSize   image.Point
 	FrameOffset image.Point
 	Hidden      bool

engine/tiles.go

@@ -7,6 +7,15 @@ import (
 	"github.com/hajimehoshi/ebiten/v2"
 )
 
+// Ensure Tilemap satisfies interfaces.
+var (
+	_ Identifier  = &Tilemap{}
+	_ Collider    = &Tilemap{}
+	_ Drawer      = &Tilemap{}
+	_ DrawOrderer = &Tilemap{}
+	_ Updater     = &Tilemap{}
+)
+
 func init() {
 	gob.Register(AnimatedTile{})
 	gob.Register(StaticTile(0))
@@ -16,7 +25,7 @@ func init() {
 // Tilemap renders a grid of tiles.
 type Tilemap struct {
 	Disabled bool
-	DrawOrder
+	ZOrder
 	Hidden bool
 	ID
 	Map      map[image.Point]Tile
@@ -114,6 +123,12 @@ type Tile interface {
 	TileIndex() int
 }
 
+// Ensure StaticTile and AnimatedTile satisfy Tile.
+var (
+	_ Tile = StaticTile(0)
+	_ Tile = &AnimatedTile{}
+)
+
 // StaticTile returns a fixed tile index.
 type StaticTile int
 

main.go

@@ -52,11 +52,18 @@ func main() {
 		Components: []interface{}{
 			&engine.Fill{
 				Color:  color.Gray{100},
-				DrawOrder: 0,
+				ZOrder: 0,
+			},
+			&engine.Image{
+				ID:       "bg_image",
+				Parallax: 0.5,
+				ZOrder:   1,
+				Pos:      image.Pt(-160, -120),
+				Src:      engine.ImageRef{Path: "assets/space.png"},
 			},
 			&engine.Tilemap{
 				ID:       "terrain",
-				DrawOrder: 1,
+				ZOrder:   2,
 				Map:      tiles,
 				Src:      engine.ImageRef{Path: "assets/boxes.png"},
 				TileSize: 16,
@@ -82,7 +89,7 @@ func main() {
 						Pos:             image.Pt(100, 100),
 						Size:            image.Pt(8, 16),
 					},
-					DrawOrder:   2,
+					ZOrder:      3,
 					FrameOffset: image.Pt(-1, 0),
 					FrameSize:   image.Pt(10, 16),
 					Src:         engine.ImageRef{Path: "assets/aw.png"},