make billboard more complicated

engine/billboard.go

@@ -2,7 +2,6 @@ package engine
 
 import (
 	"encoding/gob"
-	"image"
 
 	"drjosh.dev/gurgle/geom"
 	"github.com/hajimehoshi/ebiten/v2"
@@ -10,6 +9,7 @@ import (
 
 // Ensure Billboard satisfies interfaces.
 var _ interface {
+	BoundingBoxer
 	Identifier
 	Drawer
 	Scanner
@@ -24,9 +24,19 @@ func init() {
 type Billboard struct {
 	ID
 	Hides
-	Pos image.Point
+	Pos geom.Int3
 	Src ImageRef
-	ZPosition
+
+	game *Game
+}
+
+// BoundingBox returns a 0-depth box incorporating the image size.
+func (b *Billboard) BoundingBox() geom.Box {
+	sx, sy := b.Src.Image().Size()
+	return geom.Box{
+		Min: b.Pos,
+		Max: b.Pos.Add(geom.Pt3(sx, sy, 0)),
+	}
 }
 
 // Draw draws the image.
@@ -34,10 +44,41 @@ func (b *Billboard) Draw(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
 	screen.DrawImage(b.Src.Image(), opts)
 }
 
+// DrawAfter reports if b.Pos.Z >= x.Max.Z.
+func (b *Billboard) DrawAfter(x Drawer) bool {
+	switch x := x.(type) {
+	case BoundingBoxer:
+		return b.Pos.Z >= x.BoundingBox().Max.Z
+	case ZPositioner:
+		return b.Pos.Z > x.ZPos()
+	}
+	return false
+}
+
+// DrawBefore reports if b.Pos.Z < x.Min.Z.
+func (b *Billboard) DrawBefore(x Drawer) bool {
+	switch x := x.(type) {
+	case BoundingBoxer:
+		return b.Pos.Z < x.BoundingBox().Min.Z
+	case ZPositioner:
+		return b.Pos.Z < x.ZPos()
+	}
+	return false
+}
+
+// Prepare saves the reference to Game.
+func (b *Billboard) Prepare(g *Game) error {
+	b.game = g
+	return nil
+}
+
 // Scan returns a slice containing Src.
 func (b *Billboard) Scan() []interface{} { return []interface{}{&b.Src} }
 
+// Transform returns a translation by the projected position.
 func (b *Billboard) Transform() (opts ebiten.DrawImageOptions) {
-	opts.GeoM.Translate(geom.CFloat(b.Pos))
+	opts.GeoM.Translate(geom.CFloat(
+		b.game.Projection.Project(b.Pos),
+	))
 	return opts
 }

game/level1.go

@@ -23,8 +23,7 @@ func Level1() *engine.Scene {
 				CameraID: "game_camera",
 				Child: &engine.Billboard{
 					ID:  "bg_image",
-					ZPosition: -900,
-					Pos:       image.Pt(-160, -120),
+					Pos: geom.Pt3(-160, -120, -1),
 					Src: engine.ImageRef{Path: "assets/space.png"},
 				},
 				Factor: 0.5,