fix out-of-order detection, tweak sprite

engine/fill.go

@@ -0,0 +1,22 @@
+package engine
+
+import (
+	"image/color"
+
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+// Fill fills the screen with a colour.
+type Fill struct {
+	Color  color.Color
+	Hidden bool
+	ID
+	ZPos
+}
+
+func (f *Fill) Draw(screen *ebiten.Image, _ ebiten.GeoM) {
+	if f.Hidden {
+		return
+	}
+	screen.Fill(f.Color)
+}

engine/misc.go

@@ -1,7 +1,6 @@
 package engine
 
 import (
-	"image/color"
 	"unsafe"
 
 	"github.com/hajimehoshi/ebiten/v2"
@@ -31,12 +30,3 @@ func ToGeoMDef(m *ebiten.GeoM) *GeoMDef {
 func (d *GeoMDef) GeoM() *ebiten.GeoM {
 	return (*ebiten.GeoM)(unsafe.Pointer(d))
 }
-
-// Fill fills the image with a colour.
-type Fill struct {
-	Color color.Color
-}
-
-func (f Fill) Draw(screen *ebiten.Image, _ ebiten.GeoM) {
-	screen.Fill(color.Color(f.Color))
-}

engine/scene.go

@@ -36,9 +36,12 @@ func (s *Scene) Draw(screen *ebiten.Image, geom ebiten.GeoM) {
 	}
 }
 
+// Prepare does an initial Z-order sort.
+func (s *Scene) Prepare(*Game) { s.sortByZ() }
+
 // sortByZ sorts the components by Z position.
-// Stable sort is used to avoid Z-fighting among layers without a Z, or
-// among those with equal Z. All non-ZPositioners are sorted first.
+// Everything without a Z sorts first. Stable sort is used to avoid Z-fighting
+// (among layers without a Z, or those with equal Z).
 func (s *Scene) sortByZ() {
 	sort.SliceStable(s.Components, func(i, j int) bool {
 		a, aok := s.Components[i].(ZPositioner)
@@ -58,8 +61,7 @@ func (s *Scene) Update() error {
 	if s.Disabled {
 		return nil
 	}
-	needsSort := false
-	curZ := -math.MaxFloat64 // fun fact: this is min float64
+
 	for _, c := range s.Components {
 		// Update each updater in turn
 		if u, ok := c.(Updater); ok {
@@ -67,18 +69,20 @@ func (s *Scene) Update() error {
 				return err
 			}
 		}
-		if !needsSort {
-			// Check if the update put the components out of order
-			if z, ok := c.(ZPositioner); ok {
-				if t := z.Z(); t < curZ {
-					needsSort = true
-					curZ = t
-				}
-			}
-		}
 	}
-	if needsSort {
-		s.sortByZ()
+	// Check if the updates put the components out of order; if so, sort
+	curZ := -math.MaxFloat64 // fun fact: this is min float64
+	for _, c := range s.Components {
+		z, ok := c.(ZPositioner)
+		if !ok {
+			continue
+		}
+		t := z.Z()
+		if t < curZ {
+			s.sortByZ()
+			return nil
+		}
+		curZ = t
 	}
 	return nil
 }

main.go

@@ -44,15 +44,16 @@ func main() {
 	level1 := &engine.Scene{
 		ID: "level_1",
 		Components: []interface{}{
-			engine.Fill{
-				Color: color.White,
+			&engine.Fill{
+				Color: color.Gray{100},
+				ZPos:  0,
 			},
 			&engine.Tilemap{
 				ID:       "terrain",
 				Map:      tiles,
 				Src:      engine.ImageRef{Path: "assets/boxes.png"},
 				TileSize: 16,
-				ZPos:     0,
+				ZPos:     1,
 			},
 			&engine.SolidRect{
 				ID:   "ceiling",
@@ -75,7 +76,7 @@ func main() {
 						Size:            image.Pt(10, 16),
 					},
 					Src:  engine.ImageRef{Path: "assets/aw.png"},
-					ZPos: 1,
+					ZPos: 2,
 				},
 			},
 		},