reimplement camera constraint

engine/camera.go

@@ -25,14 +25,57 @@ type Camera struct {
 	Child interface{}
 
 	// Camera controls
+	// These directly manipulate the camera. If you want to restrict the camera
+	// view area to the child's bounding rectangle, use PointAt.
 	Centre   image.Point // world coordinates
-	Filter   ebiten.Filter
-	Rotation float64 // radians
-	Zoom     float64 // unitless
+	Rotation float64     // radians
+	Zoom     float64     // unitless
 
 	game *Game
 }
 
+// PointAt points the camera at a particular centre point and zoom, but adjusts
+// for the bounds of the child component (if available).
+func (c *Camera) PointAt(centre image.Point, zoom float64) {
+	// Special sauce: if Child has a BoundingRect, make some adjustments
+	bnd, ok := c.Child.(Bounder)
+	if !ok {
+		c.Centre, c.Zoom = centre, zoom
+		return
+	}
+
+	// The child has boundaries; respect them.
+	br := bnd.BoundingRect()
+
+	// The lower bound on zoom is the larger of
+	// { (ScreenWidth / BoundsWidth), (ScreenHeight / BoundsHeight) }
+	sz := br.Size()
+	if z := float64(c.game.ScreenSize.X) / float64(sz.X); zoom < z {
+		zoom = z
+	}
+	if z := float64(c.game.ScreenSize.Y) / float64(sz.Y); zoom < z {
+		zoom = z
+	}
+
+	// If the configured centre puts the camera out of bounds, move it.
+	// Camera frame currently Rectangle{ centre ± (screen/(2*zoom)) }.
+	sw2, sh2 := float2(c.game.ScreenSize.Div(2))
+	swz, shz := int(sw2/zoom), int(sh2/zoom)
+	if centre.X-swz < br.Min.X {
+		centre.X = br.Min.X + swz
+	}
+	if centre.Y-shz < br.Min.Y {
+		centre.Y = br.Min.Y + shz
+	}
+	if centre.X+swz > br.Max.X {
+		centre.X = br.Max.X - swz
+	}
+	if centre.Y+shz > br.Max.Y {
+		centre.Y = br.Max.Y - shz
+	}
+	c.Centre, c.Zoom = centre, zoom
+}
+
 // Prepare grabs a copy of game (needed for screen dimensions)
 func (c *Camera) Prepare(game *Game) error {
 	c.game = game
@@ -47,6 +90,6 @@ func (c *Camera) Transform() (opts ebiten.DrawImageOptions) {
 	opts.GeoM.Translate(float2(c.Centre.Mul(-1)))
 	opts.GeoM.Scale(c.Zoom, c.Zoom)
 	opts.GeoM.Rotate(c.Rotation)
-	opts.GeoM.Translate(float64(c.game.ScreenWidth/2), float64(c.game.ScreenHeight/2))
+	opts.GeoM.Translate(float2(c.game.ScreenSize.Div(2)))
 	return opts
 }

engine/game.go

@@ -4,6 +4,7 @@ import (
 	"encoding/gob"
 	"errors"
 	"fmt"
+	"image"
 	"io/fs"
 	"math"
 	"reflect"
@@ -36,9 +37,8 @@ func init() {
 type Game struct {
 	Disabled
 	Hidden
-	ScreenWidth  int
-	ScreenHeight int
-	Root         interface{} // typically a *Scene or SceneRef though
+	ScreenSize image.Point
+	Root       interface{} // typically a *Scene or SceneRef though
 
 	dbmu     sync.RWMutex
 	byID     map[string]Identifier              // Named components by ID
@@ -110,7 +110,7 @@ func (g *Game) Draw(screen *ebiten.Image) {
 
 // Layout returns the configured screen width/height.
 func (g *Game) Layout(outsideWidth, outsideHeight int) (w, h int) {
-	return g.ScreenWidth, g.ScreenHeight
+	return g.ScreenSize.X, g.ScreenSize.Y
 }
 
 // Update updates everything.

engine/scene.go

@@ -11,7 +11,7 @@ func init() {
 	gob.Register(&Scene{})
 }
 
-// Scene manages drawing and updating a bunch of components.
+// Scene just contains a bunch of components.
 type Scene struct {
 	ID
 	Bounds     // world coordinates
@@ -20,87 +20,5 @@ type Scene struct {
 	Hidden
 }
 
-/*
-// Draw draws all components in order.
-func (s *Scene) Draw(screen *ebiten.Image, opts ebiten.DrawImageOptions) {
-	if s.Hidden {
-		return
-	}
-	if s.Camera == nil {
-		// Draw everything, no camera transforms.
-		for _, i := range s.Components {
-			if d, ok := i.(Drawer); ok {
-				d.Draw(screen, opts)
-			}
-		}
-		return
-	}
-
-	// There is a camera; apply camera transforms.
-	br := s.BoundingRect()
-
-	// The lower bound on zoom is the larger of
-	// { (ScreenWidth / BoundsWidth), (ScreenHeight / BoundsHeight) }
-	zoom := s.Camera.Zoom
-	sz := br.Size()
-	if z := float64(s.Camera.game.ScreenWidth) / float64(sz.X); zoom < z {
-		zoom = z
-	}
-	if z := float64(s.Camera.game.ScreenHeight) / float64(sz.Y); zoom < z {
-		zoom = z
-	}
-
-	// If the configured centre puts the camera out of bounds, move it.
-	centre := s.Camera.Centre
-	// Camera frame currently Rectangle{ centre ± (screen/(2*zoom)) }.
-	sw2, sh2 := float64(s.Camera.game.ScreenWidth/2), float64(s.Camera.game.ScreenHeight/2)
-	swz, shz := int(sw2/zoom), int(sh2/zoom)
-	if centre.X-swz < br.Min.X {
-		centre.X = br.Min.X + swz
-	}
-	if centre.Y-shz < br.Min.Y {
-		centre.Y = br.Min.Y + shz
-	}
-	if centre.X+swz > br.Max.X {
-		centre.X = br.Max.X - swz
-	}
-	if centre.Y+shz > br.Max.Y {
-		centre.Y = br.Max.Y - shz
-	}
-
-	// Apply other options
-	opts.Filter = s.Camera.Filter
-
-	// Compute common matrix (parts independent of parallax, which is step 1).
-	// Moving centre to the origin happens per component.
-	var comm ebiten.GeoM
-	// 2. Zoom and rotate
-	comm.Scale(zoom, zoom)
-	comm.Rotate(s.Camera.Rotation)
-	// 3. Move the origin to the centre of screen space.
-	comm.Translate(sw2, sh2)
-	// 4. Apply transforms from the caller.
-	comm.Concat(opts.GeoM)
-
-	// Draw everything.
-	for _, i := range s.Components {
-		d, ok := i.(Drawer)
-		if !ok {
-			continue
-		}
-		pf := 1.0
-		if s, ok := i.(ParallaxScaler); ok {
-			pf = s.ParallaxFactor()
-		}
-		var geom ebiten.GeoM
-		// 1. Move centre to the origin, subject to parallax factor
-		geom.Translate(-float64(centre.X)*pf, -float64(centre.Y)*pf)
-		geom.Concat(comm)
-		opts.GeoM = geom
-		d.Draw(screen, opts)
-	}
-}
-*/
-
 // Scan returns all immediate subcomponents (including the camera, if not nil).
 func (s *Scene) Scan() []interface{} { return s.Components }

game/aw.go

@@ -64,12 +64,12 @@ func (aw *Awakeman) Update() error {
 	}
 
 	// Update the camera
-	aw.camera.Zoom = 1
-	if ebiten.IsKeyPressed(ebiten.KeyShift) {
-		aw.camera.Zoom = 2
-	}
 	// aw.Pos is top-left corner, so add half size to get centre
-	aw.camera.Centre = aw.Sprite.Actor.Pos.Add(aw.Sprite.Actor.Size.Div(2))
+	z := 1.0
+	if ebiten.IsKeyPressed(ebiten.KeyShift) {
+		z = 2.0
+	}
+	aw.camera.PointAt(aw.Sprite.Actor.Pos.Add(aw.Sprite.Actor.Size.Div(2)), z)
 	return nil
 }
 

main.go

@@ -38,8 +38,7 @@ func main() {
 	}
 
 	g := &engine.Game{
-		ScreenHeight: 240,
-		ScreenWidth:  320,
+		ScreenSize: image.Pt(320, 240),
 		Root: &engine.Scene{
 			ID: "root",
 			Components: []interface{}{