Reorder drawn objects

cmd/view-map/main.go

@@ -6,6 +6,7 @@ import (
 	"log"
 	"math"
 	"os"
+	"sort"
 
 	"github.com/hajimehoshi/ebiten"
 
@@ -54,6 +55,7 @@ func main() {
 	state := state{
 		zoom:   1.0,
 		origin: image.Point{0, 3000}, // FIXME: haxxx
+		zIdx:   1,
 	}
 	env := &env{
 		area:      area,
@@ -76,7 +78,7 @@ func main() {
 	win.OnMouseWheel(env.changeZoom)
 
 	for i := 0; i <= 6; i++ {
-		win.OnKeyUp(ebiten.Key1+ebiten.Key(i), env.setZIdx(i))
+		win.OnKeyUp(ebiten.Key1+ebiten.Key(i), env.setZIdx(i+1))
 	}
 
 	if err := win.Run(env.Update, env.Draw); err != nil {
@@ -103,34 +105,49 @@ func (e *env) Draw(screen *ebiten.Image) error {
 
 	sw, sh := screen.Size()
 
-	topLeftX, topLeftY := pixToCell(
+	topLeft := pixToCell(e.state.origin)
-		float64(e.state.origin.X),
+	topLeft.X -= 1 // Otherwise we miss half a cell on alternate rows on the left
-		float64(e.state.origin.Y),
-	)
-	topLeftX -= 1 // Otherwise we miss half a cell on alternate rows on the left
 
-	bottomRightX, bottomRightY := pixToCell(
+	bottomRight := pixToCell(image.Pt(e.state.origin.X+sw, e.state.origin.Y+sh))
-		float64(e.state.origin.X+sw),
-		float64(e.state.origin.Y+sh),
-	)
 
 	// X+Y is constant for all tiles in a column
 	// X-Y is constant for all tiles in a row
-	for a := int(topLeftX + topLeftY); a <= int(bottomRightX+bottomRightY); a++ {
+	// However, the drawing order is odd unless we reorder explicitly.
-		for b := int(topLeftX - topLeftY); b <= int(bottomRightX-bottomRightY); b++ {
+	toDraw := []image.Point{}
+	for a := topLeft.X + topLeft.Y; a <= bottomRight.X+bottomRight.Y; a++ {
+		for b := topLeft.X - topLeft.Y; b <= bottomRight.X-bottomRight.Y; b++ {
 			if b&1 != a&1 {
 				continue
 			}
 
-			x := (a + b) / 2
+			pt := image.Pt((a+b)/2, (a-b)/2)
-			y := (a - b) / 2
 
-			if !image.Pt(x, y).In(e.area.Rect) {
+			if !pt.In(e.area.Rect) {
 				continue
 			}
+			toDraw = append(toDraw, pt)
+		}
+	}
 
-			for z := 0; z <= e.state.zIdx; z++ {
+	sort.Slice(toDraw, func(i, j int) bool {
-				e.renderCell(x, y, z, screen)
+		iPix := cellToPix(toDraw[i])
+		jPix := cellToPix(toDraw[j])
+
+		if iPix.Y < jPix.Y {
+			return true
+		}
+
+		if iPix.Y == jPix.Y {
+			return iPix.X < jPix.X
+		}
+
+		return false
+	})
+
+	for _, pt := range toDraw {
+		for z := 0; z <= e.state.zIdx; z++ {
+			if err := e.renderCell(pt.X, pt.Y, z, screen); err != nil {
+				return err
 			}
 		}
 	}
@@ -147,8 +164,12 @@ func (e *env) renderCell(x, y, z int, screen *ebiten.Image) error {
 	iso := ebiten.GeoM{}
 	iso.Translate(-float64(e.state.origin.X), -float64(e.state.origin.Y))
 
-	fx, fy := cellToPix(float64(x), float64(y))
+	pix := cellToPix(image.Pt(x, y))
-	iso.Translate(fx, fy)
+	iso.Translate(float64(pix.X), float64(pix.Y))
+
+	if e.step%30 == 0 {
+		//	log.Printf("x=%v y=%v z=%v", pix.X-e.state.origin.X, pix.Y-e.state.origin.Y, z)
+	}
 
 	// Taking the Z index away *seems* to draw the object in the correct place.
 	// FIXME: There are some artifacts, investigate more
@@ -189,11 +210,17 @@ const (
 )
 
 // Doesn't take the camera or Z level into account
-func cellToPix(x, y float64) (float64, float64) {
+func cellToPix(pt image.Point) image.Point {
-	return (x - y) * cellWidth, (x + y) * cellHeight / 2.0
+	return image.Pt(
+		(pt.X-pt.Y)*cellWidth,
+		(pt.X+pt.Y)*cellHeight/2,
+	)
 }
 
 // Doesn't take the camera or Z level into account
-func pixToCell(x, y float64) (float64, float64) {
+func pixToCell(pt image.Point) image.Point {
-	return y/cellHeight + x/(cellWidth*2.0), y/cellHeight - x/(cellWidth*2.0)
+	return image.Pt(
+		pt.Y/cellHeight+pt.X/(cellWidth*2),
+		pt.Y/cellHeight-pt.X/(cellWidth*2),
+	)
 }