diff --git a/common/coords.go b/common/coords.go
index ae6e92b..4c32122 100644
--- a/common/coords.go
+++ b/common/coords.go
@@ -145,6 +145,10 @@ func CoordToInterleaved(c Coord) (result int64) {
 	return
 }
 
+func invert16(x int16) int16 {
+	return -x - 1
+}
+
 func InterleavedToCoord(pos int64) Coord {
 	const end = 1 << (numBitsPerComponent + 1)
 	var x, y, z int16
diff --git a/common/coords_test.go b/common/coords_test.go
index 519ac4a..011b05f 100644
--- a/common/coords_test.go
+++ b/common/coords_test.go
@@ -5,6 +5,7 @@
 package common
 
 import (
+	"math"
 	"math/rand"
 	"testing"
 )
@@ -180,6 +181,15 @@ func outsiders(zmin, zmax int64, fn func(int64)) {
 	}
 }
 
+func TestInvert16(t *testing.T) {
+	if invert16(math.MaxInt16) != math.MinInt16 {
+		t.Errorf("invert16(max) != min\n")
+	}
+	if invert16(math.MinInt16) != math.MaxInt16 {
+		t.Errorf("invert16(max) != min\n")
+	}
+}
+
 func TestBigMin(t *testing.T) {
 	const tries = 20
 	for i := 0; i < tries; i++ {
diff --git a/tilemapper/main.go b/tilemapper/main.go
index f52170b..4abbeca 100644
--- a/tilemapper/main.go
+++ b/tilemapper/main.go
@@ -64,10 +64,11 @@ func main() {
 	q2x, q2y, q2z := q1x+int16(width)-1, q1y+int16(depth)-1, q1z+int16(height)-1
 
 	renderer := NewRenderer(q1x, q1z, width, height)
+	yOrder := NewYOrder(renderer, 512)
 
 	numBlocks := 0
 	drawBlock := func(block *common.Block) {
-		if err := renderer.RenderBlock(block, colors.NameIndex); err != nil {
+		if err := yOrder.RenderBlock(block, colors.NameIndex); err != nil {
 			log.Printf("WARN: rendering block failed: %s", err)
 		}
 		numBlocks++
@@ -84,6 +85,9 @@ func main() {
 		if err = client.QueryCuboid(cuboid, drawBlock); err != nil {
 			log.Fatalf("query failed: %s", err)
 		}
+		if err = yOrder.Drain(colors.NameIndex); err != nil {
+			log.Printf("WARN: rendering block failed: %s", err)
+		}
 		if renderer.IsFilled() {
 			break
 		}
diff --git a/tilemapper/renderer.go b/tilemapper/renderer.go
index 22bac7a..80e63fe 100644
--- a/tilemapper/renderer.go
+++ b/tilemapper/renderer.go
@@ -5,6 +5,7 @@
 package main
 
 import (
+	"container/heap"
 	"image"
 	"image/color"
 	"math"
@@ -24,6 +25,95 @@ type Renderer struct {
 	Rejected int
 }
 
+type YOrder struct {
+	Renderer *Renderer
+	blocks   []*common.Block
+	capacity int
+}
+
+func NewYOrder(renderer *Renderer, capacity int) *YOrder {
+	return &YOrder{
+		Renderer: renderer,
+		blocks:   make([]*common.Block, 0, capacity),
+		capacity: capacity}
+}
+
+func max(a, b int) int {
+	if a > b {
+		return a
+	}
+	return b
+}
+
+func copyData(data []byte) []byte {
+	l := len(data)
+	ndata := make([]byte, l, max(l, 8*1024))
+	copy(ndata, data)
+	return ndata
+}
+
+func (yo *YOrder) RenderBlock(block *common.Block, nameIndex map[string]int32) (err error) {
+	var nblock *common.Block
+	if len(yo.blocks) == yo.capacity {
+		oblock := yo.blocks[0]
+		if oblock.Coord.Y < block.Coord.Y {
+			// New one is above highest old. Directly render new.
+			err = yo.Renderer.RenderBlock(block, nameIndex)
+			return
+		}
+		// Render old one. Store copy of new in heap.
+		heap.Pop(yo)
+		err = yo.Renderer.RenderBlock(oblock, nameIndex)
+		l := len(block.Data)
+		if cap(oblock.Data) < l {
+			oblock.Data = make([]byte, l, max(l, 8*1024))
+		} else {
+			oblock.Data = oblock.Data[0:l]
+		}
+		copy(oblock.Data, block.Data)
+		oblock.Coord = block.Coord
+		nblock = oblock
+	} else {
+		nblock = &common.Block{Coord: block.Coord, Data: copyData(block.Data)}
+	}
+
+	heap.Push(yo, nblock)
+	return
+}
+
+func (yo *YOrder) Drain(nameIndex map[string]int32) (err error) {
+	for len(yo.blocks) > 0 {
+		if err = yo.Renderer.RenderBlock(heap.Pop(yo).(*common.Block), nameIndex); err != nil {
+			return
+		}
+	}
+	return
+}
+
+func (yo *YOrder) Len() int {
+	return len(yo.blocks)
+}
+
+func (yo *YOrder) Swap(i, j int) {
+	yo.blocks[i], yo.blocks[j] = yo.blocks[j], yo.blocks[i]
+}
+
+func (yo *YOrder) Less(i, j int) bool {
+	// Reverse order intented.
+	return yo.blocks[i].Coord.Y > yo.blocks[j].Coord.Y
+}
+
+func (yo *YOrder) Push(x interface{}) {
+	yo.blocks = append(yo.blocks, x.(*common.Block))
+}
+
+func (yo *YOrder) Pop() (x interface{}) {
+	l := len(yo.blocks)
+	x = yo.blocks[l-1]
+	yo.blocks = yo.blocks[0 : l-1]
+	return x
+}
+
 func NewRenderer(xOfs, zOfs int16, width, height int) (renderer *Renderer) {
 	dim := width * height
 	pixSize := dim * 16 * 16