diff --git a/common/renderer.go b/common/renderer.go
index 4b82402..c60473e 100644
--- a/common/renderer.go
+++ b/common/renderer.go
@@ -11,6 +11,11 @@ import (
 	"math"
 )
 
+type Area struct {
+	X1, Z1 int16
+	X2, Z2 int16
+}
+
 type Renderer struct {
 	width    int
 	height   int
@@ -19,7 +24,6 @@ type Renderer struct {
 	yBuffer  []int32
 	yMin     []int32
 	cBuffer  []int32
-	filled   int
 	Rejected int
 }
 
@@ -140,7 +144,12 @@ func NewRenderer(xOfs, zOfs int16, width, height int) (renderer *Renderer) {
 }
 
 func (r *Renderer) IsFilled() bool {
-	return r.filled == r.width<<4*r.height<<4
+	for _, y := range r.yMin {
+		if y == math.MinInt32 {
+			return false
+		}
+	}
+	return true
 }
 
 func (r *Renderer) RenderBlock(block *Block, nameIndex map[string]int32) (err error) {
@@ -165,20 +174,18 @@ func (r *Renderer) RenderBlock(block *Block, nameIndex map[string]int32) (err er
 
 	w := r.width << 4
 	ofs := int(bz)*w<<4 + int(bx)<<4
+	yB := r.yBuffer
 
 	yMin := int32(math.MaxInt32)
 	for z := 0; z < 16; z++ {
 		for x := 0; x < 16; x++ {
-			currentY := r.yBuffer[ofs]
+			currentY := yB[ofs]
 			if currentY < blockY {
 				for y := 15; y >= 0; y-- {
 					if c, ok := db.Content(x, y, z); ok {
-						if r.cBuffer[ofs] == -1 {
-							r.filled++
-						}
 						r.cBuffer[ofs] = c
 						currentY = blockY + int32(y)
-						r.yBuffer[ofs] = currentY
+						yB[ofs] = currentY
 						break
 					}
 				}
@@ -195,6 +202,99 @@ func (r *Renderer) RenderBlock(block *Block, nameIndex map[string]int32) (err er
 	return
 }
 
+func (a Area) Contains(x, z int16) bool {
+	return x >= a.X1 && x <= a.X2 && z >= a.Z1 && z <= a.Z2
+}
+
+func (a Area) IsHigher() bool {
+	return a.Z2-a.Z1 > a.X2-a.X1
+}
+
+func areasContain(areas []Area, x, z int16) bool {
+	for _, r := range areas {
+		if r.Contains(x, z) {
+			return true
+		}
+	}
+	return false
+}
+
+// Greedy algorithm to figure out a list of disjunct areas
+// of free regions in the domain to the (x, z) block plane.
+// oldAreas are searched and found free areas are appended
+// to newAreas which ist return.
+// This is useful to spatial query only blocks from db
+// that are not below already rendered blocks.
+func (r *Renderer) UncoveredAreas(newAreas, oldAreas []Area) []Area {
+	yM := r.yMin
+
+	// Scan old areas.
+	for _, oldArea := range oldAreas {
+		for z := oldArea.Z1; z <= oldArea.Z2; z++ {
+			row := z * int16(r.width)
+			for x := oldArea.X1; x <= oldArea.X2; x++ {
+				// Uncovered and not in list of new areas?
+				if yM[row+x] > math.MinInt32 || areasContain(newAreas, x, z) {
+					continue
+				}
+				area := Area{X1: x, Z1: z, X2: x, Z2: z}
+				// Try to extend the area in x and/or z till no further extension is possible.
+				for extendDirs := 1 | 2; extendDirs != 0; {
+					var xFirst bool
+					// Try to extend in the direction with most gain
+					// of blocks.
+					if area.IsHigher() { // Higher means to win more blocks in x direction.
+						xFirst = true
+					}
+				dirs:
+					for i := 0; i < 2; i++ {
+						if xFirst {
+							// Extension in x possible?
+							if extendDirs&1 == 1 {
+								nx := area.X2 + 1
+								if nx >= int16(r.width) {
+									extendDirs &= ^1
+									continue
+								}
+								// Scan line below current area if its fully free.
+								for nz := area.Z1; nz <= area.Z2; nz++ {
+									if yM[nz*int16(r.width)+nx] > math.MinInt32 || areasContain(newAreas, nx, nz) {
+										extendDirs &= ^1
+										continue dirs
+									}
+								}
+								// free -> extend
+								area.X2 = nx
+							}
+						} else if extendDirs&2 == 2 {
+							// Symmetric case in z direction
+							nz := area.Z2 + 1
+							if nz >= int16(r.height) {
+								extendDirs &= ^2
+								continue
+							}
+							// Scan line right beside the area if its free.
+							row2 := nz * int16(r.width)
+							for nx := area.X1; nx <= area.X2; nx++ {
+								if yM[row2+nx] > math.MinInt32 || areasContain(newAreas, nx, nz) {
+									extendDirs &= ^2
+									continue dirs
+								}
+							}
+							area.Z2 = nz
+						}
+						// Switch to other search direction (x -> z or z -> x)
+						xFirst = !xFirst
+					}
+				}
+				// At this point the area is extended to max.
+				newAreas = append(newAreas, area)
+			}
+		}
+	}
+	return newAreas
+}
+
 func (r *Renderer) CreateImage(colors []color.RGBA, background color.RGBA) *image.RGBA {
 	pw, ph := r.width<<4, r.height<<4
 	image := image.NewRGBA(image.Rect(0, 0, pw, ph))