diff --git a/tilemapper/renderer.go b/tilemapper/renderer.go
index 3ccfc32..9c2d6f5 100644
--- a/tilemapper/renderer.go
+++ b/tilemapper/renderer.go
@@ -18,22 +18,32 @@ type Renderer struct {
 	xOfs     int16
 	zOfs     int16
 	yBuffer  []int32
+	yMin     []int32
 	cBuffer  []int32
 	filled   int
 	Rejected int
 }
 
+const (
+	invalid = math.MinInt32 + 1
+)
+
 func NewRenderer(xOfs, zOfs int16, width, height int) (renderer *Renderer) {
 	dim := width * height
 	pixSize := dim * 16 * 16
 	yBuffer := make([]int32, pixSize)
 	cBuffer := make([]int32, pixSize)
+	yMin := make([]int32, dim)
 
 	for i := 0; i < pixSize; i++ {
 		yBuffer[i] = math.MinInt32
 		cBuffer[i] = -1
 	}
 
+	for i := 0; i < dim; i++ {
+		yMin[i] = math.MinInt32
+	}
+
 	renderer = &Renderer{
 		width:   width,
 		height:  height,
@@ -44,24 +54,6 @@ func NewRenderer(xOfs, zOfs int16, width, height int) (renderer *Renderer) {
 	return
 }
 
-func (r *Renderer) minY(ofs, w int) (minY int32, filled bool) {
-	minY = int32(math.MaxInt32)
-	for yEnd := ofs + w<<4; ofs < yEnd; {
-		for xEnd := ofs + 16; ofs < xEnd; ofs++ {
-			y := r.yBuffer[ofs]
-			switch {
-			case y == math.MaxInt32:
-				return
-			case y < minY:
-				minY = y
-			}
-		}
-		ofs += w - 16
-	}
-	filled = true
-	return
-}
-
 func (r *Renderer) IsFilled() bool {
 	return r.filled == r.width<<4*r.height<<4
 }
@@ -70,40 +62,50 @@ func (r *Renderer) RenderBlock(block *common.Block, nameIndex map[string]int32)
 
 	bx := block.Coord.X - r.xOfs
 	bz := block.Coord.Z - r.zOfs
+
 	// We do not need to render the block if the whole 16x16 area
 	// is already filled and the block is strictly below.
 	w := r.width << 4
 	ofs := int(bz)*w<<4 + int(bx)<<4
+
 	blockY := int32(block.Coord.Y) << 4
-	if minY, filled := r.minY(ofs, w); filled && blockY < minY {
+	pos := int(bz)*r.width + int(bx)
+	if blockY < r.yMin[pos] {
 		r.Rejected++
 		return
 	}
 
-	// Decoding is pretty expensive so it that late.
+	// Decoding is pretty expensive so do it that late.
 	var db *DecodedBlock
 	if db, err = NewDecodedBlock(block.Data, nameIndex); err != nil {
 		return
 	}
 
+	yMin := int32(math.MaxInt32)
 	for z := 0; z < 16; z++ {
 		for x := 0; x < 16; x++ {
-			if currentY := r.yBuffer[ofs]; currentY < blockY {
+			currentY := r.yBuffer[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
-						r.yBuffer[ofs] = blockY + int32(y)
+						currentY = blockY + int32(y)
+						r.yBuffer[ofs] = currentY
 						break
 					}
 				}
 			}
+			if currentY < yMin {
+				yMin = currentY
+			}
 			ofs++
 		}
 		ofs += w - 16
 	}
+	r.yMin[pos] = yMin
 
 	return
 }