diff --git a/infiniteworld.py b/infiniteworld.py
index 00ce859..5af358d 100644
--- a/infiniteworld.py
+++ b/infiniteworld.py
@@ -758,7 +758,9 @@ class InfdevChunk(EntityLevel):
     def isCompressed(self):
         return self.isLoaded() and self.root_tag == None
 
-
+    def generateHeightMap(self):
+        extractLightMap(self.materials, self.Blocks, self.HeightMap)
+         
     def chunkChanged(self, calcLighting=True):
         """ You are required to call this function after you are done modifying
         the chunk. Pass False for calcLighting if you know your changes will 
@@ -770,7 +772,7 @@ class InfdevChunk(EntityLevel):
 
         self.dirty = True;
         self.needsLighting = calcLighting or self.needsLighting;
-        generateHeightMap(self);
+        self.generateHeightMap();
         if calcLighting:
             self.genFastLights()
 
@@ -910,19 +912,6 @@ class InfdevChunk(EntityLevel):
         ores and vegetation on next load"""
         self.root_tag[Level]["TerrainPopulated"].value = val;
 
-def generateHeightMap(self):
-    self.load();
-
-    blocks = self.Blocks
-    heightMap = self.HeightMap
-    heightMap[:] = 0;
-
-    lightAbsorption = self.world.materials.lightAbsorption[blocks]
-    axes = lightAbsorption.nonzero()
-    heightMap[axes[1], axes[0]] = axes[2]; #assumes the y-indices come out in increasing order
-    heightMap += 1;
-
-
 class dequeset(object):
     def __init__(self):
         self.deque = deque();
diff --git a/level.py b/level.py
index 6a55246..b3ca552 100644
--- a/level.py
+++ b/level.py
@@ -9,6 +9,21 @@ from mclevelbase import *
 import tempfile
 from collections import defaultdict
 
+def extractLightMap(materials, blocks, heightMap = None):
+    """Computes the HeightMap array for a chunk, which stores the lowest 
+    y-coordinate of each column where the sunlight is still at full strength."""
+    
+    if heightMap is None:
+        heightMap = zeros((16, 16), 'uint8')
+        
+    heightMap[:] = 0;
+
+    lightAbsorption = materials.lightAbsorption[blocks]
+    axes = lightAbsorption.nonzero()
+    heightMap[axes[1], axes[0]] = axes[2]; #assumes the y-indices come out in increasing order
+    heightMap += 1;
+    return heightMap
+
 class MCLevel(object):
     """ MCLevel is an abstract class providing many routines to the different level types, 
     including a common copyEntitiesFrom built on class-specific routines, and
@@ -143,7 +158,13 @@ class MCLevel(object):
             def chunkChanged(self):pass
             @property
             def materials(self): return self.world.materials
-
+            @property
+            def HeightMap(self):
+                if hasattr(self, "_heightMap"):
+                    return self._heightMap
+                    
+                self._heightMap = extractLightMap(self.materials, self.Blocks)
+                return self._heightMap
         f = FakeChunk()
         f.world = self;
         f.chunkPosition = (cx, cz)
diff --git a/mclevelbase.py b/mclevelbase.py
index 33eed9a..042f40e 100644
--- a/mclevelbase.py
+++ b/mclevelbase.py
@@ -102,4 +102,4 @@ else:
 
 saveFileDir = os.path.join(minecraftDir, u"saves")
 
-from level import MCLevel, EntityLevel
+from level import MCLevel, EntityLevel, extractLightMap