diff --git a/dotscape b/dotscape
index d53c824..6a86ddd 100755
Binary files a/dotscape and b/dotscape differ
diff --git a/fcore/app-init.sml b/fcore/app-init.sml
index bf5d66a..f74694a 100644
--- a/fcore/app-init.sml
+++ b/fcore/app-init.sml
@@ -48,7 +48,7 @@ struct
       , b = 0
       , a = 1
       , layer = 0
-      , layerTree = LayerTree.emptyTree
+      , layerTree = LayerTree.empty
       , modalNum = 0
       , undo = []
       , redo = []
diff --git a/fcore/app-type.sml b/fcore/app-type.sml
index 0b31a70..181eb3a 100644
--- a/fcore/app-type.sml
+++ b/fcore/app-type.sml
@@ -34,7 +34,7 @@ struct
     , b: int
     , a: int
     , layer: int
-    , layerTree: LayerTree.tree
+    , layerTree: LayerTree.t
     , modalNum: int
     , undo: square list
     , redo: square list
diff --git a/fcore/grid.sml b/fcore/grid.sml
index bee7ede..a7c016e 100644
--- a/fcore/grid.sml
+++ b/fcore/grid.sml
@@ -1,12 +1,12 @@
 structure Grid =
 struct
-  type square = {r: int, g: int, b: int, a: int}
+  type pixel = {r: int, g: int, b: int, a: int}
 
-  type t = square vector vector
+  type t = pixel vector vector
 
   val emptyPixel = {r = 0, g = 0, b = 0, a = 0}
 
-  fun isBlank ({a, ...}: square) = a = 0
+  fun isBlank ({a, ...}: pixel) = a = 0
 
   fun changeGridSize maxSide grid =
     Vector.tabulate (maxSide, fn i =>
@@ -20,4 +20,16 @@ struct
         end
       else
         Vector.tabulate (maxSide, fn _ => emptyPixel))
+
+  fun updateGrid (grid, newX, newY, pixel) =
+    let
+      val yAxis = Vector.sub (grid, newX)
+      val yAxis = Vector.update (yAxis, newY, pixel)
+    in
+      Vector.update (grid, newX, yAxis)
+    end
+
+  fun makeEmpty maxSide =
+    Vector.tabulate (maxSide, fn _ =>
+      Vector.tabulate (maxSide, fn _ => emptyPixel))
 end
diff --git a/fcore/layer-tree.sml b/fcore/layer-tree.sml
index f03103e..15fbe20 100644
--- a/fcore/layer-tree.sml
+++ b/fcore/layer-tree.sml
@@ -1,10 +1,8 @@
 structure LayerTree =
 struct
-  datatype tree =
-    NODE of {key: int, value: Grid.t, left: tree, right: tree}
-  | LEAF
+  datatype t = NODE of {key: int, value: Grid.t, left: t, right: t} | LEAF
 
-  val emptyTree = LEAF
+  val empty = LEAF
 
   fun insert (newKey, newValue, tree) =
     case tree of
@@ -65,15 +63,26 @@ struct
               else
                 valuePixel) valueYAxis) value
 
-  fun makeEmptyGrid maxSide =
-    Vector.tabulate (maxSide, fn _ =>
-      Vector.tabulate (maxSide, fn _ => Grid.emptyPixel))
-
   fun flatten (maxSide, tree) =
-    foldl (helpFlatten, tree, makeEmptyGrid maxSide)
+    foldl (helpFlatten, tree, Grid.makeEmpty maxSide)
 
   fun changeGridSize (maxSide, tree) =
     let val f = Grid.changeGridSize maxSide
     in map (f, tree)
     end
+
+  fun addPixel (key, newX, newY, maxSide, pixel, tree) =
+    let
+      val grid =
+        case get (key, tree) of
+          SOME grid => grid
+        | NONE => Grid.makeEmpty maxSide
+
+      val grid = Grid.updateGrid (grid, newX, newY, pixel)
+    in
+      insert (key, grid, tree)
+    end
+
+  fun removePixel (key, newX, newY, maxSide, tree) =
+    addPixel (key, newX, newY, maxSide, Grid.emptyPixel, tree)
 end