diff --git a/patch-image.cabal b/patch-image.cabal
--- a/patch-image.cabal
+++ b/patch-image.cabal
@@ -1,5 +1,5 @@
 Name:           patch-image
-Version:        0.3.2
+Version:        0.3.2.1
 License:        BSD3
 License-File:   LICENSE
 Author:         Henning Thielemann <haskell@henning-thielemann.de>
@@ -41,7 +41,7 @@
   README.md
 
 Source-Repository this
-  Tag:         0.3.2
+  Tag:         0.3.2.1
   Type:        darcs
   Location:    http://hub.darcs.net/thielema/patch-image/
 
diff --git a/src/Knead.hs b/src/Knead.hs
--- a/src/Knead.hs
+++ b/src/Knead.hs
@@ -16,7 +16,7 @@
    layoutFromPairDisplacements, fixAtLeastOneAnglePosition,
    )
 import Knead.Shape
-         (Size, Vec2(Vec2), Dim1, Dim2, Shape2, Index2, Ix2,
+         (Size, Vec2(Vec2), Dim1, Dim2, Shape2, Index2, Ix2, Factor2,
           verticalVal, horizontalVal)
 import Degree (Degree(Degree), getDegree)
 
@@ -244,8 +244,17 @@
 atomIx2 :: Index2 (Atom i)
 atomIx2 = Vec2 atom atom
 
+atomFactor2 :: Factor2 (Atom i)
+atomFactor2 = Vec2 atom atom
+
+decomposeDim2 :: Exp (Shape2 i) -> Shape2 (Exp i)
+decomposeDim2 = Expr.decompose atomDim2
+
+decomposeFactor2 :: Exp (Factor2 i) -> Factor2 (Exp i)
+decomposeFactor2 = Expr.decompose atomFactor2
+
 dim2 :: Exp i -> Exp i -> Exp (Shape2 i)
-dim2 y x = Expr.compose (Vec2 y x)
+dim2 h w = Expr.compose (Vec2 h w)
 
 ix2 :: Exp i -> Exp i -> Exp (Index2 i)
 ix2 y x = Expr.compose (Vec2 y x)
@@ -260,7 +269,7 @@
 
 indexLimit :: SymbPlane a -> Index2 (Exp Size) -> Exp a
 indexLimit img (Vec2 y x) =
-   let (Vec2 height width) = Expr.decompose atomDim2 $ Symb.shape img
+   let (Vec2 height width) = decomposeDim2 $ Symb.shape img
        xc = Expr.max 0 $ Expr.min (width -1) x
        yc = Expr.max 0 $ Expr.min (height-1) y
    in  img ! ix2 yc xc
@@ -270,8 +279,8 @@
    Exp Dim2 -> array sh Ix2 -> array sh Ix2
 limitIndices sh =
    Symb.map
-      (case Expr.decompose atomDim2 sh of
-         (Vec2 height width) ->
+      (case decomposeDim2 sh of
+         Vec2 height width ->
             Expr.modify atomIx2 $
                \(Vec2 y x) ->
                   let xc = Expr.max 0 $ Expr.min (width -1) x
@@ -446,7 +455,7 @@
    SymbPlane (Bool8, v)
 rotateStretchMove vec rot mov sh img =
    let coords = rotateStretchMoveCoords rot mov sh
-       (Vec2 heightSrc widthSrc) = Expr.decompose atomDim2 $ Symb.shape img
+       (Vec2 heightSrc widthSrc) = decomposeDim2 $ Symb.shape img
    in  Symb.zip
          (validCoords (widthSrc, heightSrc) coords)
          (gatherFrac vec img $
@@ -462,7 +471,7 @@
    SymbPlane v ->
    SymbPlane v
 rotate vec rot img =
-   let (Vec2 height width) = Expr.decompose atomDim2 $ Symb.shape img
+   let (Vec2 height width) = decomposeDim2 $ Symb.shape img
        ((left, right), (top, bottom)) =
          Arith.boundingBoxOfRotatedGen (Expr.min, Expr.max)
             (Expr.unzip rot) (fromSize2 (width, height))
@@ -563,14 +572,14 @@
 -- counterpart to 'clip'
 pad :: (MultiValue.C a) => Exp a -> Exp Dim2 -> SymbPlane a -> SymbPlane a
 pad a sh img =
-   let Vec2 height width = Expr.decompose atomDim2 $ Symb.shape img
+   let Vec2 height width = decomposeDim2 $ Symb.shape img
    in  generate sh $ \p ->
          let Vec2 y x = Expr.decompose atomIx2 p
          in  Expr.ifThenElse (y<*height &&* x<*width) (img ! p) a
 
 cyclicReverse2d :: (MultiValue.C a) => SymbPlane a -> SymbPlane a
 cyclicReverse2d spec =
-   let (Vec2 height width) = Expr.decompose atomDim2 $ Symb.shape spec
+   let (Vec2 height width) = decomposeDim2 $ Symb.shape spec
    in  Symb.backpermute (Symb.shape spec)
          (Expr.modify atomIx2 $ \(Vec2 y x) ->
             Vec2
@@ -642,7 +651,7 @@
 displacementMap ::
    Exp Size -> Exp Size -> Exp Dim2 -> SymbPlane (Size, Size)
 displacementMap xsplit ysplit sh =
-   let Vec2 height width = Expr.decompose atomDim2 sh
+   let Vec2 height width = decomposeDim2 sh
    in  generate sh $ Expr.modify atomIx2 $ \(Vec2 y x) ->
          (wrap width xsplit x, wrap height ysplit y)
 
@@ -683,8 +692,8 @@
    SymbPlane (Float, (Size, Size))
 allOverlapsFromCorrelation (Vec2 height width) minOverlapPortion =
    \sha shb correlated ->
-      let (Vec2 heighta widtha) = Expr.decompose atomDim2 sha
-          (Vec2 heightb widthb) = Expr.decompose atomDim2 shb
+      let (Vec2 heighta widtha) = decomposeDim2 sha
+          (Vec2 heightb widthb) = decomposeDim2 shb
           half = flip Expr.idiv 2
           minOverlap =
              fastRound $
@@ -749,7 +758,7 @@
 shrink ::
    (MultiValue.Field a, MultiValue.RationalConstant a, MultiValue.Real a,
     MultiValue.NativeFloating a ar) =>
-   Shape2 (Exp Size) -> SymbPlane a -> SymbPlane a
+   Factor2 (Exp Size) -> SymbPlane a -> SymbPlane a
 shrink (Vec2 yk xk) =
    Symb.map (/ (fromInt xk * fromInt yk)) .
    Symb.fold1 Expr.add .
@@ -767,7 +776,7 @@
 independent from whether minOverlap is zero or not.
 -}
 shrinkFactors ::
-   (Integral a) => Dim2 -> Float -> Shape2 a -> Shape2 a -> Shape2 a
+   (Integral a) => Dim2 -> Float -> Shape2 a -> Shape2 a -> Factor2 a
 shrinkFactors (Vec2 heightPad widthPad) minOverlapPortion
    (Vec2 heighta widtha) (Vec2 heightb widthb) =
       let minOverlap =
@@ -785,7 +794,7 @@
 since we expect that the user chooses an FFT friendly target size.
 -}
 shrinkFactorsAlt ::
-   (Bits a, Integral a) => Float -> Dim2 -> Shape2 a -> Shape2 a -> Shape2 a
+   (Bits a, Integral a) => Float -> Dim2 -> Shape2 a -> Shape2 a -> Factor2 a
 shrinkFactorsAlt minOverlapPortion (Vec2 heightPad widthPad) a b =
    let (widthc,heightc) =
          Arith.correlationSize minOverlapPortion $
@@ -798,7 +807,7 @@
 optimalOverlapBig ::
    Dim2 -> IO (Float -> Plane Float -> Plane Float -> IO (Float, (Size, Size)))
 optimalOverlapBig padExtent = do
-   shrnk <- RenderP.run $ shrink . Expr.decompose atomDim2
+   shrnk <- RenderP.run $ shrink . decomposeFactor2
    optOverlap <- optimalOverlap padExtent
    return $ \minimumOverlap a b -> do
       let factors@(Vec2 yk xk) =
@@ -815,8 +824,7 @@
    (Exp Size, Exp Size) ->
    SymbPlane a -> SymbPlane a
 clip (left,top) (width,height) =
-   Symb.backpermute
-      (Expr.compose $ Vec2 height width)
+   Symb.backpermute (dim2 height width)
       (Expr.modify (Vec2 atom atom) $ \(Vec2 y x) -> Vec2 (y+top) (x+left))
 
 
@@ -877,7 +885,7 @@
    IO (Float -> Maybe Float -> Plane Float -> Plane Float ->
        IO [(Float, (Size, Size), (Size, Size))])
 optimalOverlapBigMulti padExtent (Vec2 heightStamp widthStamp) numCorrs = do
-   shrnk <- RenderP.run $ shrink . Expr.decompose atomDim2
+   shrnk <- RenderP.run $ shrink . decomposeFactor2
    optOverlap <- optimalOverlap padExtent
    overDiff <- overlapDifferenceRun
    clp <- RenderP.run clip
@@ -944,8 +952,8 @@
    (Exp Size, Exp Size) ->
    SymbPlane a -> SymbPlane a -> Exp a
 overlapDifference (dx,dy) a b =
-   let (Vec2 heighta widtha) = Expr.decompose atomDim2 $ Symb.shape a
-       (Vec2 heightb widthb) = Expr.decompose atomDim2 $ Symb.shape b
+   let (Vec2 heighta widtha) = decomposeDim2 $ Symb.shape a
+       (Vec2 heightb widthb) = decomposeDim2 $ Symb.shape b
        leftOverlap = Expr.max 0 dx
        topOverlap  = Expr.max 0 dy
        rightOverlap  = Expr.min widtha  (widthb  + dx)
@@ -973,8 +981,8 @@
    (Exp Size, Exp Size) ->
    (SymbPlane v, SymbPlane v) -> SymbPlane v
 overlap2 vec (dx,dy) (a,b) =
-   let (Vec2 heighta widtha) = Expr.decompose atomDim2 $ Symb.shape a
-       (Vec2 heightb widthb) = Expr.decompose atomDim2 $ Symb.shape b
+   let (Vec2 heighta widtha) = decomposeDim2 $ Symb.shape a
+       (Vec2 heightb widthb) = decomposeDim2 $ Symb.shape b
        left = Expr.min 0 dx; right  = Expr.max widtha  (widthb  + dx)
        top  = Expr.min 0 dy; bottom = Expr.max heighta (heightb + dy)
        width  = right - left
@@ -1291,8 +1299,8 @@
    SymbPlane a -> SymbPlane Word8
 scaleDistanceMap img =
    let scale =
-         case Expr.decompose atomDim2 $ Symb.shape img of
-            Vec2 y x -> 4 / fromInt (Expr.min x y)
+         case decomposeDim2 $ Symb.shape img of
+            Vec2 h w -> 4 / fromInt (Expr.min w h)
    in  imageByteFromFloat $ Symb.map (scale*) img
 
 
diff --git a/src/Knead/Shape.hs b/src/Knead/Shape.hs
--- a/src/Knead/Shape.hs
+++ b/src/Knead/Shape.hs
@@ -51,9 +51,11 @@
 
 data ShapeTag
 data IndexTag
+data FactorTag
 
 type Shape2 = Vec2 ShapeTag
 type Index2 = Vec2 IndexTag
+type Factor2 = Vec2 FactorTag