improve on warping

src/image.jl

@@ -15,13 +15,14 @@ function interpolate_bilinear(img, x)
 end
 
 function warp_backwards(img, u)
-    axes(img) == axes(u)[2:end] ||
+    d = ndims(img)
+    axes(img) == axes(u)[2:end] && d == size(u, 1) ||
 	throw(ArgumentError("invalid dimensions"))
 
     res = similar(img)
     for I in CartesianIndices(img)
-	x = Tuple(I) .+ u[:, I]
-	res[I] = interpolate_bilinear(img, (x...,))
+	x = Tuple(I) .+ ntuple(i -> u[i, I], d)
+	res[I] = interpolate_bilinear(img, x)
     end
     return res
 end