Edge Quality

One kind of contrast to consider in your painting is edge quality.  All soft edges or all hard edges confuse the viewer.  By providing a contrast between hard and soft edges, you can direct the viewer's eye to a focal point.

The shoreline is often a place where hard edges are made, thus creating a straight, hard line.  By making a blurry, fuzzy soft edge, the viewer's eye will not stop there so readily.  Rooftops are another place where hard straight lines can over-define a shape that is not worth the attention.  Suggesting these elements with softer edges will cause the eye to seek out the harder edges.

In short, hard edges attract the eye, while soft edges keep the eye circulating around the composition.
A judicious use of both will come in aid of the focal point.