Using edge quality to create an illusion is familiar territory to most realist artists. It is especially important for watercolor painters, for whom the meaning of the shapes and strokes are realized by the behavior of the water. Look at this detail from a watercolor landscape.
You can see that the paper was wet when the pattern of the background was applied, and suddenly dried when it reached the foreground. The abrupt change of edge texture reveals which shape was meant to seem to be in front. The illusion is also supported by color temperature changes along a transition line between background and foreground, but it is definitely what happens where hard edge meets soft that accounts for most of the impression of space.
Choose a picture from your own collection or use one of these and experiment with hard and soft edges. Can you see the location of the major shapes change when you alter the edges?