The human eye has only a very small area where images are sharply in focus - about 3% of its central field. In practice this amounts to less than a hand's span at arm's length. So, we 'see' by building up a composite image of our surroundings; our eyes flick point to point in jumps called saccades, taking in small pieces of visual imagery at a time which our brain then puts together. When we look at a photograph or painting, the same process applies.
The spatial arrangement of visual components affect the inferences we make: concerning their relationships to each other, and how they might coalesce into a comprehensive entirety. Our eyes might capture discrete images of a trunk, leaves, roots and branches, and yet our brain recognise their relationships and present us with a 'perceptual whole' of a tree.
The way pieces of visual information or elements are 'placed' into a whole - an aspect of perceptual organisation - was studied by Gestalt (meaning "organised whole") theorists in 1920s Germany. Although some parts of Gestalt theory have not withstood scientific advance, its principles concerning visual organisation are still relevant today.
Principles of Gestalt Perception
1. The Principle of Proximity
Things which are seen together will be thought of as belonging together i.e. grouped.
Example: If a row four identical items has a larger space in the middle, it will be seen as two groups of two.
2. The Principle of Similarity
Things of which have characteristics in common will be grouped together.
Example: six alternating rows of blue and yellow balls will be seen as three groups of blue and three groups of yellow.
3. The Principle of Common Fate
Items grouped through their proximity and similarity to each other are assumed to behave as one, sharing the same fate. The eye will want them to move together, or stay put together. Anticipations of a 'Common Fate' can be used to create visual movement and visual tension.
4. The Principle of Continuity
The eye has momentum; once it is made to follow a path, it will try to continue along that same path. This principle predicts a viewer's preference for continuous figures.
Example: a sequence of points will be joined up by the eye to imply a line, or a curving line (which will induce an eye to follow it) leads the eye to an object placed beyond the end the line.
5. The Principle of Closure
The mind seeks completeness. It has a tendency to fill in missing bits of information to make a whole; to complete simple shapes. This is develops out of the Principle of Continuity.
Example: a curved line of constant radius with its ends closer together will be visually completed into a circle, or two convergent lines may have their other ends joined to complete a triangle.
6. The Principle of Area
If there are two overlapping figures, the smaller is perceived to be in the foreground, the larger the background. This idea explains how we perceive which of two objects is in the foreground (the figure) and which is the background (the ground).
Example: a net, formed of two sets of diagonal lines each at right-angles to each other, has a smaller blank oval outline in its centre. The oval will seen as the 'figure', and the net as the 'ground'.
7. The Principle of Symmetry
Areas bounded by symmetrical contours are seen as closed-whole figures, rendering the perception of symmetrical areas as 'figures' preferentially over asymmetrical ones. This principle is developed out of the Principle of Area.
8. The Principle of Segregation
For the 'figure' to be perceived from the 'ground' there must be sufficient difference between the two. This is derived from the Principle of Common Fate and the Principle of Area. Figure-ground images, like those of Escher, exploit this for their ambiguity.
These principles suggest how our mind creates groups and areas in the visual field - how each group has common properties internally, and how they might relate to other groups externally - giving rise to:
Concepts in Gestalt Perception
I - Concept of Emergence
The mind can join the dots. Groups, which alone do not contain sufficient information for the full context, can be brought together and their possible relations explored until the meaning of the whole emerges.
II - Concept of Reification
Lacking sufficient information, the mind will act to fill in the empty space. The Principle of Closure demonstrates this.
III - Concept of Multistability
Lacking sufficient information, the mind will spontaneously move between the possible interpretations. For example, an absence of enough depth cues will cause a figure-ground inversion.
IV - Concept of Invariance
Objects will still be recognised irrespective of orientation and scale.