How we change what others think, feel, believe and do
Mind Separates Subject From Background
As the mind is working on distinguishing objects, including tracing the outline and seeking detail, in order to fully identify a subject, the mind must separate it from the background, even if it is partially obscured, has a vague outline or hues match those of the background.
It does this using the outline, detail, pattern matching and otherwise guessing where complete information is not available.
A photographer moves the camera so the subject is surrounded by a contrasting light background, making the subject stand out more easily.
A private detective wears dark clothes and stands in shadows in order to reduce the chance of being seen by the person she is observing.
When there is clear contrast between a subject and its background, the subject is very easy to distinguish. However, this is not always the case and the mind must make rapid assessments and perhaps assumptions in order to plot a complete outline and detail that says 'yes, here is a separate thing'.
This is also a relative spectrum effect. The more a subject stands out out from a background, the more it is separate from it and the easier it is to distinguish. However, the more the subject is separated from the background, the more cognitive separation it also gains. In other words, if a subject is very different from the background it can look like it was just placed on top and has little association with the background. The background in an image provides 'glue' in holding separate subjects together, giving them a common context and consequent connective meaning.
The reverse of making a subject stand out from a background it is that it blends in with the background. This is the basis of camouflage, which breaks up outlines and uses similar hues and textures to the background.
When creating visual objects on which you want people to focus, make sure they stand out from their background. Hence have light backgrounds for darker objects and dark backgrounds for light objects. If necessary, move the subject (or viewpoint) so it is completely be surrounded by a contrasting background.
You can also deliberately make a subject harder to separate from the background, for example if it is not the main subject or if you want to create a sense of mystery.
In addition, always consider how the background gives extra meaning and what a realistic separation is (as opposed to one in which the background and subject are not mentally connected by the viewer).