A main objective of Image Processing is to help a user interpret an image. In order to seek this objective it is important to have some understanding of the Human Visual System (HVS). The HVS is responsible for transferring data into information by a viewer. It is interesting to note that perceived information may be represented by attributes such as brightness, color shades, and edges. It is known that a typical observer can discern from one to three dozen distinct levels of grey or intensity variations. If color is added, then a typical observer can discern thousands of color shades and intensities. In addition, most of the information is concentrated in the fine details, that is, abrupt changes or edges.
There is an interesting phenomena of the HVS that is called simultaneous contrast. It relates the perceived brightness of a region to its background. A region will become darker as the background becomes brighter and vice versa.
This phenomena can be visualized in the image shown below