Perception

Many image processing applications are intended to produce images that are to be viewed by human observers. It is therefore important to understand the characteristics and limitations of the human visual system to understand the “receiver” of the 2D signals. At the outset it is important to realise that (1) human visual system (HVS) is not well understood; (2) no objective measure exists for judging the quality of an image that corresponds to human assessment of image quality, and (3) the typical human observer does not exist Nevertheless, research in perceptual psychology has provided some important insights into the visual system [stock ham].

Elements of Human Visual Perception.

Figure( 2.1) The human eye

The first part of the visual system is the eye. This is shown in figure(2.1) . Its form is nearly spherical and its diameter is approximately 20 mm. Its outer cover consists of the ‘cornea' and ‘sclera'

The cornea is a tough transparent tissue in the front part of the eye. The sclera is an opaque membrane, which is continuous with cornea and covers the remainder of the eye. Directly below the sclera lies  the “choroids”, which has many blood vessels.At its anterior extreme lies the iris diaphragm. The light enters in the eye through the central opening of the iris, whose diameter varies from 2mm to 8mm, according to the illumination conditions. Behind the iris is the “lens” which consists of concentric layers of fibrous cells and contains up to 60 to 70% of water. Its operation is similar to that of the man made optical lenses. It focuses the light on the “retina” which is the innermost membrane of the eye.

Retina has two kinds of photoreceptors: cones and rods. The cones are highly sensitive to color. Their number is 6-7 million and they are mainly located at the central part of the retina. Each cone is connected to one nerve end.

     Cone vision is the photopic or bright light vision. Rods serve to view the general picture of the vision field. They are sensitive to low levels of illumination and cannot discriminate colors. This is the scotopic or dim-light vision. Their number is 75 to 150 million and they are distributed over the retinal surface. Several rods are connected to a single nerve end. This fact and their large spatial distribution explain their low resolution.

 Both cones and rods transform light to electric stimulus, which is carried through the optical nerve to the human brain for the high level image processing and perception.