Information Technology Reference
In-Depth Information
The rest of this chapter is organized as follows. Section 2 provides an introduc-
tion to the Human Visual System and how this relates to quality metrics and
evaluations. Section 3 outlines our evaluation set up and methodology for doing
subjective quality assessments in a real cinema on D-Cinema quality content. Sec-
tion 4 provides our work on developing perceptual image metrics for D-Cinema
content and correlates the results to the subjective assessment. Section 5 brings us
back to the business perspective and provides results from the NORDIC
D-Cinema trial which was run in Norway from 2005 to 2008.
2 Role of Human Visual System in the Perception of Visual
Quality
The Human Visual System (HVS) seldom responds to direct stimulation from a
light source. Rather, light is reflected by objects and thus transmits information
about certain characteristics of the object. The reflected ray of light enters the eye-
ball through the cornea. The cornea represents the strongest part of the refracting
power of the eye, providing about 80% of the total eye's refracting capacity. After
passing through the cornea and the watery aqueous humor, the photon beam enters
the inner eye through the pupil, which regulates the amount of light allowed to en-
ter. The lens focuses the light on the sensory cells of the retina [4].
The internal layer of the eyeball is made up of a nervous coat called retina. The
retina covers the inner back part of the eyeball and is where the optical image is
formed by the eye's optical system. Here, a photochemical transduction occurs:
nerve impulses are created and transmitted along the optic nerve to the brain for
higher cortical processing. The point of departure of that optic nerve through the
retina does not have any receptors, and thus produces a “blind spot”. The retina
consists of two different types of light-sensitive cells, rods and cones. There are
about 6.5 million cones in each eyeball, most of them located in the middle of the
retina, in a small area about 1.mm in diameter called the fovea or fovea centralis.
Fovea is the center of the eye's sharpest vision and the location of most color per-
ception, performing in bright light, but being fairly insensitive at lower light lev-
els. Located around the fovea centralis are about 120 million rods. They are
mainly responsible for vision in dim light and produce images consisting of vary-
ing shades of black and white. The acuity over most of that range is poor, and the
rods are multiply connected to nerve fibers, so that a single nerve fiber can be ac-
tivated by any one of about a hundred rods. In contrast, cones in the fovea cen-
tralis are more individually connected to nerve fibers [5].
The eyeball is situated in the orbital cavity, a location that protects it and pro-
vides a rigid bony origin for the six extrinsic muscles that produce ocular move-
ment. When the visual system focuses on a certain object, then the optical axes of
both eyes are adjusted toward it. According to Snell and Lemp, the sensation of
tension in the muscles serves as an indicator of the distance the object is away.
The direction of a visually perceived object corresponds directly to the position of
its image on the retina. In the determination of an object's distance to the eye,
there are a number of potential sources or cues of depth.
