ELEMENTS of VISUAL PERCEPTION

In This Lecture

• Elements of visual perception• Mechanics of the human visual system• Structure of the eye• Image formation in the eye• Brightness adaptation and discrimination

Structure of human eye

• Diameter: 20 mm

• 3 membranes enclose the eye– Cornea & sclera– Choroid– Retina

• Cornea-anterior surface • Sclera opaque membrane • The choroid

• Retina- Photoreceptors convert light into electrical impulses that are decoded by brain

Light Receptor BrainRadiant

Energy

The Choroid

• The choroid contains blood vessels for eye nutrition

• Heavily pigmented to reduce extraneous light entrance and backscatter.

• It is divided into the ciliary body and the iris diaphragm, which controls the amount of light that enters the pupil (2 mm ~ 8 mm).

• Iris-visible pigment &black pigment

The Lens• The lens is made up of fibrous cells and is

suspended by fibers that attach it to the ciliary body.– Lens -concentric layer of fiberous

cells,70%water,6%fat,and more protein– Yellow pigmentation– cataracts– Lens focuses light on to photoreceptive area (retina)

• It is slightly yellow and absorbs approx. 8% of the visible light spectrum.

• More absorption for ultra &infrared rays

The Retina

• The retina lines the entire posterior portion.

• Discrete light receptors are distributed over the surface of the retina:

– cones (6-7 million per eye) and – rods (75-150 million per eye)

Cones

• Cones are located in the fovea and are sensitive to color.

• Each one is connected to its own nerve end.• Cone cells: located at fovea shorter/thicker. Less

sensitive than rods .In normal light luminance level 1 to 10^6 cd/m²).

• The vision of cone cells dominates and is photopic vision/bright light vision. There is good visual acuity (VA) and color discrimination.

Rods• Rod cells:150million/long /thin. • Scotopic vision is produced exclusively through

rod cells so there is no color perception. • Scotopic vision/dim light vision occurs at

luminance levels of 10^-2 to 10^-6 cd/m².The human eye rod cells are functional in low light .

• Several rods are connected to a single nerve and are sensitive to low levels of illumination (scotopic or dim-light vision).

• Mesophic vision occurs in intermediate lighting conditions (luminance level 10^-2 to 1 cd/m²) and is effectively a combination of scotopic and photopic vision. This however gives inaccurate visual acuity and colour discrimination.

•Radially symmetric about fovea•Receptor density measured in degrees•Cones dense at centre•Rods increases from 20 degree•Rods increase in density from the center out to approximately 20% off axis and then decrease.•Blind spot-absence of receptors

Receptor Distribution

The Fovea

• The fovea is circular (1.5 mm in diameter) but can be assumed to be a square sensor array (1.5 mm x 1.5 mm).

• The density of cones: 150,000 elements/mm2 ~ 337,000 for the fovea.

Image Formation in the Eye• Lens in the eye is flexible• Shape controlled by muscles to focus on distance

objects - Muscles flatten lens • To focus on close objects- Muscles allow lens to thicken• Centre of lens to retina is called focal length Varies from 17mm to 14mm focal length• Refractive power min to max• Lowest refractive power when focusing on objects further than 3m• Highest refractive power when focusing on near objects

Image Formation in the Eye

• Example: – Calculation of retinal image of an object

1710015 x

mmx 55.2

Image Formation in the Eye

• Perception takes place by the relative excitation of light receptors.

• These receptors transform radiant energy into electrical impulses that are ultimately decoded by the brain.

Brightness Adaptation & Discrimination

• Range of light intensity levels to which HVS (human visual system) can adapt: on the order of 1010.

• Subjective brightness (i.e. intensity as perceived by the HVS) is a logarithmic function of the light intensity incident on the eye.

Brightness Adaptation & Discrimination

• The HVS cannot operate over such a range simultaneously.

• Total range of intensity level eye can discriminate is small when compared with total adaptation range

• For any given set of conditions, the current sensitivity level of HVS is called the brightness adaptation level.

Brightness Adaptation & Discrimination

ratioWeber IIc

Where: Ic: the increment of illumination discriminable 50% of the time and

I : background illumination

• The eye also discriminates between changes in brightness at any specific adaptation level.

Brightness Adaptation & Discrimination

• The typical observer can discern one to two dozen different intensity changes

– i.e. the number of different intensities a person can see at any one point in a monochrome image

Brightness AdaptationPerceived Intensity • Logarithmic function of light received by eye.• Range of brightness's that can be discriminated simultaneously is small in comparison to total adaptation range• For a given set of conditions the current sensitivity level of the visual system is called the brightness adaptation level. Below Bb-all stimili are perceived as black

Light Intensity vs Subjective brightness

Bright Discrimination

• Digital Images are displayed as a discrete set of intensities

• Eyes ability to discriminate intensities at a given adaptation level is an important

• Consideration when displaying images Classic Experiment

• Weber Ratio: ΔIc/I• ΔIc is the incremental illumination I +

ΔI discriminable 50% of the time• Small Weber Ratio represents “good” Brightness discrimination

The perceived brightness is not a simple function of intensity

• Mach band pattern • Simultaneous contrast• Optical illusion

•Brightness depends on the luminance of the surrounding.•Luminance is independent of the luminance of the surrounding•Simultaneous Contrast is related to the fact that a region‘s perceived brightness does not depend simply on its intensity.

Mach band effect”

• Flat shading suffers • Human eyes accentuate

the discontinuity at the boundary

• Visual system tends to under or over shoot around the boundary of two regions of different

intensity

Reduce the mach band effect – remove value discontinuity

All the inner squares have the same intensity, but they appear progressively darker as the background becomes lighter.

simultaneous contrast

Optical illusion

Wrongly perceives