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Visual Perception of Images

A processed image is usually intended to be viewed by a humanobserver.

An understanding of how humans perceive visual stimuli—the humanvisual system (HVS)—is crucial to the design of many imageprocessing algorithms.

We need to understand:I photometric properties of physical worldI spatial response of the HVSI temporal response of the HVSI spectral/color response of the HVS

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Structure of Human Eye

I cornea - hard, fixedlens

I lens - soft, flexiblelens that allows thefocus of the eye tochange

I retina - the innermembrane on whichthe image is focusedand sensed

PupilIris

Posterior chamber

Lens

Cornea

Anterior chamber(aqueous humour)

Ciliary muscle

Sclera

Choroid

Retina

Zonularfibres

Optic disc

Optic nerve Fovea

Hyaloidcanal

Suspensoryligament

Retinalblood vessels

Vitreoushumour

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Visual System BasicsI rods - a class of photoreceptor cells that do not distinguish

wavelength but are sensitive to dim light (scotopic or dim-lightvision)

I cones - a class of photoreceptor cells that are sensitive towavelength at high light levels (photopic or bright light vision)

I three types of cones, roughly corresponding to bandpassresponses around red, green and blue wavelengths (long,medium, and short)

I the eye perceives color as a weighted combination of these threetypes of cones

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Brightness Adaptation

I The HVS can adapt to distinguish up to1010 light intensity levels from the scotopicthreshold to glare level.

I The intensity range discriminated in oneinstant (the brightness adaptation level) israther small.

I Light sensitivity increases in dim light anddecreases in bright light to accommodate awide range of illumination levels.

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Weber’s Law

Weber’s Law: A just-noticeable difference (JND), denoted ∆I, in astimulus is proportional to the magnitude of the original stimulus I.Contrast sensitivity = I/∆I.

I At very low luminance, detector noise and ambient light tend toreduce sensitivity, so the stimulus appears black.

I At very high luminance, the very bright background tends tosaturate detector sensitivity, thereby reducing sensitivity byblinding the subject.

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Simultaneous Contrast

The brightness perceived by our visual system is a function of boththe illumination emitted by the object and also its surroundings.

Although the central patch is exactly the same in each case,simultaneous brightness contrast makes it appear to vary from dark tolight as the background is changed from light to dark.

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Simultaneous Contrast

The brightness perceived by our visual system is a function of boththe illumination emitted by the object and also its surroundings.

Although the central patch is exactly the same in each case,simultaneous brightness contrast makes it appear to vary from dark tolight as the background is changed from light to dark.

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Mach Band Effect

I The luminance of the above squares increases stepwise.I In each strip the right edge looks darker than left edge.I This is due to lateral inhibition. The response at a given

receptor is inhibited by the response of neighboring receptors.

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Spatial Response of the HVS

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Spatial step response of the HVS

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Spatial impulse response of the HVS

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Contrast Sensitivy Function (CSF)

Contrast sensitivity functions of seven age groups(after Schieber, 1992)

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Spatial-Frequency Effects

Jekyll and Hyde Face Example

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Brightness Constancy

I We compensate psychologically for lighting effects.I This ability allows us to see objects in terms of inherent

reflectance rather than the actual amount of light reflected.Called brightness constancy.

I A white paper appears white to us even when it is in shadow orin a relatively dark room.

I Cameras must make exposure corrections indoors; otherwise, thepicture would not look the same as we see with our eyes.

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Shadow Effects

Effect of brightness adaptation and brightness constancy

I This photo shows actual lightintensity reflected from thelocation.

I This photo shows how weinterpret object brightnessbased on prior knowledge oflighting effects and notsimply the intensit of lightreflected.

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Shadow Effects — Another Example

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Color Adaptation

Our visual system adapts its color sensitivity in a manner that tends tomake the illumination look colorless.

Color film can achieve appropriate color balance only when exposedunder specific illumination for which the film is designed.

Stare at a point on the picture for 30 s. Then look at a blank wall. Youwill see an after-image due to local color adaptation.

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Noise Masking

Noise in an image is less visible in regions of the image that are busybut more visible in smooth, flat regions.

We can see that noise is noticeable in the smooth parts of the picture(clouds and sidewalk) and less noticeable in the busy parts (bushesand road).

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Bloch’s Law of Temporal Summation

I Flashes of light will appear to be the same if they have equaltotal energy and last less than approximately 30 ms in moderateillumination.

I Flashes of light at about 50 to 60 Hz cannot be distinguishedfrom a constant light with the same average energy. Thisfrequency is known as the critical fusion frequency (CFF).

I The flickering effect can be noticed when we see a TV monitor atan angle.

I CFF is higher for larger objects than smaller.I These effects have to be considered while coding or rendering

video, as blurring will appear if motion takes place within a 20ms interval.

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