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Vision

Stefano Ferrari

Universita degli Studi di [email protected]

Methods for Image processing

academic year 2015–2016

Biological vision and image processing

I The human visual perception results from a chain ofprocessing tasks, often cooperating with other perceptualactivities.

I Image processing is a discipline (mainly) based onmathematical and statistical concepts, with constraintsimposed by the image acquisition technology.

I The study of the human vision is useful for more than onereason:

I the output of some processes is targeted for the vision of ahuman user;

I the operation of the organs devoted to the acquisition andperception of images can inspire the development of similarartificial devices; moreover, the performance of imageprocessing algorithms have to be compared;

I some known mulfunctioning of the biological apparatus do notaffect the image processing devices.

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Stefano Ferrari— Methods for Image processing— a.a. 2015/16 1

Eye

I cornea

I sclera

I pupil

I iris

I lens

I choroid

I retina

I fovea

I blind spot

Blind spot: close the left eye, sta-re to the cross, come closer to thescreen till the spot disappears.

Distribution of the receptors on the retina

On the retina, twokinds of receptorscan be found:

I cones;

I rods.

I Cones:I about 7 millions;I perception of details and rapid changes;I sensitive to colors;I sensitive in high illuminance conditions (photopic vision).

I Rods:I about 100 millions;I provide a large scale vision;I sensitive in low illuminance conditions (scotopic vision).

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Image formation

I The distance the lens and the retina is fixed (about 17 mm).

I The scene is focused by changing shape of the lens (fordistances up to three meters).

I The dimension of the object onto the retina is a function ofits distance (no zoom!).

Perceived brightnessI The human eye can operate in

a very large range of lightintensity level, from scotopicvision to glare.

I 1010 discriminated levels.I Perceived brightness is a

logartimic function of thelight intensity.

I However, this interval is notcovered simultaneously, butonly a subset of levels close tothe brightness of the scene canbe discriminated.

I The sensitivity range is shiftedas the brightness changes(brightness adaptation).

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Perceived brightness (2)

I The perceivedbrightness (subjectivebrightness) dependson the environmentalbrightness, Ba.

I Under the lightintensity thresholdBb, black isperceived.

I The upper limit is lessmeaningful: abrighter light canshift Ba.

Discrimination of intensities

I Experiment for evaluating thesensitivity to brightness changes: on auniformly illuminated screen atintensity I , a short-duration spot isprojected at intensity I + ∆I .

I As ∆I is progressively increased, thespot is more esily perceived: ∆Ic is theintensity change that makes the spotdiscriminated at 50% of the time.

I ∆Ic/I is called Weber ratio.

I A high value means low sensitivity: thespot have to be much brighter thanthe background to be discriminated.

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Discrimination of intensities (2)

a

b

I The eye can discriminate few(10–25) shades with respect tothe environmental brightness.

I However, this does not meansthat all the images can bereported using few shades:maybe locally they can beenough, but the brightness ofthe scene can change.

The same image at 256 (a) and 24(b) gray levels. In the regions whe-re the intensity changes slowly (skyand clouds), the lack of intermediateshades is apparent.

Perceived intensity

I The perceived brightnessdoes not depends only bythe intensity.

I The Mach bands areconstant intensity region.

I However, the boundarybetween two bands seemsdarker in the darkerregion and brighter in thebrighter band side.

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Contrast

a

b

I The phenomenon calledsimultaneous contrast isanother clue that theperceived brightness is notonly a function of theintensity.

I The center squares of figure(a) have the same intensity,but the perceived brightnessdepends on the backgroundintensity.

I Similarly, despite it seemsimpossible, in figure (b) thesquares A and B share thesame intensity.

Optical illusions

a b

c d

I The optical illusions areperceptions that mistakenlyrepresent the observedreality.

I The are caused bypeculiarities of the humanvision system and are aphenomenon activelystudied.

(a) e (b) perception of inexistentshapes. (c) wrong estimation ofthe length. (d) diagonal parallellines perceived as non-parallel.

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Optical illusions (2)

The optical illusions can occur also at higher level of abstraction.

The ability of recognizing shapes under vague stimulation (e.g.,animal-like shaped clouds) is well known and is called pareidolia.


a b

c d

The human visualsystem operates fa-ce recognition veryefficiently.The perceived dif-ference between theimages (a) and (b)is less than thatbetween (c) and (d).

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The perceived expression changes with the distance.

We see with the brain

I The optical illusions are very variegated and include bothstatic and dynamic phenomena.

I A good collection (with explanations):I 84 Optical Illusions & Visual Phenomena by Michael Bach,

http://www.michaelbach.de/ot.

I The wrong perception phenomena also affects the othersenses. For instance, the third sound of Tartini and binauralsounds are well known audio illusions.

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Homeworks and suggested readings

DIP, Section 2.1

I pp. 35–43

Mirage with parabolic mirrors

I http://www.ap.smu.ca/demos/index.

php?option=com_content&view=

article&id=122&Itemid=85

Autostereograms

I http://en.wikipedia.org/wiki/

Autostereogram

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